A Treatise on Geology/Chapter 7
CHAP VII.
General Remarks.
IN a former part of this work[1] a general view is given
of the reasons which have guided modern geologists in
ascribing to a large class of rocks in the crust of the
earth an original state of igneous fusion; and in connection
with each system of strata some notice is taken
of the distribution and characteristic phenomena of the
igneous rocks locally associated therewith. We must
now take up the subject in a comprehensive point of
view, and elucidate its bearings on the general problem
of the effects of heat in the crust of the globe. We
must unite into one contemplation the history of the
whole series of igneous rocks of every age, from the supposed
" fundamental granite" to the volcanic mounds,
heaped up under daily observation. And in this review
care must be taken, both to combine and to analyse the
knowledge of igneous effects, so as to obtain from the
whole investigation trustworthy conclusions regarding
the true condition of the globe, in respect of heat, at
and below the surface, in successive geological periods.
Igneous Origin.—In asserting, concerning granite, basalt, porphyry, and other rocks, that they are of igneous origin, we must be careful to explain that it is not meant to affirm, that the materials of which these rocks consist have not existed together in any other combination, or been subject to other conditions previously. Fusion obliterates all or most of the marks of earlier states of material arrangement, and it is only in a few cases that direct or indirect evidence remains, by which to form a correct judgment respecting them. Granite may have been derived from the fusion of previously formed strata, a mode of origin confidently ascribed to certain ancient porphyritic rocks, and probable with regard to some modern lava. The origin of all natural phenomena is obscure; and with regard to the rocks above named, and others like them, all that it is now necessary to admit, is that, through whatever previous conditions the matter of which they consist has passed, their last combination, in which they now appear, has been caused by the agency of heat.
Geological Age.—Heat, though a simple cause, is productive of most complicated effects; not only because of the unequal action of different degrees of heat, or the various habitudes of the substances operated on, taken singly or in combination, but because extraneous circumstances, such as pressure, the passage of electrical currents, &c., affect the condition of the fused mass, and modify the aspect and arrangement of the solidified products. The mere antiquity of an igneous rock is a circumstance absolutely inefficient in accounting for any other of its characters than the degree of superficial waste, or internal change by particular agencies; and therefore an inquiry into the composition and structure of such rocks must in the first instance include the whole series of igneous products, if we wish to determine, in the first place, the conditions to which particular phenomena are due, and, finally, to obtain a correct general history of the change of these conditions in the order of geological time.
Composition.—Reduced to their last molecules, all igneous rocks appear to be oxides of various metallic and metalloid bodies, oxygen constituting about one half of their weight; silicium, aluminum, magnesium, calcium, potassium, sodium, iron, &c., are the most prevalent elementary bases, of even the most dissimilar rocks.
Silica, or silicium combined with oxygen, is found abundantly in perhaps every igneous rock, and very commonly Is combined in definite atomic proportions with lime, alumina, &c., so as to form a peculiar class of compounds, called silicates, bi silicates, and trisilicates, according to the atomic proportion of silica in the mineral. So general is this fact that, considering the easy fusion of most earthy substances in contact with silica, and the well known fact that in most of the igneous rocks some superabundant silica remains (in the state of quartz), we may contemplate the whole mass of these rocks as having existed in the state of a siliceous glass, from which, according to the admixture of other elements, silicates, bisilicates, &c., would be formed by crystallisation; or, according to the rate of cooling, pressure, and other circumstances, earthy aggregates, compact stones, or glassy products, result.
According to this view, the differences between some of the most remarkable igneous rocks are merely in the degrees of arrangement to which their particles have been subjected. As lava, obsidian, and pumice, are merely three states of the same volcanic product, so probably the granitic, porphyritic, and homogeneous rocks, generated by heat in ancient times, have derived their characteristic structures from the conditions of their solidification. On this subject it is satisfactory to refer to the capital experiments of Mr. Gregory Watt (Phil. Trans. 1804), which are among the most interesting and instructive on record, and have been repeated by other observers with like success.
Mr. Watt's experiments were made on the amorphous basalt of Rowley, in Staffordshire, a fusible, fine grained, confusedly crystalline stone, of dark colour, and opaque. It affects the magnetic needle, and has a specific gravity of 2.868.
Seven hundred weight of this rock was placed in a reverberatory furnace, on the elevated part of the interior, between the fire and the chimney, from whence, as it melted, it flowed into the deeper part, where the melted iron is usually collected. When the whole was melted, it formed a liquid glass, rather tenacious. From this a large ladleful was taken; which being allowed to cool, retained the characters of perfect glass. The fire was maintained throughout, with gradual diminution, for more than six hours, after which time the draught of the chimney was intercepted; the surface of the glass was covered with heated sand, and the furnace was filled with coals, which were consumed very slowly. By these precautions the heat was so slowly conducted away, that it was eight days before the mass in the furnace was sufficiently cool to be extracted, and even then it retained considerable warmth.
The form of the mass, being derived from the bottom of the furnace, was considerably irregular, approaching to the shape of a wedge, whose lower angles were rounded. It was nearly three feet and a half long, two feet and a half wide, about four inches thick at one end, and above eighteen inches at the other. From this diversity of thickness, and from the unequal action of the heat of the furnace, too great an irregularity had prevailed in the refrigeration of the glass to permit the attainment of a homogeneous texture. These circumstances might probably have been counteracted by better devised precautions; but the inequality of the product is not to be regretted, since it disclosed some very singular peculiarities in the arrangement of bodies passing from a vitreous to a stony state, which might have remained unobserved, if the desired homogeneity of the result had been obtained.
1. This substance is easily fused into glass, with few air-bubbles; it then possesses an undulated conchoidal fracture, is black and opaque, except in thin fragments, and harder than felspar. Its sp. gr. is 2.743, and it has no action on the magnetic needle.
2. The tendency towards arrangement, in the particles of the fluid glass, is first developed by the formation of minute globules, which are generally nearly spherical, but sometimes elongated, and which are thickly disseminated through the mass. The colour of these globules is considerably lighter than that of the glass; they are commonly greyish brown, sometimes inclining to chocolate-brown; and when they have been formed near the interior surface of the cavities in the glass, they project, and resemble a cluster of small seeds. Their diameter rarely exceeds a line, and seldom attains that size, as in general they are so near to one another that their surfaces touch before they can acquire considerable magnitude. In the process of cooling, they adapt their form to their confined situation, fill up every interstice, and finally present a homogeneous body wholly unlike glass, and equally unlike the parent basalt. When the union of the little globules has been imperfectly effected, the fracture of the mass indicates its structure by numerous minute conchoidal surfaces, which display the form of each globule.
But, if the arrangement has extended a little farther, all these subdivisions are entirely lost; the mass becomes perfectly compact; has an even or a flat conchoidal fracture; is nearly of the same hardness as the glass; is commonly of a chocolate colour, graduating into a brownish black; and the intensity of the colour increases in proportion to the degree to which the arrangement has extended. Its aspect is rather greasy; and it much resembles some varieties of jasper in the compactness of its texture, and in its opacity. Its magnetic action is extremely feeble. Sp. gr. 2.938.
3. If the mass were now rapidly cooled, it is obvious that the result would be the substance just described; but if the temperature adapted to the further arrangement of its particles be continued, another change is immediately commenced, by the progress of which it acquires a more stony texture, and much greater tenacity, and its colour deepens as these changes advance, till it becomes absolutely black. Sometimes this alteration is effected by a gradual transition, the limits of which cannot be assigned, but more generally by the formation of secondary spheroids in the heart of the compact jaspideous substance. These spheroids differ essentially from those first described; the centres of their formation are more remote from each other, and their magnitude is proportionably greater, sometimes extending to a diameter of two inches, and seeming only to be limited by contact with the peripheries of other spheroids. They are radiated, with distinct fibres: sometimes the fibres resemble those of brown haematites, and sometimes they are fasciculated irregularly, so as to be very similar in appearance to the argillaceous iron ores rendered prismatic by torrefaction. They are generally well defined, and easily separable from the mass they are engaged in; and often the fibres divide at equal distances from the centre, so as to detach portions of the spheroid in concentric coats. The transverse fracture of the fibres is compact and fine-grained; the colour black; and the hardness somewhat inferior to that of the basaltic glass. When two of the spheroids come into contact by mutual enlargement, no intermixture of their fibres seems to take place: they appear equally impenetrable, and in consequence both are compressed; their limits are defined by a plane, at which a separation readily takes place, and each of the sides is invested with a rusty colour. When several spheroids come in contact on the same level, they are formed by mutual pressure into pretty regular prisms, whose division is perfectly defined; and when a spheroid is surrounded on all sides by others, it is compressed into an irregular polyhedron.
4. The transition from this fibrous state to a different arrangement seems to be very rapid; for the centre of most of the spheroids becomes compact before they attain the diameter of half an inch. As the fibrous structure propagates itself by radiating into the unarranged mass, the compact nucleus which supplies its place gradually extends till it finally attains the limits of the spheroids; and the same arrangement pervades the matter comprehended between them. The mass has now assumed a compact stony texture, and possesses great tenacity. Its hardness is somewhat inferior to that of the glass from which it was formed. Its action on the magnetic needle is very considerable. Sp. grav. 2.938. Its colour is black, inclining to steel grey; it is absolutely opaque, and only reflects light from a few minute points. Though the divisions between the spheroids are rendered imperceptible to the eye, they are not obliterated, and their rusty surfaces are often disclosed by an attempt to fracture the mass.
5. A continuation of the temperature favourable to
arrangement speedily induces another change. The texture
of the mass becomes more granular, its colour rather
more grey, and the brilliant points larger and more numerous;
nor is it long before these brilliant molecules
arrange themselves into regular forms; and, finally, the
whole mass becomes pervaded by thin crystalline laminæ,
which intersect it in every direction, and form projecting
crystals in the cavities. The hardness of the basis
seems to continue nearly the same; but the aggregate
action of the basis and of the imbedded crystals on the
magnetic needle is prodigiously increased. The substance
now appears to possess some polarity, and minute
fragments of it are suspended by a magnet. Its specific
gravity is somewhat increased, as it is now 2.949.
The crystals contained in it, when examined by a microscope,
appear to be fasciculi of slender prisms, nearly
rectangular, terminated by planes perpendicular to the
axis: they are extremely brilliant; their colour is greenish
black; they are harder than glass, and fusible at the
blowpipe; they are suspended by the action of a magnet.
They are arranged nearly side by side, but not accumulated
in thickness, so that they present the appearance
of broad thin laminæ; they cross one another at all
angles, but always on nearly the same plane; and the
lamina thus formed is often three or four lines long, and
from a line to a line and a half broad, but always extremely
thin.
The cavities which existed in the glass are not obliterated during the subsequent processes, though changed on the surfaces.
All these steps in this remarkable experiment may be compared with parallel instances in the products of volcanos.
Thus, from homogeneous obsidian we pass to that variety of it which envelopes small globular concretions; and these, by increasing in number and size, convert the whole into a finely granular mass.
The increase of arrangement is traced through the lavas with interspersed crystals, becoming decidedly porphyritic, until at length we find the whole a congeries of crystals.
In the older rocks of igneous origin a similar gradation is observable—through homogeneous pitchstone, pitchstone with globules, to pitchstone with crystals;—through claystone, claystone with concretions, with felspar crystals, with felspar, and quartz crystals;—through amorphous felspar, with felspar crystals, with felspar and quartz crystals, with felspar, quartz, and hornblende crystals, passing to sienite,—with felspar, quartz, and mica, scarcely distinct from granite.
The process of crystallisation being determined by the attractions of the particles, it by no means follows that the most in fusible substance in an igneous fluid, or the most insoluble in an aqueous solution, should be the first to crystallise. In either case the particles of different kinds are mixed together; and it depends upon their relative elective attractions and cohesive forces, what crystals shall be the first generated. Now as the elective attractions between particles of different nature, superadded to the common force of cohesion, will tend to bring these together with more energy than the homogeneous particles, it follows that, in most instances, crystals compounded of several ingredients should be formed before those which consist of one simple substance; and this seems to explain the remarkable general fact, that quartz, the most in fusible portion of granite, should be impressed by the previously formed crystals of felspar and mica.
Nevertheless, the degree of infusibility of the ingredients must be allowed to have a considerable influence in determining the order of crystallisation; because, in the first place, no crystal can be formed at a heat sufficient for its entire fusibility; and, 2dly, the action of heat seeming to be directly opposed both to elective attraction and the force of cohesion, if the fusing points of the materials be very unequal, the refractory substance may be collected together at a heat too great to permit any other part of the compound to solidify.
However, as in real solution and fusion we must in general suppose the materials resolved into their atomic constituents, the former state of things seems likely to be most common; and we ought in consequence to expect that a portion of the most abundant substance should remain till the last, and appear as a homogeneous enveloping base, whether crystallised or not.
This is remarkably the case with granite, which appears to have been once a melted fluid, consisting of the ingredients of felspar and mica, with an excess of silica; and this often remains not exactly as an enveloping paste, but in detached and irregular masses, filling the vacuities between the crystals of felspar and mica.
The rate of cooling is shown by Mr. Watt's experiments to have a most decided influence on the ultimate condition of earthy masses solidified from igneous fusion; the degree of pressure under which the solidification happens is also influential, by introducing a new force, to modify the relative molecular attractions. Of this sir James Hall's experiments on powdered limestone offer a satisfactory proof. Under a pressure which prevents the escape of its carbonic acid, limestone undergoes fusion, and assumes different degrees of consolidation and crystallisation, according to the pressure.
The principal products of volcanic action are known to us in the form of slender lava currents, and scattered scoria and ashes, which are all cooled and solidified in the air with greater rapidity, and under less pressure, than under the deep roots of a volcanic mountain. The same materials which, cooled at the surface of the earth, may be of glassy nature, as obsidian, or cellular, as most lava, may be, and probably are, at great depths in the earth's crust, or even under the sea, solidified with structures as highly crystalline, and in masses as dense, as those of granite or greenstone. And as in fact we know, from careful observation, that granites, greenstones, and other ancient rocks of igneous origin, were solidified under the pressure of the sea, and generally below a great mass of strata on its bed, it is not without good reason that modern geologists have drawn a line of distinction between the plutonic rocks, elaborated in the deep recesses of the earth, and the volcanic products, which are solidified at or near the surface. This distinction is indeed one of degree, and may be misapplied, and is neither complete nor exact when used, as it frequently is, absolutely to separate the consideration of the old and the modern products of heat. There are crystallised rocks among the products of modern volcanos, and glassy lavas among the ancient strata; basalt is both an ancient and a modern product; yet, as a general rule, it is true that the ancient igneous rocks possess those characters which we may believe to belong to slower cooling under greater pressure than the lavas which flow from subaerial volcanos have experienced. A philosophical consideration of the subject will always recognise the essential differences of subterranean, submarine, and subaerial solidification, as independent of geological antiquity; and philosophical observation will gradually enable us to detect these differences, and to employ them in tracing the changing conditions of the terraqueous globe.
Mineral Composition of Unstratified Rocks.
In those rocks of igneous origin, which permit the ingredients of which they are composed to be clearly distinguished, one mineral substance is almost universally found, viz. felspar, which equally abounds in the oldest granites and most recent lavas, and occurs, though not in equal abundance, in rocks of very different weight, colour, and chemical composition.
