A Treatise on Geology/Chapter 8
CHAP VIII.
WERNER, in his valuable treatise on veins, distinguishes
between "true veins" and some other appearances which
he thinks undeserving of the title. "Veins" he declares
to be particular mineral repositories, of a flat or a tabular
shape, which in general traverse the strata of mountains,
and are filled with mineral matter differing more
or less from the nature of the rocks in which they occur.
They cross the strata, and have a direction different from
theirs; they are rents which have been formed in mountains,
and have been afterwards filled up by mineral
matter.
In this definition rock dykes are included, and it sometimes happens that those dykes are metalliferous; but the substances associated with tin, copper, lead, and the other minerals for which veins are valued, are usually quite different from the matter of rock dykes. Felspar and augite, so common in trap rocks, are almost unknown in metalliferous veins, which contain, in fact, few silicates of any kind, though quartz (of a peculiar aspect) is very frequent therein. Besides the metals, in combination with sulphur, carbonic acid, &c., salts of lime and barytes abound, and clays of different qualities appear.
Thus the distinction between rock veins and rock dykes is in their contents; and since we find both in the same districts, in similar fissures, and under similar circumstances, this difference is of such importance, that, however strong may be the arguments which tend to show that mineral veins are the result of igneous action among the masses of the globe, we cannot fail to perceive that this action was materially different in the two cases.
Geographical Distribution.
On no part of the history of veins has observation pronounced a more positive decision, than on the relation borne by their distribution to physical geography. The truth is universally recognised, that while extensive plain countries are utterly deprived of all indications of these valuable mineral deposits, and others contain them but rarely and in small quantity, there are few mountain countries in which mineral veins are not found in abundance and variety. They are, indeed, not equally nor uniformly distributed even in their more favoured regions: their occurrence is sufficiently dependent on other causes, besides the mere form of the surface, to keep alive the curiosity and inflame the enterprise of the miner, as well as to conduct the philosopher one step further in his research into the mysterious structure of the earth. Taking a general view of the mining districts (not herein counting the collieries) of Great Britain, we see the Grampians, and Lammermuir, and Cumbrian mountains; the great ridges of Northumberland, Durham, Yorkshire, and Derbyshire; the anticlinal axes of the Isle of Man, Anglesea, Snowdonia, and Shropshire; the elevated boundaries of the coal tracts of Wales, and Somerset; the mountain chain of Devon and Cornwall; the elevated ranges of Wicklow, and Wexford, of Leitrim, Sligo, Mayo, and Galway; all rich in lead, copper, zinc, tin, &c., with some silver, and traces of gold. On the other hand, the broad valleys of the Forth, Clyde, and Tweed; the wide vales which surround the Cumbrian, Yorkshire, Welsh, and Devonian mountains, contain almost no mines; and the central plains of Ireland hardly yield any metallic treasures. The same contrast appears on the continent of Europe, between the mountainous and metalliferous tracts of Brittany, the Pyrenees, the Harz, Eregebirge, Oural, &c., and the great Plains of France, Germany, and Russia.
In considering further the situations of mineral veins, we are struck by another feature of their geographical distribution. There are some general directions, common not to all, but yet to a very great majority of the veins of the British islands. More than half of the productive veins pass in east and west lines, or rather a little N. of East, and S. of West, in the mining districts of Cumberland, Yorkshire, Derbyshire, North Wales, Shropshire, and Cornwall. The same directions prevail in Brittany, the Harz, Hungary, and, according to Mr. J. Taylor, in Mexico. Hence, veins running east and west are commonly called "right running" veins, while others, which in the same districts are generally unproductive, and run very often north and south, across the productive veins, are often called "cross" veins. (For proofs of these truths, Werner on Veins, Williams's Mineral Kingdom, Forster and Sopwith's Accounts of Aldstone Moor, and Farcy's Derbyshire; Mr. Carne, in the Geol. Trans, of Cornwall; Mr. J. Taylor on Veins, in the Brit. Assoc. Reports, may be consulted.) Now as the directions of the mountain masses to which these veins are geographically related are various; the greater number ranging N. E. and S. W.; some (Yorkshire, Derbyshire, Flintshire) north and south; others, Pyrenees, Harz, Carpathians, E. S. E.; it is requisite to take other circumstances into account, before deciding to what extent these prevalent directions of the mineral veins are dependent on the direction of the mountains which they enrich.
One of the most obvious and interesting points of inquiry is the dependence of the occurrence of metalliferous veins on the age of the rocks; and Werner, as might be expected from the tenor of his generalisations, ventured boldly to pronounce concerning many metals, the order of their antiquity in the crust of the earth. Judging from the rocks in which they frequently occur, tin, molybdena, tungsten, and wolfram are ranked as the most ancient metals; uranium and bismuth stand next, "having been found in veins in transition or secondary strata." Gold and silver are considered comparatively new; copper, lead, and zinc occur in deposits of various ages; arsenical pyrites ranks as an old product, cobalt as new, magnesia is of intermediate, and iron ores are of all ages.