Very frequently, though not universally, we detect another mineral, which, under two forms, has been called by two distinct names, augite and hornblende (pyroxene and amphibole of Haüy). These, by the admirable researches of Rose and Mitscherlich, have been shown to acquire their characteristic differences of crystallisation from the rate of cooling to which they have been subjected. This protean mineral (which varies greatly in its chemical composition, by the substitution of different ingredients in combination with silica) constitutes a great proportion of the substance of greenstone and basalt, and many congeneric rocks. In general they present themselves under different circumstances from those which accompany rocks allied to granite, but offer near approximations to some of the products of actual volcanos, the flags of melting furnaces, and other fruits of artificial heat.
These two minerals, felspar and hornblende, appear at opposite points of the circle of plutonic and volcanic, of ancient and modern igneous products; so that mineralogists have generally found reason to coincide with the opinions of Cordier and Scrope, and to adopt them as the elements for a fundamental classification of the rocks of fusion.
Thus we have two series of rocks, viz. felspathic and augitic (or hornblendic) rocks, of every geological age, which, in the extremes (as granite and basalt, among the ancient, and trachyte and basalt, among the modern rocks), are perfectly and strikingly different; but yet graduate into one another by innumerable variations, which demonstrate the similarity of origin of all the unstratified rocks, and at the same time open wide fields of inquiry into the causes and effects of their differences.
Besides these predominant and typical minerals, others are frequently observed to modify very much the characters of igneous rocks, as mica, quartz, garnet, schorl zircon, olivine, mesotype, epidote, hypersthene, diallage oxydulous iron, iron pyrites; cyanite, pinite, spodumene, topaz, beryl, corundum, chromate of iron, prehnite apatite, sphene, molybdena, &c. also occur—in particular rocks even abundantly.
According to the views previously established, every definite chemical mixture of earthy substances in fusion may be of crystalline, earthy, or vitreous texture; of uniform or unequal aspect in its parts; compact, cellular, or spumous; according to the circumstances of solidification.
The most correct way of describing a rock would be to give the formula of its mineral composition; but in uncrystalline masses this cannot be done, and the chemical composition of the same rock is not the same in even neighbouring parts. Geologists, therefore, whose more immediate object is to record the principal phenomena associated with rocks, have generally preferred to give distinctive names to those aspects of solidified igneous products which depend rather on the circumstances of their solidification, and indicate characteristic physical conditions of the globe, than on original and real differences of their own nature. Thus igneous rocks, with crystals lying detached in an uncrystallised basis, are generally called porphyries (as felspar porphyry, clay porphyry, trap porphyry, &c.); such as have concretions of quartz or mesotype, in place of those cavities which occur in modern lavas, are called amygdaloids. This method, though not strictly scientific, will perhaps always prevail; because the variations to which these rocks are subject are such as to baffle all mineralogical strictness; and because the most prominent and characteristic circumstances which accompany them, the form and manner of their exhibition, their relative antiquity, and the induration, metamorphism, and elevation of strata, appear but very indistinctly related to the formulae which represent their chemical or mineralogical nature. On this ground Dr. MacCulloch justifies his classification; in which rocks are often grouped under one head, not because they consist of the same ingredients, or of similar combinations of related minerals, but because they are related in age or position with regard to the strata, or fulfil other geological functions in common. In popular language, the mutual mixture of the crystals constitutes granitic; the separation of certain crystals defines the porphyritic; and peculiar divisional planes characterise the basaltic rocks; but every one of these circumstances belongs to almost every combination of felspar, quartz, mica, and hornblende. If we bear in mind that, in describing phenomena (for which chiefly technical names are useful), the first question to be answered is always with what these phenomena are associated, we shall see great reason to regret the neglect of eminent modern observers, who are satisfied with such terms as "trap" (which may be felspathic or hornblendic, porphyritic or amygdaloidal), or "granite," which may be a binary compound of felspar and quartz; a ternary mixture of quartz, felspar and mica; a quaternary union of quartz, felspar, mica, and hornblende, with or without large interspersed crystals of felspar, titaniferous iron, molybdena, apatite, &c., or may have the mica replaced by other congeneric substances.
This has been forcibly pointed out by Mr. Scrope, who has proposed a very intelligible plan of arrangement for volcanic rocks, on the basis of the relative abundance of the two conspicuous minerals felspar and hornblende (or augite), which, as before observed, compose the greater part of the igneous rocks of every age.
Mr. Scrope's synopsis of the species of volcanic rocks is as follows. (Journal of Science, vol. xxi.)
Trachyte.
Greystone.
Basalt.
If our knowledge of the true composition of many of the old rocks of fusion were perfect, we might propose for them a scale of classification parallel to that which Mr. Scrope has given for volcanic rocks. Of such a scale the following would appear to be the elements:—
Division I.—Felspathic.
Rocks in which the characteristic and most abundant mineral, felspar, is not at all or but slightly mixed with hornblende, augite, or their conveners, hypersthene, diallage, &c.
| Ancient. | Modern. |
Granitic and most porphyritic rocks. |
Trachytic rocks of Von Buch, Cordier, Scrope, &c.
|
| Division II.— | Hornblende, | Felspathic. | ||
| Augite, &c. |
Rocks in which felspar is mixed in nearly equal proportion with hornblende or augite, or their conveners, hypersthene, diallage, &c.
| Ancient. | Modern. |
Sienitic and greenstone rocks. |
Greystones of Mr. Scrope.
|
Division III.—Hornblendic, Augitic, &c.
Rocks in which hornblende, augite, hypersthene, or diallage predominates over the felspar (or its representative olivine, &c.), and sometimes constitutes the whole mass of the rock.
| Ancient. | Modern. |
Basaltic series of most authors. |
Basaltic series of Scrope.
|
To each of these three divisions belong the granular, earthy, compact, resinous, and vitreous textures; porphyritic, concretionary, amygdaloidal, and cellular structures; cuboidal, prismatic, spheroidal, or irregular divisional planes. (Among recent igneous rocks the cellular and vitreous structure passes to spumous and filamentous:—pumice and scoria.)
To each of them belongs also a peculiar set of stratified analogues—as gneiss to granite; some hornblende slates to greenstones; wacké to basalt,—which are often embarrassing to the observer, and perplexing to the reasoner, even with the advantage of Mr. Lyell's views of "metamorphic" rocks, (for which consult a future section).
Exposed to the wasting agency of the atmosphere and water, few resist decomposition, and then yield clay or sand, often of great fertility.
A classification and nomenclature upon this system, which should embrace the igneous rocks of all ages, might, if accepted generally among observers, confer great benefits on geology. It would, however, necessitate an almost total change of descriptive names, and would render it indispensable for geologists to study mineralogy with more care than is now given to that rather difficult subject. It seems therefore unlikely that success would attend such a system if proposed at this time, more especially when we remember how very little egard has been paid in England to the classification and nomenclature of mixed rocks devised by M. Brongniart. The system alluded to is, however, well worthy of consideration; and being much and usefully employed on the Continent, it appears proper to offer the following brief account of that portion which relates to our present subject.
Mixed Rocks.
Gradations among Igneous Rocks.
The rocks of igneous origin exhibit among one another particular relations and gradations, which it is important to attend to before proceeding to discuss some other points of their history. That such variations should take place among the felspathic rocks on the one hand, and among the augitic rocks on the other, was quite to be expected; but, in fact, between these generally opposite groups some transitions are known. Dr. Hibbert Ware has noticed, in his work on the Shetland islands, a gradation from binary granite (composed of quartz and felspar) to a basaltic rock (composed of hornblende and some felspar). He also describes a transition from felspar porphyry into granite, near Hillswick Ness.
M. Necker informs us, that, in the depth of the valley of the Valteline, which is in the anticlinal axis of the Alps north of Como, three great protuberances of granite arise, surmounted by gneiss and mica schist. The granite resembles that of the Valorsine and Mittenwald, in the Tyrol, being composed of grey quartz, white felspar, and black mica, and it throws up veins into the schist ore rocks. This granite is seen to pass, by an easy gradation, first to common sienite, then to sienitic hypersthene, some of which has white felspar and black hypersthene, some green hypersthene, and greenish felspar. This rock varies also in the size of the grain and the reflections of the hypersthene; it partly resembles diallage rock and partly greenstone; the different varieties are intermingled, and the complication is augmented by contemporaneous veins of fine-grained granite entering the hypersthene. The granite is traversed by veins of quartz enclosing black tourmaline. (Bibliothèque Universelle, 1829.)
This description of M. Necker will remind the geologist who has examined the granitic region of the Caldew, in Cumberland, of what is there a probable, but not a certain, inference, the connection of the granite of the base of Saddleback (which, like that of the Valteline, is composed of grey quartz, white felspar, and black mica) with the hypersthenic sienite of Carrock Fell, which passes into common sienite, and in places cannot be distinguished from diallage rock or greenstone. It often encloses magnetic iron ore. Von Buch speaks of the transition of "gabbro," or diallage rock, to granite, in the island of Kielvig.
No author has given more attention to the transitions which obtain between the various pyrogenous rocks, nor with greater success, than the late Dr. MacCulloch, to whom, indeed, modern geologists owe a large debt, for the clear and masterly conceptions he has published on this subject. He tells us, concerning the granites of Aberdeen shire, which are generally composed of quartz, felspar, and mica, that in this compound hornblende is occasionally substituted for mica; that the quartz sometimes fails; that this hornblendic mass becomes fine grained, and passes to greenstone, basalt, and an earthy trap-like claystone.
Von Dechen, in the German translation of De la Beche's manual, expresses very clearly the state of opinion among geological observers, as to the gradation in character from one to another, of all the igneous rocks. Thus granite, by replacement of its mica with hornblende, changes to sienite; by containing detached felspar crystals, it becomes porphyritic; and when reduced to very fine grains, we can entirely corroborate Von Dechen in saying that it is indistinguishable from felspar porphyry. A more earthy basis gives us clay porphyry; a concentric internal arrangement makes globular porphyry (kugel porphyr).
Trachyte and porphyritic trachyte are a parallel series to granite and porphyritic granite; in an earthy state they constitute domite.
Sienite and felspar porphyry pass by variation of mineral ingredients to the vague group of greenstones or traps, in which hornblende or augite forms a prominent part of the mass. Of these, diorite (diabase or greenstone) is related to sienite (the gradations being called greenstone sienite, and sienitic greenstone, &c.). The total absence of felspar turns such greenstones into hornblende rocks; diorites with extremely fine grains are called aphanite, and these cannot often be separated from the more quartzose rocks, usually called hornstone (by other writers, petrosilex or cornean). Such a basis, with crystals of felspar and hornblende, is often called greenstone porphyry, green porphyry, &c.
Dolerite (mimose of Brongniart) differs from diorite by holding augite instead of hornblende; its fine-grained varieties pass into the vague group of basalts or whinstones, which, if restricted to a common definition, should contain magnetic (titaniferous) iron ore.
Augite alone rarely constitutes a rock (lherzolite, or augite rock). The compact rocks, like aphanite, compact basalts, &c., change to amygdaloids, when they include masses of extraneous minerals, which fill, or appear to fill, cavities in the stone like those common in lava; the basis of many amygdaloids is earthy, and is called wacké. The rock called gabbro (euphotide, diallage rock, hypersthene rock) is characterised by its mixed felspar and diallage, or hypersthene; and serpentine is a corresponding but uncrystallised mates of felspar and schiller spar, usually enclosing several talcose minerals.
Felspar, the most abundant of all the minerals in rocks of igneous origin, is variable as to the alkaline portion of it; for in some (common felspar), potash—in others (labradorite), soda—in others (albite), lime and soda, are found. Von Dechen tells us that common felspar is mostly found in quartziferous and hornblendic mixtures; labradorite in mixtures with augitic minerals; while albite, though sometimes mixed with common felspar, constitutes but a small part of the masses of igneous rocks.
Chemical Composition of the Rocks of Igneous Origin.
The permutations which take place among the mineral ingredients of igneous rocks are easily and clearly intelligible by considering the chemical composition of these minerals, which, as in the case of hornblende, augite, hypersthene, and diallage, often differ from one another, rather by the crystalline arrangement of the parts, or the substitution of mutually replacing substances, than by any essential and constant characters. If the pyrogenous rocks of every age be restored in imagination to their ancient state of fluidity, and their chemical constitution in this state be calculated from the analysis of their integrand minerals, we shall find a remarkable general analogy running through them, and be able to perceive, in some instances, the reason of those gradations in mineral characters, which link into one system a long series of seemingly different rocks.
Mr. De la Beche has given some calculations on this subject, founded on the assumed elementary composition of minerals which are of frequent occurrence in igneous rocks. Some of the analyses adopted by Mr. De la Beche, and the calculations founded on them, are appended, with a few additions of our own.
Analysis of Minerals in Igneous Products.
| Silice. | Alumine. | Lime. | Magnesia. | Soda. | Soda. | Oxides of Iron and Manganese. | Fluoric Acid of Boracic acid. | Water. | |
| Felspar, common | 64.0 | 18.9 | 0.8 | — | 13.7 | — | 0.7 | ||
| Felspar, albite | 69.5 | 19.4 | 0.2 | 0.1 | — | 10.0 | 0.3 | ||
| Mica | 46.1 | 26.2 | 0.4 | 5.0 | 10.0 | — | 8.8 | 1.1 | 2.0 |
| Hornblende | 45.7 | 12.2 | 13.8 | 18.8 | — | — | 7.5[3] | 6.5 | — |
| Augite of Etna[4] | 52.0 | 3.3 | 13.2 | 10.0 | — | — | 16.7 | — | 4.8 |
| Tourmaline | 36.0 | 35.8 | 0.3 | 4.4 | 0.7 | 2.0 | 15.3 | 3.5[5] | — |
| Hypersthene | 54.2 | 2.3 | 1.5 | 14.0 | — | — | 24.5 | — | 1.0 |
| Diallage | 47.2 | 3.7 | 13.1 | 24.4 | — | — | 7.4[3] | — | 3.2 |
In this list, the most variable substances are mica, augite, and hornblende; the most uniform is felspar. The variety of composition in mica is extraordinary, as the following comparative table, in which the four varieties are classed according to the predominance of magnesia, alumina, potash, or oxide of iron, will show.
| Silica. | Alum. | Magnesia. | Potash. | Oxide or Iron and Manganese. | Fluoric Acid. | Analysts | |
| Mica, magnesian of Siberia | 42.5 | 16.0 | 26.0 | 7.6 | 5.0 | 0.7 | Rose |
| aluminous of Sweden | 46.4 | 34.8 | — | 8.8 | 5.8 | 0.8 | Rose (3 analys.) |
| ferruginous of Siberia | 42.5 | 11.5 | 9.0 | 10.0 | 22.0 | — | Klaporth. |
| Potash of Moscow | 40.0 | 11.0 | 19.0 | 20.0 | 8.0 | — | Vauquelin |
| 171.4 | 73.3 | 54.0 | 46.4 | 40.8 | |||
| Average | 42.8 | 18.3 | 13.5 | 11.3 | 10.5 | 0.4 | = 96.5 |
| The sum = 1OO gives | 44.3 | 18.9 | 14.9 | 11.7 | 10.6 | 0.4 | = 99.9 |
Granite, of the ordinary kind, compounded of quartz, felspar, and mica, varies greatly in the proportion of these substances, yet the fused glasses from which these various products have crystallised, might differ only by small variations in the proportions of the ingredients.
Granite, composed of quartz 2 parts, felspar 2 parts, and mica 1 part, would, according to Mr. de la Beche's calculation, be represented in column 1. of the table below; and porphyritic granite, composed of quartz 2 parts, felspar 3 parts, and mica 1 part, in column 2.; and we have added binary granite (felspar 3 parts, and quartz 2 parts) in column 3.
| 1. | 2. | 3. | |
| Silica | 74.84 | 73.04 | 75.1 |
| Alumina | 12.80 | 13.63 | 10.9 |
| Potash | 7.48 | 8.51 | 9.8 |
| Magnesia | 0.99 | 0.83 | |
| Lime | 0.37 | 0.44 | 0.5 |
| Oxide of iron | 1.93 | 1.73 | 0.4 |
| Oxide of Manganese | 0.12 | 0.10 | |
| Fluoric acid | 0.21 | 0.18 |
The differences of the ultimate analysis are very much smaller than the different aspect of the rocks might lead us to expect.
Sienite, composed of quartz, felspar, and hornblende, in equal proportions, would be represented in the subjoined table by column 1.; sienitic granite in which quartz, felspar, and mica should appear in equal proportions, in column 2.; schorl rock, composed of equal parts of quartz and schorl, in column 3.
| 1. | 2. | 3. | |||||
| Silica | 69.91 | 63.96 | 68.01 | ||||
| Alumina | 10.37 | 14.32 | 17.91 | ||||
| Potash | 4.55 | 5.94 |
| ||||
| Lime | 4.86 | 3.73 | 0.14 | ||||
| Magnesia | 6.26 | 5.94 | 2.22 | ||||
| Oxide of iron | 2.69 | 4.06 | 6.85 | ||||
| Oxide of Manganese | 0.07 | 0.21 | 0.81 | ||||
| Fluoric acid | 0.50 | 0.65 | 1.79 |
Turning from these rocks, in which quartz is an essential constituent, to those which are composed of felspar united with hornblende or some analogous mineral, we have greenstone (felspar and hornblende in equal parts) represented in the first column of the next table;
| 1. | 2. | 3. | |
| Silica | 54.86 | 59.14 | 58.42 |
| Alumina | 15.56 | 10.59 | 13.86 |
| Potash | 6.83 | 6.83 | 9.10 |
| Lime | 7.29 | 1.13 | 4.87 |
| Magnesia | 9.39 | 7.00 | 8.13 |
| Oxide of iron | 4.03 | 12.62 | 2.00 |
| Oxide of Manganese | 0.11 | ||
| Fluoric acid | 0.75 | ||
| 0.50 | 1.06 |
equal parts) in column 2.; and diallage rock (two thirds of common felspar and one third of diallage) in column 3.
Serpentine, usually considered to be little else than diallage or schiller spar, seems to be well represented in general, by supposing it a hydrated sub silicate of magnesia; and contains besides chrome and other metals, alumina, &c. (in all 5 per cent.):—
| Silica | about | 42 |
| Magnesia | —— | 38 |
| Water | —— | 15 |
A sub silicate of magnesia would contain very nearly the same proportions of the earths.
Among the rocks known to be of volcanic origin, porphyry, which graduates to claystone, and trachyte,— trachyte, which in a vitreous state becomes one kind of obsidian,—and pumice, which is a spumous or filamentous form of obsidian,—appear to compose one long series of felspathic compounds, remarkably analogous to granite, both by mineral variations (where these can be clearly seen) and by chemical composition. The analysis of obsidian from Hecla, by Vauquelin, yielded the results in column 1.; while in column 2. is the composition of a siliceous granite, which we have calculated from the proportions of quartz 3 parts, common felspar, albite, and mica, each 1 part. In column 3. is the analysis of
| 1. | 2. | 3. | 4. | |
| Silica | 78 | 80.1 | 80.2 | 80.9 |
| Alumina | 10 | 10.0 | 12.7 | 10.2 |
| Potash | 6 | 4.0 | — | 1.7 |
| Lime | 1 | 0.7 | 1.1 | 0.2 |
| Magnesia | — | — | — | 0.8 |
| Soda | 1.6 | 1.7 | 1.9 | 3.3 |
| Oxide of iron and manganese | 1.0 | 1.5 | 1.1 | 1.5 |
| Fluoric acid & water | — | 0.5 | — | 0.5 |
the bitumen and water omitted); and in column 4. a granite of 3 parts quartz, 2 parts albite, and 1 part mica.
Pumice, the last term of this siliceous series, is stated by W. Phillips to be composed of
| Silica, 77.5 | Potash and soda, 3.0. |
| Alumina, 17.5. | Oxide of iron, 1.7. |
As an example of greystone? lava, Dr. Kennedy's analysis of the compact lava of Calabria may be quoted:—
| Silica, 51. | Soda, 4. |
| Alumina, 19. | Iron, 14. |
| Lime, 10. | Water, 1. |
Basalt, which belongs to almost every geological age, constitutes the last term of this series, in which silica is diminishing continually. It is very irregular in composition, as might be expected from the character of its predominant ingredient, hornblende or augite. The basalt of Hasenberg in Saxony, according to Klaproth, is composed of
| Silica, 44.50. | Soda, 2.60. |
| Alumina, 16.75. | Oxide of iron, 20.00. |
| Lime, 9.50. | Oxide of manganese, 0.12. |
| Magnesia, 2.25. | Water, 2.00. |
That of Staffa, according to Dr. Kennedy,
| Silica, 48. | Soda, 4. |
| Alumina, 16. | Muriatic acid, 1. |
| Oxide of iron, 16. | Water, &c., 5. |
| Lime, 9. |
Exterior Forms of the Masses of Igneous Rocks.
Interposed Beds.—As before observed, the want of stratification is one of the characters of igneous rocks; yet there are two cases in which they show themselves in stratiform masses, which seem exceptions to the rule. One of these cases has been amply treated by Macculloch, in his account of the Island of Skye; examples of it may also be seen in the Island of Arran. The reader will understand the circumstance alluded to by consulting fig. 81., where (d) represents a vertical mass
of igneous rock (greenstone in Skye, pitchstone in Arran) filling & fissure in the stratified rocks, (s) and (b) an interposed bed of the same igneous rock forced in a liquid state between two strata originally contiguous.
The second case is exemplified in the basaltic formation of Antrim, where several successive layers of melted rock, the fruit of many successive volcanic eruptions, are heaped one upon another as they were originally poured out upon the chalky bed of the ancient sea.— Another example is furnished by the volcanic rock called "toadstone[6]," which in Derbyshire lies in one or more stratiformed masses between the beds of mountain limestone, as probably it was originally effused on the surface of the lower bed. The upper surface of the toadstone is said to be remarkably undulated. A third example is found in the region round Crossfell, where a basaltic formation, called the "whin sill," is widely spread in the midst of the limestones and sandstones, over some of which it appears to have poured as a submarine current of lava, while through and amongst others it was perhaps forcibly injected. The diagram No. 82. shows the manner in which the basaltic mass (b) grows thinner
in one direction (towards the west), and also the occurrence of a mineral vein (v) (yielding sulphuret of lead) in a fissure which divides equally the limestone and the "whin sill," and yields valuable metallic ores in each.
Overlying Masses.—In the preceding instances, igneous rocks are included between sedimentary strata; overlying masses, as they are called, spread irregularly over a surface of other rocks without being themselves covered by any. The same overflow of melted rock may, in one part, appear an overlying mass, and, in another, an interposed bed, as in the Clee Hills, in Salisbury Craig, near Edinburgh, &c. The porphyritic summit of Ben Nevis is an overlying mass, which has burst up through the granitic base of the mountain; the porphyritic mass at the lower end of St. John's Vale, Cumberland, is similarly circumstanced in relation to the slate rocks of that region; and the phenomenon is common. It is perfectly paralleled by what happens in many eruptions of lava, and was well illustrated by the great Icelandic lava currents in 1783.
Fissures.—In all these cases the situation of the once melted rocks is easily explicable by supposing, what in some cases is known to be the fact, that the horizontally extended masses of igneous rocks have been forced upwards through tubular passages or fissures, as happens at this day at the summit or on the sides of active volcanos. Such fissures or tubular passages occasionally appear connected in one long or in several short parallel lines; as, for example, among the silurian strata the line of eruptions marked by the trap rocks of the Wrekin, the Lawley, Caer-Caradoc, &c.; and, among existing volcanos on a greater scale, the linear volcanos, to which Von Buch was the first to direct attention. Great fissures, such as here alluded to, may be extremely irregular; the strata through which they break may be thrown into great confusion; their parts may be disjoined and separated by cavities. Into these irregular hollows the fused matter sometimes has been forced; and not infrequently large and small portions of the broken strata are inclosed in the midst of the igneous rock; while sometimes portions of the latter have flowed into cavities in the stratified masses, from which it is difficult to trace their connection with the main stream.
Such phenomena may be well studied in Salisbury Craigs, and other localities near Edinburgh; in Teesdale; the Caradoc Hill, &c.
Dykes.—A still more common form of appearance among igneous rocks is what is called a dyke, which agrees with the general description of similar rocks occupying a fissure; nor in some cases is there any distinction. But dykes, when seen in perfection, as in the Island of Arran, the coal-field of Durham and Newcastle, the limestone of Teesdale, the lias near Stokesley, the silurian rocks of Shropshire, or the slates of Snowdonia, present characters of greater symmetry, and claim a somewhat different origin. The fissures which inclose these trap dykes present often no trace of violent movement of the strata, which, on the contrary, sometimes appear level and undisturbed on both sides; these sides are remarkably parallel, plane, and either vertical, or slightly inclined, so that the inclosed mass of rock looks like a continuous wall. On the surface the dyke lies usually in a straight line from a few hundred yards to ten, twenty, and more miles in length.
Archdeacon Verschoyle has described several trap dykes which range on the coast of Mayo and Sligo: one of them extends altogether, in an east and west direction, sixty or seventy miles. One of the dykes, which
is represented in the diagram No. 83., continues in a 
d. A straight dyke passing East 20 North.
d′. Another, passing generally to the South of East.
perfectly straight line, across the Durham coal-fields, twenty miles, in a direction E.N.E.; the other, starting from the same point (near Middleton in Teesdale), extends into the eastern part of Yorkshire, nearly reaching Robin Hood's Bay, a distance of seventy miles, in an E.S.E. direction.
In some districts, rock dykes are wonderfully numerous. Forty-four trap dykes of various kinds were carefully noticed and measured by the author of these remarks, in a few miles of the coast of the Island of Arran, between Brodick and Lamlash. They abound no less on the western side of the same island at Tormore.
Veins.—One of the most interesting forms of occurrence of igneous rocks is that of veins, which penetrate and ramify irregularly in the fissures of the neighbouring rocks. These veins sometimes appear insulated in the midst of rocks more or less different from them in composition, except at the common surfaces, where the substance of the vein and the in closing rock are intimately united by intermediate characters of mineral composition or indistinguishable blending of the parts. In this manner granite frequently incloses parts in which hornblende, or mica, are particularly abundant or remarkably deficient; the redundancy and defect being equally referrible to circumstances which operated during the crystallisation of the stone. To such spherical, nodular, or elongated parts of a rock, the title of contemporaneous veins has been given by professor Jameson: they may also be called veins of segregation.
But the veins to which attention is now directed had a different origin, and disclose a different history. They sometimes may appear insulated in a mass of quite different rock, but there is little, or no gradation of mineral character at the common surface, and, when carefully traced, the veins are found connected with larger masses of their own substance at no great distance. (See diagram No. 84. p. 76.) Recollecting that all the igneous rocks, found intermixed with the strata, have been pressed by considerable mechanical force, it is an unexpected fact that veins, such as are now described, branching off into the minute cracks and fissures of the stratified masses, should be witnessed almost exclusively in granitic and sienitic compounds. Nor is our surprise lessened, when we find the lava or existing volcanos occasionally assuming the shape of veins, as well as of dykes, in the fissured substance of the crater and sides of the mountain.
Why, for example, should it almost never occur that the substance of porphyritic and basaltic dykes, whether they pass through slate, coal, sandstone, or limestone, is extended from the main body into the numerous small cracks and fissures which margin the dyke; while, on the other hand, there are few situations where granite comes in contact with gneiss, clay slate, limestone, mica slate, or hornblende slate, without throwing off many branches into those rocks?
One reason may be, that the porphyritic and other 'trappean' dykes, injected among the strata while they were cold, lost, like lava at the surface, their heat and fluidity too rapidly to penetrate the small fissures; while the enormous masses of granite in contact with the strata which they penetrate, may have retained their fluidity through a considerable period. But this is probably not the whole truth. One effect of the igneous rocks is to produce fissures in the stratified masses; and it is very conceivable that the small lateral fissures alluded to did not exist till after the partial or complete solidification of the rock which filled the dyke.
Examples of granite veins are innumerable, though
a few years only have passed since they were deemed
too rare to be of much value in supporting the Huttonian
doctrine of the crystallisation of this rock from
igneous fusion. Their importance was most fully understood
by Dr. Hutton, and his able supporter Playfair,
whose notices have not lost their value in the eyes of
modern inquirers. Distinguishing between the veins
which are clearly and completely traced to the large
masses of granite rock, and such as appear insulated,
Playfair describes the latter class as occurring in the
Western Islands, particularly in Coll, where they traverse
the beds of gneiss and hornblende schist. They are several
fathoms in thickness, obliquely intersecting the nearly
vertical planes of the strata. The beautiful Portsoy
granite is a vein or dyke; a similar granite is found
inland, near Huntly. The bed of the river Tilt, in the
distance of little more than a mile, is intersected by no
less than six very powerful veins of granite, all of them
accompanied with such marks of disorder and confusion
in the strata, as indicate very strongly the violence
with which the granite was here introduced into
its place. (Dr. Macculloch's view of these phenomena
in Glen Tilt is different.) "The second kind of granite
vein is one which proceeds visibly from a mass of that
rock, and penetrates into the contiguous strata. The
importance of this class of veins, for ascertaining the
relation between granite and other mineral bodies, has
been pointed out (§ 82.); and by means of them it has
been shown that the granite, though inferior in position,
is of more recent formation than the schistus incumbent
on it; and that the latter, instead of having been quietly
deposited on the former, has been, long after its deposition
and consolidation, heaved up from its horizontal
position by the liquid body of the granite forcibly impelled
against it from below."[7]
Among the cases quoted by Playfair in his further discussion of this subject, is the series of veins which accompany the junction of the granite and schist of Galloway. Sir J. Hall and Mr. Douglas, following the previous indications of Dr. Hutton and Mr. Clerk, traced the line of separation between the granite and schist all round a tract of country about eleven miles by seven, extending from the banks of Loch Ken westward; and in all this tract they found that wherever the junction of the granite with the schistus was visible, veins of the former, from fifty yards to the tenth of an inch in width, were to be seen running into the latter, and pervading it in all directions, so as to put it beyond all doubt that the granite of these veins, and consequently of the great body itself, which was observed to form with the veins one uninterrupted mass, must have flowed in a soft or liquid state into its present position.