Though these doctrines of the relative antiquity of the metals must now be greatly modified, the subject of the distribution of metallic ores according to the place of the rocks in the scale of stratification, is still one of the most curious in geology, and valuable in mining. It is certain that such a dependence exists, and probable that the accurate knowledge of it would be important in clearing up some great difficulties in the theory of mining. The variety of metallic and earthy minerals in the veins which traverse primary slates in Cornwall, Cumberland, and the Lead Hills, is very great and remarkable when compared to the small catalogue of these found in the secondary limestones of Flintshire, Derbyshire, and Durham. While argentiferous lead ore, and salts of lead, copper ore, blende, calamine, pyrites, carbonate of iron, quartz, carbonate of lime, sulphate of barytes, fluor spar, &c., are common to these and the Cornish districts, the latter yield ores of silver, tin, bismuth, cobalt, arsenic, antimony, uranium, &c., opal, jasper, garnet, zoolites, tourmaline, schorl, epidote, asbestus, steatite, &c.
There is a remarkable circumstance in the distribution of metallic veins in the same class of stratified rocks,—a peculiarity depending on local influences; such, that while the slates of Cornwall near the granitic eruptions, yield tin and copper, and the Snowdonian slates, and those of Coniston Water Head yield copper; those of Loweswater, Borrovvdale, Patterdale, and Caldbeck fells yield lead, or lead and copper. Copper ore and red oxide of iron occur in the limestone of Furness; lead ore and calamine in that of Derbyshire, Flintshire, and Mendip. In the same manner the veinstones vary; even the calcareous spar is crystallised with quite different planes in the mines of Aldstone and Derbyshire.
The limits of mining districts are often very decided. In the rich mining tract round Cross fell, dissected like a map by mineral veins, and worked with an enterprise worthy of all praise, no instance (we believe) has yet occurred of a single vein being traced to the western side of the mountain range, across the great Penine fault, so as to penetrate the slaty rocks that rise in the line of dislocation. The same fact is witnessed again, in almost precisely similar circumstances, in the Flintshire veins, which do not, in a single instance, enter the subjacent silurian rocks of the Moel Fammau range, which rises on the line of a great axis of movement. Numerous instances of this remarkable dependence of the occurrence of mineral veins, in limited portions of country definitely related to particular lines of disturbed strata, are well and familiarly known.
Occurrence of Mineral Veins near Centres of Igneous Action.
Ever since the analogy of mineral veins and rock dykes has been clearly perceived, and the dependence of these latter on disturbance of subterranean temperature recognised, the dependence of the occurrence of mineral veins on the general influence of heat has been continually more and more apparent. This appears to have been strongly felt by Boué and Humboldt; there are also passages in the writings of Von Buch which conduct to the same conclusion. M. Necker presented to the Geological Society, in 1832, an attempt to bring under general geological laws the relative position of metalliferous deposits with regard to the rock formations of which the crust of the earth is constructed. The doctrine of the sublimation of the metalliferous contents of veins from igneous matter occurred to the author, twelve years previously, from observing the deposition of specular iron on the crust of a stream of lava flowing down the side of Vesuvius; and he was induced, from that circumstance, to institute an investigation of the subject with reference to the following questions:—
First, Is there, near each of the known metalliferous deposits, any unstratified rock?
Secondly, If none is to be found in the immediate vicinity of such deposits, is there no evidence, derived from the geological constitution of the district, which would lead to the belief that an unstratified rock may extend under the metalliferous district, and at no great distance from the surface of the country?
Thirdly, Do there exist metalliferous deposits entirely disconnected from unstratified rocks?
With respect to the first of these questions, the author showed, by copious references to England, Scotland, Ireland, Norway, France, Germany, Hungary, the Southern Alps, Russia, and the northern shores of the Black Sea, that the great mining districts of all these countries are immediately connected with unstratified rocks: and in further support of this solution of the first question, he mentions the metalliferous porphyries of Mexico, and the auriferous granite of the Orinoco; but he observes, that his knowledge of the mining countries of South America is not sufficient to enable him to state their general geological connection. Locally, this truth is well known. Mr. R. Fox, in his excellent summary of facts regarding the veins of Cornwall, observes:—"The copper and tin mines are generally situated at or near some of the junctions of the granite and killas, or of killas and elvan," &c.: and both of these metals have been found in great abundance in each of these rocks; and it is, perhaps, difficult to decide in which of them either metal has, upon the whole, predominated.