Perhaps no better example of granite veins is known than in the mountain of Tornidneon, above Loch Ranza, which was examined by sir J. Hall. From a careful personal survey of this case, in 1826, the following notes and diagram (No. 84.) are extracted. The junction of granite and a dark quartziferous clay slate, with rather wavy laminæ, takes place nearly in a vertical line, rudely parallel to the lamination of the slate. The granite at a distance from the slate is very coarse grained (composed of quartz, felspar, and mica, occasionally with cavities in closing those minerals distinctly crystallised), and sometimes porphyritic; but where it touches the slate it appears fine grained and much more compact. Veins pass from the granitic mass in various directions: a great vein, which incloses fragments of slate, divides itself, and crosses at different angles the slaty laminæ, but is not ramified into many small strings. In the large vein the granite is coarse, but in the small veins it is fine grained.
The substance of granite veins is sometimes indistinguishable from that of the great mass whence they spring,
as in some of the veins which surround the granitic 
G. The mass of granite.
g. One of the veins. The style of dotting is intended to express the fineness or coarseness of grain in the granitic mass and veins.
s. Portions of slate included in the granite vein.
region of Galloway, and some of the veins in Glen Tilt; in other cases it is very much more fine in grain, and otherwise dissimilar to the parent rock, as at St. Michael's Mount, and in the ease already mentioned at Tornidneon; and sometimes it is said by Playfair to be more crystallised in the veins than in the mass. These differences probably depend on several circumstances. The rate of cooling, if at all rapid, would cause the thinker veins to be of fine grain, while the broader veins would more nearly approximate to the parent rock. This is exactly what occurs at Tornidneon. The remoteness of the point in the vein from the mass of igneous rock, and the nature of the strata penetrated, may also have influenced the particular mode of aggregation of the substance of the veins.
No part of the world equals Cornwall in the abundance of opportunities afforded by its sea cliffs, streams, and mines, for studying the veins which at almost every point branch off from the great subjacent masses of granite into the everywhere incumbent "killas." Professor Sedgwick, Dr. Forbes, sir H. Davy, Mr. Carne, Mr. Kenwood, Mr. De la Beche, Dr. Boase, Von Dechen, and many other eminent geologists, have paid great attention to their occurrence and characters, which certainly are very complex, and, to judge from the diversity of the published opinions concerning them, very perplexing. When, indeed, we see on the plans given by Von Dechen (Phil. Mag. 1829), granite veins ramified in almost every direction, of almost every size and form of sides, plane or indescribably twisted, of large or small grain, pure or holding fragments of the neighbouring killas, or mixed with greenstone crossed by quartz and schorl veins, and by metallic lodes which displace the veins of granite and quartz variously connected with serpentine masses and veins of steatite, it is surely not surprising that phenomena so various and remarkable, exhibited incompletely, should, if studied without reference to other and less complicated examples, be the source of confusion and discord between perfectly impartial observers and reasoners. Nor is this all the difficulty: the general relation of the laminar structure of killas to the faces of the granite masses is extremely difficult to reduce to a clear statement. The killas is of most indefinite composition; the granite includes contemporaneous veins; and this same country is broken into innumerable parts by metallic lodes, elvan courses, and other accompaniments of subterranean dislocations.
Most thankful, therefore, should geologists be, that further investigation of the facts, on which so many hands have been employed, has been performed by Mr. De la Beche, whose report, accompanying the geological survey of Devon and Cornwall, has now passed through the press.
From professor Sedgwick's description of the magnificent phenomena of granite veins at Trewavas Head, about two miles west of Forth Leven (Cambridge Philosophical Trans.), we extract the following notice:—
"On reaching the beach, we first found the killas rocks intersected by many contemporaneous veins of quartz. Not many feet farther west we were surprised to observe an appearance of alternation between the slate on which we were advancing, and several thin beds of granite. One more especially, which towards its southern extremity was lost under the waters, preserved its thickness and conformity to the laminæ of the schist for more than 100 feet. But its true nature was easily determined in the other direction; for it gave out several smaller veins, then cut obliquely through the laminæ of slate, and at length contracted its dimensions, started entirely from its previous direction, and ran in a flicker, ing line across the perpendicular cliffs. This vein is in no part more than two feet wide; yet it may be traced from the edge of the water to its termination in the cliff, nearly 400 feet.
"In the cliffs further west there are several granitic veins, which would be considered of no great interest if they had not been intersected by two other veins of different character, which must be classed either with the metalliferous lodes or the cross course of the country. One of them ranges nearly in the magnetic meridian, is about one foot and a half wide, and underlies east, two feet in a fathom. The other underlies in an opposite direction. They both contain quartz, oxide of iron, and apparently some fragments of clay slate. At the time of their formation, the mineral masses which they traverse must have undergone a considerable disturbance; for the broken ends of the schistose beds and granite veins, where they pass, are distinctly heaved from their original position.
"Still further west we found the rocks beautifully intersected by granitic veins; the higher part being traversed by innumerable ramifications, while the lower part is cut through by one well-defined vein about a foot thick, which, after keeping nearly in the direction of the beds of slate for about sixty feet, suddenly starts off at right angles to its former direction, and rises up to the top of the cliff. The whole system of veins here described afterwards unites in one trunk, which traverses a projecting ledge of rock, and descends obliquely into a mass of granite which forms the eastern side of the entrance into a singular natural cavern. Both sides of its entrance are of granite, but the roof is formed by undisturbed beds of killas. The granitic masses, however, soon contract their dimensions, and wedge out in the schistose rocks, which form both the roof and walls of the cavern, about 50 feet from its commencement.
"From the very point which is marked by so much confusion, two large veins, separated by a lancet-shaped mass of slate, rise towards the west at an angle of about 15°. Within a few feet of the other two, a third vein starts out nearly at the same angle, and proceeds in the same direction. These three veins are throughout nearly of the same thickness, viz. each about five feet. The highest, at some distance from its base, begins to ascend more rapidly, and is lost in the alluvial soil at the summit. The other two preserve their course, without being much deflected, for some hundred feet from the place where we first remarked them, and disappear behind a projecting part of the cliff. On turning this projecting ledge, we suddenly reached a recess, the lower part of which was filled with the ruins from the higher part of the overhanging rocks. The western side of this recess is composed of killas intersected by some small granitic veins. A protruding mass of granite forms the base of the eastern side to the height of twenty-five or thirty feet. It is of a very singular outline, yet does not appear to have thrown the slaty laminæ reposing on it out of their usual direction.
"The mound of rubbish in the recess enables us to ascend more than half way up the cliff, and trace the two large veins before mentioned into an enormous bunch of granite, which here reposes on the top of the cliff, and is supported by undisturbed beds of slate; the line of demarcation being nearly horizontal, and at an elevation of sixty or seventy feet above the level of the beach. The denuded face of this bunch of granite is thirty or forty feet thick, and, in a section made farther from the cliffs, would probably be much more considerable; for the ground rises rapidly to the north, and it is impossible even to form a conjecture how far the cap of granite may extend in that direction.
"Two or three veins appear to take their origin from this anomalous overlying mass. One spreads out in minute ramifications towards the part of the cliffs which abuts against Trewavas Point, at the termination of the killas in that direction. Two others descend obliquely, and are lost behind the large mound of rubbish before mentioned."
Granitic veins, which ramify and pass irregularly for short distances from the great mass, are frequent; but dykes, which are of simple form, and cross with a certain regularity great breadths of strata, where no parent mass of the same nature is known, are very rarely granitic. If this seem a paradox, its solution may lead to important results. Could we behold enormous masses of porphyry, or basalt, below vast breadths of stratified sediments, as granite is commonly seen, there would probably be found porphyritic or basaltic veins passing from them into the cracks of the strata. If this is never the case, does it not show the peculiar mineral character of granite, and its peculiar effects on the adjoining rocks, to be the fruit of the local circumstances of its deep 'plutonic' origin? It is a 'hypogene' rock very slowly cooled; in other circumstances it would not appear as granite. In thin veins and parts remote from the great body it becomes a fine-grained or even compact mass, hardly different from the base of porphyry. What then prevents us from believing that many felspathic dykes, like the elvans of Cornwall and Cumberland, which are so very generally found on the borders of granitic districts, are really of granitic origin? This is a view which has become familiar to our minds, while traversing the vale of St. John's, Wastdale, and Shapfells, and which has already been advanced by MM. Oeynhausen and Von Dechen, while speaking of the geology of Cornwall. (Geol. Proceedings, vol. i.)
Amorphous Masses under all the Strata.—If granitic veins surprise us by their smallness and the perfection with which they have been injected into all the ramifications of a stratified rock, the vastness of the masses from which they arise is even more remarkable. For it is certainly true, that in every place, yet completely explored, the veins end downwards in granite formations, so extensive and unbounded, and appearing at so many points beneath the lowest strata, as to deserve, more than any other assemblages of mineral masses yet made known, the title of an universal formation. The differences which obtain between different sorts of granite are more striking to the eye than important in reasoning; for it has already appeared, that even when one of the constituent minerals, mica, is wholly absent, the chemical contents of this remarkable stone vary almost imperceptibly. (See p. 92.)
Internal Divisions of Igneous Rocks.
On this head it has not been found necessary to add
to the remarks which will be found in Vol. I. p. 62.
Phenomena observed where Igneous Rocks come in contact with Stratified Masses.
Induration of Stratified Rocks.
One of the most usual effects of moderate heat upon argillaceous and arenaceous compounds is to indurate and condense their substance: considerable heat causes the grains to agglutinate into a "grit;" extreme heat fuses most argillaceous and many arenaceous rocks into a slaggy or glassy matter, which upon cooling remains vitreous, earthy, or crystalline, just as Mr. Watt found to happen to the basalt of Rowley Hills. In the slags of furnaces, several minerals have been found crystallised. The effects of the heated rocks which fill veins and dykes, and spread above and below argillaceous strata, are very similar. When dykes are of small breadth, the alteration which is seen in the neighbouring rocks is very slight. Of forty-four dykes composed of greenstone, claystone, and other igneous rocks, which were carefully observed and described by the author, as they occur on the shore of the Island of Arran, between Brodick and Lamlash, very few were found to have produced in the adjacent red sandstone more than a slight induration, in a very narrow space close to the dyke. Where two dykes crossed, it happened sometimes that a vitreous substance ran along the line of intersection.[8] But on the sides of large dykes, 20 to 60 feet wide (as, for example, the great dyke of Cockfield fell in Durham), the shales are highly indurated and otherwise altered, and the sandstones rendered as hard and solid as some sorts of quartz rock.[9] In Salisbury Craigs, the greenstone which is intermixed with the soft sandstones and shales of the coal formation has hardened these beds at the surfaces of contact so as to convert them into a kind of jasper, which takes a good polish. Under Stirling Castle, in Teesdale, on the flanks of the Caradoc, by the Plas Newydd dykes on the Menai, and indeed generally where the rocks of igneous origin appear in great masses, this effect of consolidating the stratified rocks is conspicuous, and leads to important reflections concerning the changes which, on a greater scale, the whole series of stratified rocks may have undergone.
The induration of the strata is an effect quite distinct from their deposition, and appears to require the supposition of long continued application of heat. In surveying the different systems of strata in succession, we readily perceive that, independent of the local influence of particular masses of igneous rocks, whose influence extends only a few yards at most from their bounding surfaces, the formations of different ages are unequally indurated,—the oldest being by far the most consolidated, while the newest appear but little harder than the analogous deposits which at this day are known to be produced in freshwater lakes, at the mouths of rivers, on the sea coast, or on the bed of the ocean.
This may be satisfactorily proved by a short comparison
of the three principal varieties of stratified rocks,
viz. arenaceous, argillaceous, and calcareous beds. In
the tertiary series loose sands not only occur, but, in
fact, constitute a large part of the whole series in Europe;
for the sandstones of Fontainbleau, and the "grey-weathers"
of the Wiltshire downs, and the molasse of
Switzerland, seem only exceptions to the general rule.
Clays abound under London, in Hampshire, and the
sub-Apennine hills; and even the limestones, as the
stony crag of England, the Leitha kalk of Transylvania,
and the calcaire grossier of Paris, have a softness and
looseness of texture not common in strata below the
chalk. (Some freshwater beds in the Cantal, and near
Weimar, are hard.)
In the oolitic system there are still some beds of sand, but sandstones predominate; there are also clays, but they grow denser toward the lower or lias formation; and the limestones exhibit the same gradations. In these respects the saliferous system differs but little, and still shows clays and sands and soft limestones; among the carboniferous rocks we lose almost totally the trace of loose sands, and soft clays (until brought to the surface) and the limestones acquire that compact and solid character which belongs to almost all the strata below the old red sandstone. Below the Silurian rocks the induration of the strata is rapidly accelerated; the clays have become slate, the sandstones are changed to quartz rocks, and the limestones have undergone an equal metamorphosis. The superior consolidation of the primary strata has struck every intelligent observer, and, allowance being made for difference of materials and local igneous agency, there can be no doubt of the justice of referring this quality to the higher degree in which they have been influenced by general subterranean heat.
Alteration of the Structure of Rocks by Heat.
The influence of heat in altering the structure of rocks is no less decided than in condensing their substance. For by this agency the original stratified arrangement of rocks is greatly obscured, and in some cases almost wholly extinguished, while entirely new structures are introduced to supplant those formerly imparted by water. The general character of the divisional planes in rocks has been already noticed[10]; it is desirable, however, to extend the description formerly given of slaty cleavage, the most striking and important of all these structural changes. We shall previously give some illustrations of the evidence on which some geologists have attributed these effects to subterranean heat, and others to electrical currents.
A case which has fallen under our own observation at the celebrated waterfall called the "High Force," in the upper end of Teesdale, Yorkshire, will first require attention. At this romantic spot the river Tees dashes down a precipice of 69 feet, which to the artist
shows two distinct forms of rocks: the upper part is boldly prismatic, and the lower part stratified. Across the prisms run bands of stratification, and to the hasty observer this will appear a case of stratified basalt. But careful inspection demonstrates a more curious truth. The annexed sketch (taken from the Illustrations of the Geology of Yorkshire, vol. ii. pi. xxiii.) will explain the peculiar circumstances alluded to.
a. Basalt, rudely prismatic, grey with lichen.
b. Thin "plate," not very much indurated.
c. Bed of plate, sub-prismatic.
d. Beds of plate, laminated.
e. Thin limestone bed with a superficial layer of pyrites.
f. Bed of hard pyritous limestone.
g. Several beds of common dark limestone, with white shells and corals.Here we see a new structure, commonly found in great masses of igneous rocks, communicated to the adjoining strata; but this is not very obvious in Teesdale, except where the basaltic rock is in very great quantity and thickness. At a distance from the heated rock, the shale or "plate" resumes its usual divisional surfaces, caused by nearly vertical joints which cross and intersect in rhomboidal or rectangular figures. (Compare cuts No. 86. and No. 87.) Both of these differ from those produced by the local application of heat, but neither of them is the effect of violent disturbance; both arise from the condensation of the matter of the strata, under the influence of heat or other causes competent to induce particular arrangements,—prismatic,
cubical, rhomboidal, &c., according to the nature, thickness, and position of the rocks, the degree in which the polarities of their particles are controlled by the different qualities of neighbouring mineral masses, and other important circumstances. Another case which also fell under the author's notice at Coley Hill, near Newcastle, appears strongly to confirm the view here presented, and at the same time to remove part of the obscurity which has always been supposed to overhang the origin of the "cleavage" of slate (see Vol. I. p. 67, &c.). In the annexed cut,
d, is a basaltic dyke, nearly vertical, and between twenty and thirty feet across, ranging east and west, and appearing at the surface.
s, is the ordinary coal shale, which is, as usual, very much laminated at a moderate distance (a few yards) from the dyke, and contain fern leaves and other plants between the laminæ.
obscured, the shaly mass is indurated, and traversed by numerous vertical divisional planes parallel to the faces of the dyke, most numerous near the dyke, so as to occur in every half inch of breadth, but becoming less and less abundant in the parts removed from the dyke, till they entirely vanish. On the surface section, the lines of these vertical planes would, on a minute scale, represent the "cleavage" edges of slate.
This fact is an example of a large class of phenomena, often to be seen on the sides of basaltic and porphyritic dykes, which traverse argillaceous strata; and it is one of the most prominent illustrations which we have, ever met with in favour of the opinion that the cleavage of slate is a metamorphic structure produced by the action of heat. Heat, however, is certainly not the only agent for generating cleavage.
We are acquainted with instances in which a similar structure (though certainly less perfect) is found parallel to, and limited to the region of, great fractures of the strata where no dyke of basaltic or other pyrogenous rocks occurs. This is seen in limestone cliffs which border the north side of the Great Craven fault in Yorkshire, where it crosses Giggleswick Scar, near Settle, and certainly no igneous action is otherwise indicated or probable there.
Mr. R. Fox, in prosecuting his curious researches regarding the changes effected in metallic bodies by electrical currents, has been conducted to an unexpected result, which appears to be of importance in reasoning on the laminated structures of mineral masses generally, and especially on the "cleavage" planes. The following notice of the experiments is extracted from the Report of the Royal Cornwall Polytechnic Society for 1837.
"Some clay was exhibited by R. W. Fox, esq., which had become laminated by long continued voltaic
action, so as to resemble clay slate in its structure. 
"The above figure may serve to illustrate the process by which this was accomplished. Let abcd represent the top or rim of an earthen ware cup or basin; e, a piece of copper pyrites; f, the upper edge of a plate of zinc; i, copper wire by which the two latter were connected; and g, h, the top of a mass or wall of clay between the copper ore and the zinc, and forming for each of them a watertight cell. The cell containing the copper ore was filled with a metallic solution—the sulphate of zinc, for instance—and the other with water mixed with a little sulphuric acid. The water with which the clay was worked up was also acidulated. Thus circumstanced, the apparatus was set aside three or four months, and was not disturbed till some little time after the water had evaporated, and the clay had become perfectly dry throughout.
"It then exhibited, on breaking off a portion of its upper part, lines of cleavage of a schistose character, parallel to the sides of the clay and plate of zinc, or at least as nearly so as was consistent with their undulatory form. In other words, the lines or laminæ were at right angles to the direction of the electrical forces.
"They are indicated by the lines on g, h; and the strongly marked line a c represents a principal line of division which separated the clay into two portions from the top to the bottom.
"These seemed to form, as it were, two voltaic plates, in opposite states of electricity, and one of them, consequently, more favourable than the other for the reception of metallic deposits and other bases from their solutions.
"Indeed, the general laminated structure of the clay appears to indicate that a series of voltaic poles were produced throughout the clay, the symmetrical arrangement of which had a corresponding effect on the structure of the clay. This view is still more strikingly confirmed by the occurrence, in several instances, of veins, or rather laminæ, of oxide of iron, the edges of which are shown by the shaded lines k, l, m. In these cases sulphate of iron was substituted for sulphate of zinc; and laminæ of oxide of copper were sometimes formed, in like manner, when a solution of that metal was employed; and moreover, numerous minute insulated portions or specks of the oxide of copper were detected in different parts of the mass of clay when broken."
These facts appear highly favourable to the opinion that the direction of cleavage planes in slate depends on some form of electrical excitement, and currents of electricity passing in given directions; but they do not at all negative the probability, from other and more general facts, that it is to the application of heat that the electrical currents owed their origin. In fact, when we remember that it is only among dislocated primary strata that real clay slate occurs, and that it is only in the vicinity of pyrogenous rocks, or fractures of the strata, that rocks of later date assume, however imperfectly, the slaty aspect, and that dislocations of the strata with unequal conducting powers for heat and electricity necessarily generate electrical disturbance and currents to restore the equilibrium, we see that the general opinion which geologists had adopted, of the dependence of the directions of cleavage and other symmetrical structures in rocks, upon local or general application of heat, may be very correct, though certainly it is incomplete. Mr. Fox's experiments will doubtless be repeated in other forms, but their present value is great, and they may, as he suggests, lead to practical results of value in mining operations.
We must, however, add some further details of the phenomena of cleavage, and discuss their bearing on another hypothesis, which ascribes to pressure this beautiful superposition of structure.[11]
The occurrence of cleavage at all in any given district is in some degree dependent on the nature of the rocks therein. Still more obvious is it that perfect examples of it only occur in certain argillaceous deposits. In a country consisting of alternations of thick argillaceous beds with coarse conglomerates, hard sandstones, limestones, quartz rock, and felstone, or greenstone, we shall find the cleavage, after passing through the argillaceous bed, more or less constantly interrupted by the other strata,—through which, however, a certain fissility, occasionally twisted and otherwise modified, is often traceable.
We have also for many years observed a beautiful case of the bending of the cleavage surfaces when they pass from one bed, or part of a bed, to another bed or mineralogically different part of a bed. Mr. Sharpe also admits this fact. This bending is always in such a manner as to render the angle of intersection between the cleavage and the stratification more acute, just as sometimes happens when a mineral vein crosses obliquely a strong throw, or when strata rise with an uplifting fault. The law is the same in all cases. These phenomena deserve the utmost attention from those who speculate on the theory of cleavage.
Beyond these completely or partially interrupting layers the cleavage recurs in the next band of argillaceous rock, with planes parallel to those first observed.
There are, however, cases in which alternating beds of more and less argillaceous rock manifest cleavage, but not in parallel planes, and when this happens, the angular difference of the planes is such that those in the
finer grained or more argillaceous bed meet the planes of stratification at more acute angles than those do which traverse the coarser and more sandy or more indurated bed. An example of this in secondary cleavage ("bate") is given in the author's paper on Craven rocks (Geol. Trans. 1828). He has since collected examples more obviously dependent on the difference of the mineral quality of the adjacent beds. Mr. Sharpe has admitted this peculiarity (Geol. Proc. 1848).
The above diagram shows a remarkable case observed in old red sandstone near Cork, (1843); s being soft red marly beds, h harder beds, l a laminated sandstone without cleavage, but jointed.
Another of the characteristic phenomena of cleavage was frequently presented to us, while surveying (in 1839) the Palæozoic strata of North Devon. Surfaces of stratification are usually found to be ridged and furrowed
by the edges of cleavage, in such a way that the 
and their surfaces crumpled and waved. Thus the symmetrical forms of leptænæ, orthides and spiriferæ become abbreviated in one direction, and (relatively) lengthened in another, and if they were laid obliquely to the direction of cleavage they have become distorted. So the trilobites of Llandeilo appear, in some instances, much narrowed, in others much widened, and in other instances obliquely elliptical, but in every instance the result was a contraction of the space across the edges of cleavage, and what may be called a minute folding, or furrowing; in fact, a "creep," in the direction of the dip. This "creep" is such as in the case of specimens of Ogygia Buchii from Llandeilo to contract them ¼ and even ½ an inch.
We may illustrate this by a diagram. Let ss be the line of strike, and D the line of dip on the surface of the stratum. Let o be the semicircular smaller valve of an orthis, with r its radius, perpendicular to the diameter d. Let such a figure be placed in 1 with its diameter in the line of the dip, in 2 at right angles to it, and in 3 at some lesser angle, say 50° to it. Then let all the stratum be subject to compression along the line of the dip, the result will be that 1 becomes shortened diametrically, r remaining unchanged, 2 becomes shortened on the radius r, but unchanged on the diameter d, while 3 is shortened both on the line r and on the line d, and is distorted[12], so that r is no longer at right angles to d. If we had assumed an extension of the rock in the line D, we should have had d lengthened in 1, r in 2, both d and r in 3; 1 and 2 retaining their symmetry, and 3 being distorted in a different manner.
Hence arises distinctly the idea of pressure as a cause of cleavage; an idea which has been the subject of elaborate illustration by Mr. Sharpe.[13]
Mr. Sharpe has given examples of elongation due to expansion in the direction of the dip of the cleavage; but it does not yet appear that any change of dimensions can be shown in the direction of the strike of the cleavage. No one can doubt that here we have indications of exact mechanical laws, operating on masses of matter, so regularly as to emulate the results of crystalline force on the molecules. But the latter force is free to arrange molecules singly by polar attractions, the former is constrained to obey certain axes in the mass.
Cleavage is remarkably developed in some districts which are formed upon one or more axes of anticlinal elevation and synclinal depression: for example, in Cumbria, Wales, and Devonshire. In each of these cases the fact is patent that the cleavage runs for 20 or 30 miles in one continuous direction, which is observed by all the cleavage planes over a considerable breadth of country. This direction is parallel to the great axes of movement in that district, almost exactly so on a great scale, though deviating slightly from the strike of the beds in particular places, especially when the strata are in any degree twisted.
The cleavage is in fact but little, if at all, affected by small irregular twists of the beds, and is, on the whole, more regular in its strike than they are. It is related to the great axes, not to the local bedding. May we from this infer that the general pressure on the axes of movement has been a determining cause of the new structures parallel to these axes?
Another thing is remarkable. The cleavage is less frequently vertical than inclined at a high angle, say 70°. It is also found at 45°, 30°, 20°, and even at much lower angles. Most frequently, when the strata are much inclined, the cleavage is inclined still more; but this has exceptions.
As the cleavage strike is not really dependent on the strike of the beds at a particular place, so is its dip not really dependent on the dip of the strata there: there may be more than one anticlinal and synclinal of strata (besides minor folds) and yet only one cleavage system. According to Mr. Sharpe, a cleavage system may be regarded as bounded by parallel lines along which the cleavage is vertical, and in all intermediate points less than vertical, in the middle of the space horizontal or nearly so; and he imagines these cleavage surfaces to be portions of great curves, everywhere perpendicular to pressures emanating from the axis of that space; that is to say, they would be so many parts of cylindrical sheets of uniform tension.
Upon this view we are not perhaps obliged to take into account any one of the axes of movement in a district, but the pressure on a whole district; and we are even released from referring the slaty cleavage to the date of their axes; it may be posterior to them all, and be only related to a general subterranean cause, of which they are some of the external manifestations.
A curious investigation of the component parts of non-fossiliferous slates has convinced Mr. Sharpe that the parts of such rocks have undergone that compression across the planes of cleavage, and extension in the direction of the dip, which had been inferred for other slates from evidence of altered fossils.
On the foundation of facts which have thus brought out the idea of internal pressure as an antecedent to the production of cleavage, Mr. Hopkins[14] has endeavoured to point out the accurate mechanical conditions of the problem, and to indicate the points to which the attention of future observers should be specially directed for the purpose of ascertaining the data required for a complete theory. Though we cannot here give an analysis of this investigation, a short statement of the bearing of it may suffice to put geologists on the right track for further inquiry, and perhaps to show them how much of beautiful illustration of geology is lost by those who permit themselves to be deterred by mathematical expressions from a close survey of physical truths. Pressure and tension being taken as of opposite meanings, and coexistent in a mass of rock, we may admit as representing their directions three coordinate axes passing through a central point at right angles to each other. Along these axes the effects of pressure and of tension will be direct and total, so that a small plane situated at right angles to one of these lines will be subject to the pressures or tensions of that line only, and will be moved, if at all, along that line; but a small plane placed in some other position will be moved in a line not having the same direction. As these pressures and tensions are assumed to be general, and so to affect all the particles, it is obvious that we shall have three co-ordinate planes, parallel to which direct forward or backward motion is possible, and between them other (tangential) planes in which the possible motions are oblique.
Now in the case before us one of the axes of direct pressure or tension may be regarded as of little or no effect, viz., that which coincides with the strike of the cleavage and the strike of the strata. And from this it follows, that direct motion from pressure should take place along lines lying in one plane only, viz., that which is perpendicular to the anticlinal, and only in two directions crossing each other at right angles in this plane, one of these directions being that of pressure, the other that of tension. Planes perpendicular to these two lines will be planes of direct tension or pressure, and their strike will be that of the beds. There will be also crossing these planes (but having the same strike with them) two other (tangential) planes, making with them angles of 45°, but with each other angles of 90°, in which oblique motions will be at a maximum. One of these (tangential) planes, therefore, will dip in the same direction (but not necessarily at the same angle) as the beds, and the other in the opposite direction.
These things premised, we may by referring the pressures to the plane of stratification discover their effect on the outlines of organic remains, which, for the sake of comparison with our observations already recorded, we shall assume to be semicircular, and always laid with the hinge or diametral line parallel to the line of dip.
First, let it be supposed that pressure is applied perpendicular to the stratification and tension produced parallel to the strata. In this case there will be a symmetrical extension of the figure in the direction of the line of dip. If tension be applied perpendicular to the strata and pressure exerted parallel to them, the semicircle will be symmetrically contracted to a semi-ellipse, as in the example already represented in the diagram (p. 119.); and, finally, if either pressure or tension be applied in a direction meeting the plane of stratification at 45°, so that a plane of maximum tangential action shall coincide with the stratification, the semicircle will be unsymmetrically changed in form, so as to become elliptical with its diameter lying obliquely across the line of dip—in fact, to be angularly distorted.
When, therefore, angular distortion occurs in an
equilateral shell placed symmetrically with respect to
the line of dip, we may be sure the result is due to the
tangential movements developed by pressure. If this
happens chiefly or exclusively when the cleavage nearly
coincides with the stratification, and happens rarely or
not at all when the cleavage meets the strata at or about
an angle of 45°, we may conclude that the cleavage plane
is not perpendicular to any axis of antecedent pressure
or tension, but is coincident with one of the two planes
of maximum tangential movement.
Thus a delicate and critical inquiry is opened out for geologists who may have been trained to the accurate use of graduated instruments.
Metamorphic Rocks.
For the application of the useful term, "Metamorphic Rocks," in the description of phenomena connected with the occurrence of igneous rocks, and reasoning on their causes, we are indebted to Mr. Lyell; and there is, perhaps, no part of the study of ancient nature more worthy of attention from philosophic minds. For thus, and thus only in many instances, are we enabled to arrive at probable and intelligible views of the course of changes which even the most solid materials of the globe have undergone. The Pythagorean maxim,
"Nihil est toto quod perstet in orbe,"
comes into full credit when we approach the great masses
of felspathic and augitic rocks which have been effused
in a melted state above and amongst the ordinary products
of water. As we pass from the districts where no
igneous rocks appear at the surface, towards the mountain
regions where they abound, the strata acquire hardness,
assume new structures, and in their innermost
texture and substance appear under new and peculiar
aspects. In order to trace these phenomena, so that the
picture may not only be interesting but instructive, it
will be necessary to distinguish the effects which we call
"metamorphic" into three classes.
1. There are rocks which, by the local influence of heated rocks, are locally changed as to the arrangement of their mineral ingredients; so that earthy substances become crystalline; and the view thus arising is capable of being generalised so as to explain the corresponding appearances of similar rocks, by a similar but more general cause.
2. There are stratified masses which have undergone, near pyrogenous rocks, a loss of some portions of their substance.
3. There are cases in which the rocks near igneous dykes have not only been hardened, fissured in a certain manner, and subjected to re-arrangement of their ingredients; but further, there have been introduced into their substance, minerals not known in the same rocks elsewhere. This is also found to have a general application to rocks exhibiting like phenomena, but upon a scale so vast as to require the supposition of very general application of heat.
From these facts and inferences we pass immediately and inevitably to the great geological problem naturally arising out of such data, viz. the degree in which the peculiar mineral characters and admitted absence of monuments of organic life among the oldest strata are to be relied on as conclusive testimony concerning the primeval condition of the globe.
Re-arrangement of the Particles of Rocks.
One of the earliest notices of an extensive mass of limestone changed by the action of igneous rock, is that of the district of Strath in the Isle of Skye. Dr. Macculloch's observations in this island led him, in 1816, to believe that certain laminated shelly limestones, which occupy a considerable breadth, and cross the island from Broadford to Loch Slapin, are altered in various ways, by contact with and proximity to sienitic rocks, so as, in a considerable space of country, to have lost all stratification, and in same instances to have assumed the character of a pure white marble of fine grain. In its chemical composition it is generally a pure carbonate of lime; but where in contact with the sienite or the trap veins, becomes overloaded with silica, magnesia, and argil. In such situations it often contains veins and nodules of greenish transparent serpentine, and appears in a variety of colours, grey, dove-colour, dark blue, grey, striped, mottled, veined, pure white. At points removed from the sienite, the pectines, and other shells which this rock contains, and its position with regard to other secondary rocks above and below, have satisfied not only Dr. Macculloch, but Mr. Murchison and professor Sedgwick, that it is a part of the lias formation, which also occurs in Pabba, &c.
A case of the same kind, on an equally extensive scale, which occurs in connection with the "whin sill," or stratiform basalt of High Teesdale, especially in that portion of the country where the trap rock is very thick, has been made known by Professor Sedgwick. The limestone of this district, both above and below the basalt, is usually of a very dark grey or even blackish colour (some beds are very black); but in contact with that rock it loses its obscure blackness (probably by loss of bitumen), becomes of a clear blue tint, and finally, the change being complete, of a clear or greyish white. The arrangement of the particles is altered in an equal degree. The stone usually is compact, or partially varied by laminar shells, or crystallised plates of calcareous spar, representing the stems of crinoïdes. Near the "whin" these characters change; the stone becomes granular and crystalline (in the sense that statuary marble deserves this term), and in some cases the crystalline grains separate by disintegration of the mass. In these metamorphic limestones small cavities sometimes occur; but the most interesting fact that remains to be noticed, is the occurrence of crinoidal columns in the midst of the granular crystalline mass (our own observation). They are, however, not common. These phenomena may be seen over some square miles of surface in the vicinity of the High Force and Caldron Snout, chiefly, perhaps, in the limestone which overlies the basalt.
In connection with the ancient volcanic rocks of the Kaiserstuhl mountain (in the Rhine valley), limestone of the Jura formation (or oolitic system) is similarly altered to a really crystallised mass of calcareous spar; and, in addition, mica and other minerals are intermixed with the limestone. The broad flakes of carbonate of lime are here very remarkable.
The basaltic district of Antrim furnishes abundant and precise evidence of the conversion of chalk into granular marble by the action of basaltic dykes.
"The Irish chalk is seldom of a texture sufficiently loose to soil the hand; and in the few instances where this does take place, it is in a very slight degree: its general colour is either perfectly white, or white with a very slight tinge of yellow; towards the lower part it passes into a uniform ash-colour; the texture then becomes still more compact." "At many points near Belfast, Glenarm, Moira, &c., the chalk is frequently traversed by basaltic dykes, and often undergoes a remarkable alteration near the point of contact; where this is the case, the change sometimes extends 8 or 10 feet from the wall of the dyke, being at that point greatest, and thence gradually decreasing till it becomes evanescent. The extreme effect presents a dark brown crystalline limestone, the crystals running in flakes as large as those of coarse primitive limestone; the next state is saccharine, then fine grained and arenaceous; a compact variety having a porcellanous aspect, and bluish grey colour succeeds; this towards the outer edge becomes yellowish white, and insensibly graduates into the unaltered chalk. The flints in the altered chalk usually assume a grey yellowish colour; the altered chalk is highly phosphorescent when subjected to heat."[15]
In the island of Raghlin, directly over against Kenbaan Head, a singular combination of dykes occur (seeming to be a continuation of those which at the latter place have been attended by such extraordinary disturbances). Here, within a distance of 90 feet, these dykes may be seen traversing the chalk, which is converted into a finely granular marble, where contiguous to the two outer dykes, and through the whole of the masses included between them and the central one.
The following diagram, copied from Mr. Conybeare's section (Geological Transactions, vol. i. pl. 10.), will be useful for reference. It represents the ground plan of the dykes as they appear on the shore.
| d 1. Dyke, 35 feet wide. | d 2. Dyke, 1 foot wide, |
| d 3. Dyke, 20 feet wide. | c. Chalk. |
| m. Granular marble. |
One of the most direct objections to that part of the Huttonian system of geology in which the induration of rocks is attributed to the action of heat, was drawn from the calcareous strata, which, it was said, would have parted with their carbonic acid, and thereby have ceased to be limestone. That such an effect would take place in the open air, in the ordinary state of limestone (not perfectly dry), is a matter of invariable experience; but Dr. Hutton, with his accustomed sagacity, proposed the hypothesis that the carbonic acid gas would not be liberated by heat under great pressure, such as the weight of the ocean pressing on its bed. This hypothesis, sir J. Hall, with equal sagacity, put to the test of accurate and conclusive experiments. In the breech of a gun-barrel he placed an earthen tube half filled with calcareous matter in powder, and strongly compressed, the rest of the space being filled with powdered silica. The tube was then closed hermetically by a mixture of fusible metal. The end of the barrel where the powdered earth was, being heated in a furnace, a part of the fusible racial yielded to the heat, and came nearly into contact with the porcelain tube (separated by aqueous vapour and air); the rest remained solid.
After the heat had been sufficiently applied, and the whole had become cool, the fusible metal which stopped the tube was melted out by moderate heat, and the calcareous powder in the porcelain tube was examined. Similar experiments were made in porcelain tubes alone, with different modes of hermetical sealing. The general result was, that, under mechanical pressure, carbonate of lime may be exposed to great heat without calcination; while, by the effect of great heat and pressure combined, the calcareous powder was agglutinated into a solid limestone, nearly as hard and as heavy as the natural rock. Some portions might even be polished as marble.
By a mechanical contrivance, the degree of pressure on the materials exposed to heat was varied and measured; and it appeared, that with a pressure of 52 atmospheres, equal to a column of 1700 feet of sea-water, powdered limestone was converted to hard stone; with 86 atmospheres, equal to a column of 3000 feet of sea-water, it is changed to marble; with a pressure of 173 atmospheres, equal to a column of 5700 feet of sea-water, it is completely fused, so as to act strongly on other earthy substances.
The celebrated marble of Carrara is probably an altered limestone of the oolitic era.
Having now seen many examples of the conversion of common limestone into crystalline marble, both by actual experiment, by volcanic action, and the heat communicated from pyrogenous rocks of different kinds, the application of these truths to the history of the "Primary Strata" is obvious. For primary limestones differ from secondary and tertiary calcareous deposits merely by their mode of aggregation, which is not such as water ever produces in carbonate of lime, but is exactly comparable to that occasioned by heat. And this general analogy is strengthened by collateral circumstances, as, for example, the frequent occurrence of serpentine in some of the "primary" limestones is a fact exactly parallel to the introduction of serpentine among the crystallised metamorphic limestone of Skye (noticed by Macculloch). In Radnorshire, Mr. Murchison observed the ramification of serpentinous strings through limestone which was otherwise altered in contact with a felspathic trap; and in this and other places, anthracitic coatings and nests, and crystals of copper and iron pyrites, complicate the effects. In one place, a serpentinous rock of this kind is 20 or 30 feet wide. If therefore, in conformity with so many and such strong analogies, we admit the inference that the crystalline primary limestones have acquired this character by the action of heat, it must follow that this heat was of a very general, if not universal, application below the primary strata, for there is, perhaps, no considerable district known where the gneiss and mica schist systems are devoid of such crystalline limestone, and the occurrence of it is not specially connected with the local appearance of igneous rocks. This important inference will, however, be invested with a higher degree of probability if it be also found, as a matter of fact, that the other strata with which this limestone is associated show independent signs of having been subjected to a general heat.
Alteration of the Chemical Nature of Rocks.
One of the most popular of all the proofs of the pyrogenous origin of basalt and greenstone, is the effect they produce on coal and bituminous shales, for by their action the coal is often turned to coke, and the dark shales assume a very light colour. These effects are almost too common in Scotland and the North of England to deserve especial notice. Thus the Kyloe dyke, which crosses the Tweed below Lennel, has converted the coal seams intersected by it into a sort of cinder, the bituminous matter having been entirely dissipated. (Milne on the Geology of Berwickshire.} Several of the dykes in the collieries of Newcastle and Durham (as the dyke in Walker colliery, the Coley Hill dyke, the Cockfield fell dyke, &c.) have expelled the bitumen from the coals and shales, to various distances, according to the width of the dykes, and other less known conditions of the adjoining strata. The anthracite has in some instances been injected into the cracks of neighbouring sandstones. Analogous facts on a smaller scale are found in connection with the trap dykes in Radnorshire, &c., which are frequently accompanied by anthracitic nests and coatings.
The following notice of the effects of the remarkable Cockfield dyke is from an eye witness, whose observations were communicated to me by my friend John Ford, esq.
"In working the coal towards the dyke, when within 50 yards of it the coal begins to change. It first loses the white spar in its joints and faces; looks dull, tender, and short; and loses its quality for producing flame. Nearer the dyke it has the appearance of half-burnt cinder: still nearer it decreases in thickness, and becomes a hard cinder 2 feet 6 inches thick. Eight yards from the point at which the coal becomes a real cinder, that is, 8 yards nearer the dyke, the coal assumes the appearance of soot caked together; it is called 'dawk' or 'swad:' when it touches the dyke, the coal is reduced from 6 feet to 9 inches."
"On each side of the dyke, betwixt it and the regular strata, there is a thin layer of clay, or, as it is called, a 'gut' or 'core,' about 6 inches thick, which turns the water from the rise to the dip side of the dyke, and forces it to the surface in several springs, in the direction of the dyke, where it crosses the country." The damage done by the dyke is thus estimated: "25 yards of tender, short, spoiled coal; 16 yards of cinder; and 10 yards of dawk or swad," making a total of 100 yards of spoiled coal throughout Cockfield fell. The dyke is nearly vertical, and 18 yards in width; the strata of coal, sandstone, &c. are dislocated by it about three fathoms. In other situations the "throw" is greater.
The application of these facts to the explanation of the condition of ancient strata is important. For it is a general fact, that the carbonaceous substances which are associated with any part of the primary or transition strata, are of the nature of anthracite, which is devoid of bitumen. Whether it will be proper to extend this explanation to the large anthracitic beds of Pennsylvania, South Wales, Devonshire, Brittany, &c., some of which lie among secondary strata, is at present uncertain.
Dolomitic Limestone.—One of the effects of the sienite of Skye, in contact with the lias limestone, which it converts to fine granular marble of many colours, is the introduction of silica, alumina, and magnesia, into its composition. (Macculloch, Geol. Trans. vol. iii. p. 42.) This sienite is principally a felspathic mass, varying from claystone to clinkstone and compact felspar, from which no transfer of magnesia could be supposed. Von Buch, in the course of his extensive and laborious examinations of plutonic and volcanic rocks, was led to attribute to a rock of quite another kind, the melaphyre (black or pyroxenic porphyry) of the southern flank of the Alps, not only an important function in the elevation of mountain ranges like the Alps, but the peculiar chemical and mineral change which is locally noticed in some of the limestones. By this change carbonate of lime becomes a double carbonate of lime and magnesia; the compound is crystalline, and often of a dazzling whiteness. This is the case with the dolomite of St. Gothard, and with much of that which occurs on the Lago Lugano.
This last is the vicinity to which Von Buch has specially directed the attention of geologists, and as melaphyre, granite, dolomite, and common limestone here occur in abundance and in varied circumstances of exposure, perhaps no better locality can be chosen for investigating the truth and applicability of the opinion of this eminent geologist.
Between Varese and Tresa is seen the section presented below. In this section the main facts commonly
| 1. Gneiss. | 2. Mica schist. | 3. Granite. | 5. Melaphyre. |
| 6. Tuff. | 8. Sand and gravel. | 9. Limestone. | 10. Dolomite. |
noticed as to the association of melaphyre with the other rocks are well typified, and it is seen that the occurrence of dolomitic limestone is not uniformly connected with the appearance of melaphyre; sometimes it adjoins granite, in other localities mica schist: it also appears that limestone is not always dolomitised in contact with melaphyre, or granite; and those geologists who have imagined that Von Buch supposed there was a real transfer of carbonate of magnesia from the augitic rock, have very naturally arrived at the inference that this district lends no countenance to the speculation. But we learn from M. Elie de Beaumont (Ann. des Sc. Nat. vol. xviii.), that this was not Von Buch's meaning, and indeed, that would easily appear from the facts quoted by him in support of his opinion. The true notion advocated by Von Buch, of these transformations of limestone, is that the eruption of melaphyre was coincident with violent disturbances and fractures of the country in a particular line parallel to the melaphyre; and that along the fissures then produced, gaseous sublimations of different kinds found their way to the surface, and altered particular rocks in their passage. The ordinary and obvious form of objection above noticed therefore fails; and it remains to be seen whether the occurrence of dolomite in definite relation to lines of melaphyre, or, to take the problem in a still more general sense, to fractures of the earth's crust, is a circumstance well proved; and if so, whether the sublimation of carbonate of magnesia would be chemically probable. On this latter subject, Dr. Daubeny and Dr. Dalton stated facts in confirmation of the view of Von Buch (Reports of the British Association, for 1835), and on the former question, we have related, in describing the geology of Yorkshire, the dolomitisation of common limestone by the sides of faults and mineral veins, far away from igneous rocks of any kind. It seems, therefore, unsafe to reject Von Buch's remarkable hypothesis, without a patient investigation of many collateral points; and, on the other hand, the dolomitic masses of Franconia, which form a part of the Jura kalk, and the magnesian limestones of England (extensive deposits which are unconnected with pyrogenous rocks), appear to show that subterranean heat is not the only nor the principal means of introducing magnesia as an ingredient of limestone. We may, indeed, choose further to suppose that the submarine springs, which probably gave origin to the magnesian limestones of Durham, were a consequence of that great disturbance of the earth's crust which is so manifest in the coal districts of England; and this easy and probable explanation for these cases, while it recognises the general principle which connects magnesian limestones with dislocations of the strata, may possibly be found applicable to other examples.
One of the points favourable to investigation of the relation of dolomitic limestones to volcanic forces, is Gerolstein in the Eifel, where, round a particular vent, for a considerable space, the "transition" limestone (corresponding exactly to the "Wenlock" limestone in the silurian system of England) is converted to dolomite, and appears in the usual unstratified, fissured, and antiquated forms of that rock, while further off it is a thin-bedded rock; organic remains occur in both the common and dolomitic limestone (observed 1829). To the facts which appear in this volcanic region, Von Buch appealed in proof of his hypothesis; but Dr. Daubeny (On Volcanos, p. 51.), after describing the cellular character of the lava, and the way in which it is related to the present form of the surface, observes, "It seems difficult to reconcile the hypothesis of Von Buch with the age which we are compelled to assign to the volcanic operations here, as well as in other parts of the Eifel. As it is evident that no foreign ingredient could penetrate the substance of the rock in its present hardened condition, so as to unite with the other constituents, and diffuse itself uniformly through the mass, it seems necessary for Von Buch's hypothesis to suppose the limestone to have previously been at least softened by the heat, which occasioned the sublimation of the magnesia. Hence we should be obliged to fix the period at which this process took place as antecedent to the formation of the valleys, for these would be necessarily obliterated by any softening of the limestone which now overhangs them.
"Indeed it would be necessary to carry back this supposed softening of the calcareous rocks to some period antecedent to the retirement of the ocean, when sufficient pressure might be exerted to prevent the carbonic acid from being driven off from the limestone when exposed to the heat required for softening it.
"But all this is contradicted by the phenomena of the volcanic products in question, the cellular appearance of which plainly indicates the absence of pressure, and which even seem, from the existence in them of craters, and by the manner in which they have accommodated themselves to the present slope of the valleys, to have been formed since the commencement of the present order of things."
Dolomitic limestone is not at all common among primary strata, though these early limestones often contain serpentine in strings and veins, augite (as at Tiree) mica, and other magnesian minerals.
Generation of New Minerals.
Perhaps no more interesting or satisfactory evidence of the generation of new minerals in strata which adjoin a "trap" rock has ever appeared than in the description of the great dyke south of Plas Newydd in Anglesea, by professor Henslow. (See Camb. Phil. Trans, vol. i.) The substance of this dyke is basalt, composed of felspar and pyroxene; its width is 134 feet, and it cuts perpendicularly through strata of shale and limestone. The strata on each side form an abrupt cliff, against the Menai shore, about 15 feet high, but the dyke, through decomposition, offers a gradual slope.
The Plas Newydd dyke crosses the Menai. The cliff which bounds the dyke at Plas Newydd is composed of clay shale, and argillaceous limestone. The lowest portion (thin calcareous shaly bed), on approaching the dyke, undergoes various changes. At 15 feet from the contact it forms a compact bluish grey mass, with spots of a fainter colour. In contact it is bluish green, very compact and hard. The shaly structure disappears, in a great measure, near the dyke (as at Coley Hill).
The next portion of the cliff, proceeding upwards, consists, at 50 feet from the dyke, of a soft dark-coloured plastic clay shale, thinly laminated. At 35 feet from the dyke this becomes indurated; at 10 feet it is a cherty mass, in closing patches of highly crystalline limestone; in contact it is a hard porcellanous jasper of various colours. (Impressions of shells remain in it.)
The third division of the cliff consists of dark argillaceous limestone, which in contact is found of a speckled dull green and brown colour.
Above this is a thick body of clay shale, which, near the dyke, is partially turned to a flinty mass, while the rest of the shale assumes a confused appearance of crystallisation and globular structure. Perfect crystals are recognised in this mass of two distinct kinds, and exhibiting every gradation of aspect from a globular and concretionary to a perfectly crystalline character. Some of the crystals (analcime) have twenty-four trapezoidal faces. Shells of brachiopoda are enveloped in globules and crystals. Other crystals have twelve rhomboidal faces, and prove to be garnet of specific gravity 3.353. The crystals were examined by professor Gumming, and those of analcime analysed by him, and found to have a specific gravity of 2.293, or 2.394. Minute garnets in the form of rhombic dodecahedron's were found by the Rev. J. Harrison under the basaltic mass which overhangs the Tees, below Caldron Snout in Teesdale, in altered shale and limestone.
The segregation of mineral substances in rocks adjoining trap dykes is noticed by Mr. Milne, in his account of the geology of Dumfriesshire.
Since it thus appears that in many instances where the masses of igneous rock were considerable, perfect garnets have been produced by heat in the neighbouring sedimentary strata, though these were not in other respects re-crystallised, we turn with interest to the well known and general (though not universal) fact of the occurrence of garnets in the ancient strata of gneiss and mica schist, as a valuable addition to the evidence brought by the crystalline limestone associated with the same strata, in favour of the opinion that the whole mass of these rocks has been subjected to a pervading high temperature. For the occurrence of garnets in mica schist and gneiss is entirely unconnected with any local effect of heat derived from particular masses of granite, greenstone, &c.; nor can their occurrence be often accounted for by any supposition of their having formed part of more ancient rocks, which by disintegration yielded them to the watery currents concerned in accumulating the primary strata; for they are in general perfectly crystallised, among fragmentary scales of mica, and worn and broken felspar and quartz, or granular aggregates of those substances, scarcely differing in arrangement or aspect of the parts from particular sandstones and coarse argillaceous slates. The term so commonly employed of "crystalline schists," for mica schist, gneiss, &c., appears to be seldom justified by accurate examination; for frequently, we believe, the parts of these rocks are not individually crystals (as mica and felspar are in granite), nor envelop crystals (as quartz often envelops the other substances in granite), nor are in a state of crystalline aggregation, as the grains and plates of most primary limestone, but are parts of crystallised bodies fragmented and worn in various degrees, aggregated in laminæ under the influence of water (perhaps in a peculiar state), and subsequently consolidated, but not melted, nor re-crystallised, by the application of heat.
It is, however, thought by some geologists that the whole mass of the primary schistose rocks is to be viewed as metamorphic; as transformed from some other sort of sedimentary rock—grauwacke, for instance—and rearranged into a crystalline rock of granitic aspect and affinity. We must therefore pay attention to some of the evidence which is adduced in support of this important hypothesis.
Metamorphic Slates.
As containing examples of metamorphic rocks, on a considerable scale, and of interesting if not remarkable variety, the district of the Cumbrian mountains may be advantageously quoted. In connection with the granite of the Caldew occurs the remarkable mass of chiastolitic and hornblendic slates, which form the base of the clay slate system of Cumberland; and it is thought that these rocks are, at least in part, metamorphic, similar combinations being found in analogous situations elsewhere. Dr. Macculloch ascribes a metamorphic origin to hornblende schist, viewing this rock as the extreme term of a series of changes commencing with clay or shale, and passing through siliceous schist or Lydian stone. Argillaceous schist, when in contact with granite, is sometimes (as in Shetland) converted into hornblende schist.
The hornblende schist of the Cumbrian granitic district is in places similar to that which adjoins the granite of Glen Tilt; and in each case its slaty structure is parallel to the crystalline faces of the prisms of hornblende. Some of this rock is almost pure crystallised hornblende; in other parts hornblende and felspar appear; but in Cumberland at least, and, judging from specimens, we think also in Cornwall, it is not quite correct to call another metamorphic rock gneiss. There appears to be produced, in connection with the granite of the Caldew, a combination (in small quantity) of crystallised mica and uncrystallised quartz, which has been called mica slate. The main fact to be attended to with regard to these phenomena of contact is, whether the parts of the altered rocks called gneiss, hornblende schist, mica schist, &c., are really crystals, and in crystalline aggregation,—circumstances often erroneously admitted with regard to primary strata, in consequence of the very inaccurate use of these important and characteristic terms.
In professor Sedgwick's account of the succession of the strata above the granite of the Caldew, given below, he seems to refer all the interpolated crystals of the upper part of the series to chiastolite. Some of the rocks appear, however, to be genuine hornblende slate crystallised, and one of our specimens is traversed by a granite vein.
A different series of changes may be traced among different rocks in Borrowdale and Wastdale, where the members of the middle division of slate rocks abut against the granitic mass which forms the base of Seafell, and occupies considerable breadths in Eskdale.
The slaty rocks alluded to are bedded and laminated; but besides the cleavage structure, which has been superadded, and which crosses all the beds of fine, coarse, and laminated grauwacke, we notice (as in the rocks which overhang the Bowder Stone) extreme induration, and the plentiful occurrence of spots and strings of epidote. In other beds the stratification remains, but the mineral composition is complicated by the segregation of spots of green earth, and nodules of green earth, calcareous spar, quartz, or even chalcedony, so that the stone would, by most persons, be called amygdaloid. It is, however, a widely stratified rock, and passes by perfect gradation in Borrowdale, near Ulpha Park, on Grasmere, and in Patterdale, to the common bedded and spotted slate. On approaching yet nearer to the granitic mass, other changes appear; the slaty stone becomes very hard and compact, is traversed by abundance of fissures, acquires a peculiar spotting, which finally assumes the character of felspar, till the whole mass becomes what is often called clay porphyry, and at length can in no manner be distinguished from variolites and porphyries with a compact base. (Some of these rocks have been called greenstones.) This series of changes may be traced in a breadth of two miles, by walking over the summit of drainage between Borrowdale and Wastdale, called Stye Head.
What renders these alterations the more interesting, is the abundant occurrence of garnets of a fine red colour and perfect crystallisation (rhombic dodecahedron) in the porphyritic, partially porphyritic, and even brecciated rocks. Such specimens may be gathered on the slopes of the Gable Mountain, or obtained from the rocks near the summit of the pass of Stye Head (observed by the author 1838). How many of the porphyritic masses of this interesting region may hereafter be ranked as metamorphic slates, we cannot predict; but many rocks at the base of Helvellyn and in the Vale of St. John's (some of which contain garnets) appear to the author to deserve examination in this respect.
On a great scale, the alternation of porphyritic and schistose rocks in this region is established by professor Sedgwick's laborious researches, still only partially known to geologists. The results of his corresponding examination of the parallel series of rocks in North Wales appear very similar to those obtained in the Cumbrian mountains. (See Geol. Proceedings, vol. i. p. 400.)
The alterations produced upon the argillaceous slaty rocks of Cornwall, by the proximity of granite, are differently reported by different observers; but in general they appear to be inconspicuous, and perhaps cannot be described in a smaller compass than in the words of Oeynhausen and Von Dechen, who say,—"The killas is, at its junction with the granite, rather hornblende slate and greenstone than clay slate. The transition from clay slate into hornblende slate and greenstone is commonly so gradual, that we have not been able to trace any where a line of junction between both rocks." (Phil. Mag. and Annals, 1829.) The slightness of the changes which are remarked near many of the granite veins of Cornwall is not an unusual circumstance elsewhere, among argillaceous slates in closing greenstones and porphyries; and perhaps the reason may be, that these substances had already undergone great heat, and suffered a great degree of change from their first condition.
Speaking with reference to the granite of Cligga point, and the porphyritic elvan courses of St. Agnes, the Rev. J. Conybeare observes,—"The killas, which is traversed and covered by these more crystalline rocks has, for the most part, the character usually ascribed to clay state, and its strata occasionally present singular curvatures; in many places it passes into chlorite slate, and in the immediate neighbourhood of these dykes it usually presents either a highly crystallised form of that rock, or such an intermixture of it with quartz and felspar as might fairly be esteemed a variety of gneiss." (Geol. Trans. iv. p. 403.)
Metamorphic Mica Schist, Gneiss, &c.
From cases like those already mentioned, where argillaceous slates, on approaching granite, appear in every intermediate state of change till they finally are converted to clay porphyry or to hornblende slate, we pass to consider other supposed transformations, in which the original substances are similar, but the product is different. Speaking of the altered rocks round Dartmoor, Mr. De la Beche (Manual, p. 479.) observes,— "The grauwacke slates in many parts of the country surrounding the granite of Dartmoor have suffered from its intrusion, some being simply micaceous, others more indurated and with the characters of mica schist and gneiss, while others again appear converted into a hard zoned rock strongly impregnated with felspar."
Von Dechen's account of the changes effected by the granite of the Hartz on the grauwacke of that region, appears not dissimilar to the description we have given of the Cumbrian rocks, for flinty slate, quartz rock, greenstone, &c. are stated to be the result of the igneous action. Mr. Griffith has found it convenient to express by a particular colour the metamorphic portion of the slaty series of the South-east of Ireland which surrounds the granite of Wicklow and Wexford. He describes them as "altered rocks in the neighbourhood of granite, clay slate, passing into greenstone or greenstone slate, or serpentine, or crystalline micaceous slate, or micaceous shining slate, or flinty slate." Similar phenomena are recorded by the same geologist, in a considerable breadth round the Mourne mountains. (See his Map, 1838.)
Von Buch first made known the interesting circumstances under which the syenite of Christiania touches and partially overlies the "transition" rocks of that country, which yield trilobites, orthocerata, &c. in considerable abundance. Mr. Lyell has recently explored this district, and, fully confirming the important inference of Von Buch, that the sienite was of posterior date to these transition strata, observed those changes which are now known to be the frequent concomitants of the contact of igneous and stratified rocks. The limestone, usually of very dark colour, is turned into white marble, the schist into Lydian stone, and "sometimes into mica schist," of which Mr. Lyell saw one striking example at Grorud, north-east of Christiania. Traces of fossils are not infrequently discoverable in some of the crystalline and altered rocks of the transition formation, so that the actual conversion of the latter into metamorphic strata is unequivocal. (Lyell, in Brit. Assoc. Reports, 1837.) The rocks here termed syenite are considered by Mr. Lyell to be (geologically speaking) of the granitic family; they seem to pass into trap porphyry, and divide the gneiss and less ancient schists in a very irregular manner, but do not spread widely over them in any part of the district. Tabular masses of igneous rocks are no where seen to spread over the fossiliferous rocks, except where they have assumed the usual aspect and characters of trap.
The oolitic system of strata, as described by De Beaumont, Necker de Saussure, and Brochant de Villers, in the Tarentaise, Dauphiné, and the valleys near Mont Blanc, puts on a very different aspect from that which is usual in the more level regions of Germany, France, and England; and this difference appears similar to some occurrences mentioned by Studer and De Beaumont, which are obviously dependent on the heat of contiguous granitic rocks. In the Tarentaise, siliceous limestones, micaceous quartz rocks, and gypsum, correspond to the lias and lower oolitic rocks of England; and contain the fossils common in these rocks. It is further remarkable, that at the Col du Chardonnet (Hautes Alpes), plants, supposed to be of species which also occur in the coal formation, lie in beds which alternate with others containing belemnites of the lias. In the upper part of the Buet, Necker de Saussure has observed the following series of strata: viz., mica schist covered by various sandstones and schists; black slaty beds with talcose impressions of ferns; dark impure limestones; black slaty clay with nodules of Lydian stone, alternating with talcose slaty clay, both containing ammonites; and over all a grey calcareous belemnitic shale, to the top of the Buet.
Relative Antiquity of Pyrogenous Rocks.
The determination of the relative antiquity of the unstratified rocks is a point of much importance, and of great difficulty. Taken generally, it is an indeterminate problem; for though, in a vague sense, we may easily be satisfied that granitic and other felspathic rocks are more ancient than basaltic and other augitic rocks, yet there can be little doubt that some of these latter, as, for example, the bedded greenstones of Wales and Cumberland, are of higher antiquity than the granitic rock of Weinbohla, which rests on members of the cretaceous formation.
When we consider this question with reference to a small district, as, for example, the Island of Arran, so rich in various rocks of igneous origin, the result to be looked for is like that which may be gained by examining a volcanic mountain, where certain different rocks have at different times been ejected by the same volcanic forces. In Arran, for instance, we have granite, sienite, porphyries of many kinds, claystone, hornstone, pitchstone, greenstone, basalt. These cross and complicate one another; and it is possible, upon certain suppositions or admissions, to determine their relative antiquity. If the conclusion be substantially correct, and the order of production among these rocks be known, the interpretation may be trusted to the small extent of inferring, that below this small tract, at different successive times, rocks of different chemical composition existed in a melted state, and were forced upwards through rifts in the strata. The same thing is known with respect to modern volcanic accumulations, which change with time; and there remains for each case the same further question of the cause of these mineral changes under a given area of the earth's face.
The principle upon which the inquiry proceeds in the case of the older rocks, was strongly enforced and applied by Werner; but is not universally, though perhaps it is generally, admitted. Dykes fill fissures in stratified and unstratified rocks; mineral veins appear under the same circumstances. Where the rocks are distinctly stratified, and are of different qualities in the different beds, and contain organic remains in some or all of the beds, the proof that the fissures alluded to are of later date than the formation of the rocks is conclusive: therefore the dykes, which fill these fissures, are of still later date; and the same conclusion is extended to unstratified rocks: nor is it limited to the great masses of rocks. When dykes or veins intersect one another, that which is divided is the older, that which cuts through another is the newer. Thus, in the diagram (No. 92.), taken from Dr. Macculloch's drawing in the Geological Transactions, vol. iv. pl. 6. (S) the schist rock is divided by veins of granite (G),
| S. Schist. | G. Granite veins. | P. Porphyry dykes. |
cut through by straight dykes of porphyry (P). This occurs in Ben Cruachan, by the shore of Loch Awe.
Upon this principle Werner speaks confidently of the relative age of mineral veins; and it is the general impression of miners and geologists, that he is right in so doing.
On a greater scale, the same problem is presented to
us by examinations of large districts, like Ireland, the
Pyrenees, Cornwall, or the Bohemian mountains. But
the data necessary for the solution of this problem are
quite different, and the result becomes a part of the
history of the formation of the crust of the globe. It
is requisite to know in this case what relation the several
rocks of igneous origin bear to the stratified rocks among
which they appear. In this inquiry we must not assume
that all the masses of igneous rocks of the same nature
have been forced among the strata at the same time;
this would be sometimes erroneous, always insecure.
One of the most certain proofs of the exact age of a
particular mass of igneous rock, is also one of the
rarest. When strata a, b, c, d are traversed by a trap
dyke, and these strata, together with the dyke, are overlaid
by the next stratified rock in order of time e, it is
evident that the dyke was formed in the interval
(whether long or short) between the deposition of d
and e. Such a case is believed to occur on the line of
a trap dyke which crosses the Durham coalfield from
Eggleston to Quarrington, dividing the coal strata, but
not the superincumbent magnesian limestone. A similar
dyke, starting from near the same point, passes into the
oolitic system; and thus we learn that the igneous
action in Teesdale, which commenced in the early carboniferous
period, continued to produce similar basaltic
rocks till after the deposition of part of the oolites;
and there is nothing which prevents us from supposing
that this last eruption may have been of much later
date, as the great eruption in the north of Ireland is
known to have been.
The great basaltic plateau in the counties of Antrim and Londonderry rests upon chalk; there are no tertiary strata above it: its date is therefore only known approximately: it was effused during the tertiary eras. The great basaltic masses of Mull and Skye, of Arran and Ayrshire, the Ochill Hills, &c., appear in directions and under circumstances which seem to connect them with the same seat of volcanic action as the Irish basalts; but data are wanted for determining the age of their eruption.
Mere association of igneous rocks with particular strata only proves that such rocks are at least not older than these strata: the case of the dyke traced by Mr. Murchison from the Breiddyn Hills (amid primary strata) and under and into the new red sandstone of Acton Reynolds, shows how very little propriety there is in classing trap rocks by the strata among which they have been injected; since this is, in fact, "a geological accident."
It is remarkable with regard to granite and rocks closely allied to it, that, excepting at a very few spots, among which Weinbohla on the Danube is the most remarkable, these igneous products are not seen in contact with any of the strata of the secondary or tertiary class. Granite touches gneiss at Strontian; mica schist in Ben Nevis; hornblende schist, argillaceous schist, and primary limestone in Glen Tilt; clay slate and grauwacke slate in Wicklow, Anglesea, Devon, and Cornwall.
It has been supposed that granites of different antiquity possess distinguishable mineral characters. The opinion is not improbable; but it is difficult to assure ourselves of its truth, because, as Humboldt confesses, it would be difficult to mention a granite which geognosts unanimously consider as anterior to every other rock. The same author observes, while speaking of "primitive" granite, "it appears to me that in both hemispheres, particularly in the New World, granite is most ancient when it is richer in quartz and less abundant in mica; and he notices the addition of hornblende as characterising the most modern granites. As before observed, the three granitic masses in the midst of the Cumbrian mountains present as many distinct sorts of granite, and each belongs to a distinct place in the series of slates. The Skiddaw granite is quartzose and micaceous, and underlies the lowest slate rocks; the Eskdale granite is quartzose with little or no mica, and lies among green slates of the middle division; the Shap granite contains but little quartz, is porphyritic in structure, and lies near the base of the upper Cumbrian series of slates. Whether these granites are of the same or very different geological eras, cannot be known without the most careful study of the district undertaken for the purpose.
Those geologists who think that the culmiferous strata of Devon form part of the carboniferous system of England, which overlies old red sandstone, may believe the granite of Dartmoor to have been erupted since the age of the mountain limestone; for the culm measures are greatly contorted where they approach the igneous rock.
At Weinbohla on the Danube, according to professor Weiss, confirmed by many subsequent authorities, occurs a real superposition of granite (or sienite) on chalk and green sand, which strata, usually horizontal, dip suddenly beneath the granite in some places, and rest upon it in others. (See De la Beche's Manual, for a detailed account.)
In the Pyrenees we learn from M. Dufrenoy, that granite sends veins into chalk, and converts it into granular crystallised limestone, and generates in it valuable veins of iron ore. This range of mountains is remarkable for showing contacts of granite with calcareous beds of the several eras of transition rocks, lias, and chalk, and in each of these cases the limestone become crystalline and metalliferous.
Our view of the history of igneous rocks will be both more complete and accurate by considering them in connection with the lines and points where the strata have been subjected to remarkable disturbance. By this means their true origin becomes, if possible, more clear, their relative antiquity less doubtful, their affinity to the products of modern volcanos more definite. As by the modern earthquake the ground is opened far beyond the reach of lava currents, so in earlier times great fractures were not every where filled with melted rock; but yet it is only along and near to lines of subterranean disturbance that the "hypogene" rocks have risen to the day. Their dependence on such dislocations is very unequal: granitic rocks show themselves in distinct connection with the principal ranges of mountains which mark the most considerable effects of modern subterranean disturbance. Minute scrutiny may show in many mountain chains that the granite, which is almost universally present, does not uniformly occupy the mineral or geographical axis or centre of the rocky group;—amidst the complicated displacements which there occur, this could seldom be exactly the case; but a glance at all good geological maps will satisfy the impartial student that the connection of granitic elevation, uplifted primary strata, and mountain country, is real, if not necessary, and of high theoretical importance. (See Vol. I. p. 38.)
Rocks which in some degree share with granite this character of central position, with respect to mountain ranges of primary strata, are hypersthenic syenite and common syenite, and certain porphyries which graduate into granite, and share its geological history. But the trap rocks generally, including in this term the augitic and hornblendic rocks, and the porphyries which are related to them, are differently circumstanced. Von Buch has remarked, concerning augitic porphyry, that it ranges parallel to, and is found constantly at the base of, great chains of mountains; and he attributes to this porphyry a powerful influence in the elevation of the mountains. If we consider the granites as supporting lines of principal movement among the stratified masses, and recollect that, on a great scale, the angle of elevation quickly diminishes as we proceed from the mountains, till, in plains not far remote, the strata retain their horizontality, we may say that the trap rocks are most abundant in points and in lines distributed between the granitic axis and the level plains. In some instances trap rocks occupy an extent of country not inferior to the area of granite. The Ochill Hills, the Campsie Hills the Pentland Hills, and others connected with them, form one great trappean country filling the vale of the Forth and Clyde, which is a great natural hollow between the ranges of the Grampians and the Lammermuir mountains, both elevated on axes of granite and syenite. Large breadths of trap rocks appear in Skye, Rum, Eigg, Mull, Arran, and Antrim; but in none of these cases is their appearance connected with ridges of stratified rocks, as granitic masses almost invariably are. Moreover these trap rocks, whether in the shape of dykes or overlying masses, are usually so disposed as to suggest the idea of volcanic action, determined to particular points, and bursting out and overflowing from particular lines, rather than a general expansion beneath immense areas of strata which seems best to agree with granitic elevations.
Trap dykes are frequently manifested along the lines of faults, and these may sometimes be determined in geological age by the circumstances which accompany the disturbed strata.
Keeping in mind these general facts, but disregarding the crude notions which attribute to granite or trap rocks the elevations and fractures which have merely opened to us their subterranean repository, or given them channels to the surface, we shall be able to construct a table of the relative antiquity of igneous rocks, by comparing their distribution with the principal phenomena of convulsion in the crust of the earth. Such a table, however, would be very incomplete if founded upon small geographical areas; as the imperfection of the following sketch, based on the examination of the British islands, will abundantly prove.
Table of the Principal Disturbance of the stratification of the British Islands, with the Igneous Rocks observed in connections therewith.
| class I.—Before the Deposition of Old Red Sandstone. | |||||
| Strata disturbed | In What manner. | Localities | Igneous Rocks in connection. | ||
| a. Affecting Hypozoic strata chiefly | Long ranges of gneiss, mica schist, chlorite schist, clay slate, primary limestone, &c. | The Grampians mountains generally | Granite, sometimes, as in Ben Cruachan, Strontian, &c. | ||
| The primary mountains of Donegal. | Serpentine at Portsoy, &c., porphyry and greenstone not unfrequent. | ||||
| b. Affecting Lower Silurian, &c. | Anticlinal axes of clay slate and grauwacke. | The Lammermuir mountains. | Syenitic granite, claystone, porphyry | ||
| The Cavan mountains. | Granite. | ||||
| The Wicklow and Wexford mountains. | Granite. | ||||
| Isle of Man. | Granite. | ||||
| Longmynd, near Shrewsbury. | Various traps. | ||||
| Snowdon range | Greenstone, porphyry. | ||||
| Berwyn range | Greenstone, porphyry. | ||||
| c. Affecting all the Siluarion Strata. | Anticlinal axes of Cambrian and silurian rocks. | Cumberland and Westmoreland generally | Granite of different kinds, porphyry, greenstone. | ||
| class II.—Before the Deposition of the Lias. | |||||
| Strata disturbed | In What manner. | Localities | Igneous Rocks in connection. | ||
| a. Affecting the old red sandstone and carboniferous generally, but not the magnesian limestone. | By anticlinal and synclinals, and great faults. | South Wales. | None | ||
| Woolhope, Malvern Hills. | |||||
| Abberford, in Yorkshire | None | ||||
| Quarrington, in Durham. | Basaltic dyke from Tuesdale | ||||
| Manchester | |||||
| Somersetshire coalfields. | ? | ||||
| South Wales | Feldspathic, epidotic, and greenstone trap | ||||
| Caradoc and Wrekin | |||||
| b. Affecting the Magnesian limestone, or its equivalent | By great faults | The Penine Fault (Vol. I. p. 194.) | Granitic porphyry (Dufton Pike) | ||
| The 9 fathom dyke (Vol. I. p. 194.) | None | ||||
| The Craven fault (Vol. I. p. 194.) | |||||
| In the Coalbrookdale coalfield. | ? | ||||
| South Wales. | |||||
| c. Affecting beds of the new red sandstone formation | Fault or small anticlinal | Acton Reynolds, Shropshire. | Dyke of greenstone from the Breidddyn Hills. | ||
| Island of Arran | Dykes of porphyry, claystone, hornstone, pitchstone, basalt, greenstone, &c. | ||||
| Great uplifting. | Arran. | Granite of Goatfield, &c. | |||
| class III.—Before the Deposition of the lower Green Sands. | ||||||
| Strata disturbed | In What manner. | Localities | Igneous Rocks in connection. | |||
| Affecting the oolite rocks generally, but not any part of the cretaceous system | A broad axis of elevation | Yorkshire (Bishop Wilton) | None | |||
| Unconformity of dips | Bowood | None | ||||
| Blackdown | None | |||||
| Great dyke and fault | Yorkshire (Moors near Whitby) (Vol. I. p. 231.) | Basaltic dyke from Teesdakle | ||||
| Complicated effects | Skye, Eigg | Sienite, hyperstheine and other trap rocks, pitchstone of Eigg | ||||
| Ord of Caithness (Vol. I. p. 231) | Granite | |||||
| class IV.—Before the Deposition of the Chalk. | |||||
| Strata disturbed | In What manner. | Localities | Igneous Rocks in connection. | ||
| Affecting, perhaps, all the tertiary strata which occur in the district | Anticlinal axes. (See Vol. I p. 276.) | Weald of Kent and Sussex | None | ||
| Isle of Wight | None | ||||
- ↑ See VoL I. p. 45.
- ↑ From ισος, equal, and μεζος, a portion.
- ↑ 3.0 3.1 Protoxide of iron.
- ↑ Boracic acid.
- ↑ Black augite analysed by Vauquelin.
- ↑ Is this word originally todtstein, derived from German miners? It would in this case signify rock, which in a mining country is dead, or unproductive of mineral treasures, a character generally applicable to this rock.
- ↑ Illustrations of the Huttonian Theory, Works, p. 312.
- ↑ These descriptions are unpublished.
- ↑ Mr. Murchison has found numerous examples of this effect in his survey of the trap rocks of the Silurian system, as in Caer Caradoc, the Corndon Hills, the Stiperstone ridge, and many others. One of the Corndon dykes, forty feet wide, with prisms lying across'the dyke, composed of greenstone varying to felspar, has indurated the neighbouring argillaceous beds for two or three inches, so as to make them like the substance known as porcelain jasper; and for twelve feet the induration is remarkable.
- ↑ Vol. I p. 65, &c.
- ↑ Consult on this subject, besides Memoirs by the Author, (Geol. Trans. 1820, and Brit. Assoc. 1843), and Professor Sedgwick (Geol. Trans. 1835), the later writings of Sharpe, (Proceedings of Geol. Soc. 1847. 1849), and Hopkins (Phil. Mag. &c.) See also De la Beche, in Geol. Observer, 1851.
- ↑ Brit. Assoc. Reports for 1843.
- ↑ Geol. Proceedings, 1847, 1849.
- ↑ Camb. Phil. Trans. 1847.
- ↑ Dr. Berger on the Geological Features of the North of Ireland, Geol. Trans, vol. iii. p. 172.