With reference to the second question, the probable association of metallic veins with unstratified rocks, though the latter are not visible in the immediate neighbourhood of the former,—the author gives a section of the country between Valorsine and Servoz, and points out the probable extension of the granite of Valorsine under the Aiguelles Rouges and Mont Breven, composed of protogine, chlorite, and talcose schists, to the immediate vicinity of the mines of Servoz, which are situated in the latter formation. He also refers the reader for further illustration to the metallic deposits of Wanlockhead and the Lead Hills; to the mines of Huelgoet and Poullauen in Brittany; to those of Macagnaga and Allayna at the foot of Mount Rosa; to those of Sardinia, Corsica, and Elba; to the metalliferous veins of the Vosges, Brescina in the Alps, and the Altai chain; all of which occur in districts where unstratified rocks are known to exist.
In reply to the third question,—Do their exist metalliferous deposits entirely disconnected from unstratified rocks?—the author enumerates the mines of the Netherlands, those of quicksilver at Idria, the lead mines of Poggau in the valley of the Mur; Pezay and Macoz in the Tarentaise, and the veins of galena in the mountain limestone of the south-west of England. (See Geological Proceedings, 1832.)
On considering the cases mentioned by Mr. Necker, of metalliferous veins entirely unconnected with great masses of unstratified rocks, we perceive they are not unaccompanied by great dislocations of the strata, such as are usually associated with the appearance of trap rocks at the surface. It is probably not to the "Whin Sill" that the rich and abundant lead mines of the whole district extending from the Tyne to the Aire are due, —for indeed, through all the southern portion of this tract, almost no igneous rock appears,—but to the mighty and continuous disruption of strata caused by disturbance of interior heat, which bounds the mining district. In like manner, the very rich mining tract of Flintshire is unconnected with igneous rocks, but is defined, and is obviously dependent on the great disruption of strata along the eastern side of the vale of Clwydd. The Mendip hills offer a similar example of veins which depend on an axis of movement, though no igneous rocks appear on the line.
Again, in several smaller instances, the relation of lead and copper veins to axes of dislocation is obvious; witness the lead veins which cross the anticlinals of Greenhow Hill, Bolton Bridge, Bolland, &c. (see Illustrations of the Geology of Yorkshire), in none of which situations is there the smallest indication of igneous rocks near the surface.
Now, as in all these cases the subterranean movement has opened a passage to the interior regions of the earth, we see that M. Necker's propositions are not negatived, provided we suppose these communications to have been traversed by the sublimations to which he ascribes the origin of the substances in veins. Whether the particular mode of igneous action (sublimation from heated rocks), proposed by Mr. Necker for investigation, be the true method of nature or not, it is clear that his researches, followed out, justify a confident belief that proximity to, or communication with, masses of igneous rock, is a condition remarkably and generally influential on the production of metalliferous veins in the stratified rocks.
Taking, then, the element of heat as of great importance in explaining the leading facts connected with mineral veins, we are prepared at once with answers to the obvious question, Why are the metalliferous veins, beyond all comparison, most plentiful in primary and early secondary (transition rocks of Buckland) strata?—Because these rocks, as being nearer to the ignigenous masses below, must have experienced, more than those of later origin, the general influence of heat, We are also enabled to account for the exceptions to this rule in the Pyrenees, where, according to M. Dufrenoy's interesting examination (Mémoires sur les Mines de Fer des Pyrenées, 1834), ores of iron accompany the ramifications of granite even in the cretaceous formation. There are, in fact, in the Pyrenees, three repositories of iron ores.
All these deposits of iron ore are found where the rocks touch or approach very near to the granite; and from all the circumstances, M. Dufrenoy is apparently well justified in viewing the occurrence of the ores as dependent on the proximity of granite, and independent of the antiquity or other characteristic differences of the rocks in which they lie.
Lest this result should lead us too far, and confound all the variety of phenomena connected with mineral veins in the vague and valueless notion of "the effects of heat," it appears right to point the reader's attention to such localities as the Island of Arran, where the proximity of the granite is marked by abundance of rock dykes, but shows almost no trace of mineral veins. The dependence of metallic veins upon local centres of igneous action, is certainly very different from that of rock dykes, as might be safely inferred from many essential differences between them in countries where they occur together.
Relation of Veins to the Substance and Structure of the neighbouring Rocks.
Before proceeding to trace the relations which really exist between the substance of the veins and the neighbouring rocks, a more minute description of the forms and contents of veins must be attempted than was necessary for the preceding inquiries.
The fissures now occupied by veins pass through all the rocks met with in their downward descent. Though a few instances are supposed to be known of their termination, at some considerable depth, all large veins continue beyond the reach of the deepest mine. Their horizontal extent is various: some veins run 5, 10, or more miles through a country; and, in fact, their termination is not really known, except that they are lost in mere cracks not worth the miner's attention. But so variable is the breadth of veins, that extreme contractions and considerable expansions sometimes confuse all regularity, and render doubtful even the connection of the seemingly disunited parts of such veins. "If we take a vein of 3 or 4 feet to represent a fair average size, it may be only an inch or two wide in one place and 8 or 10 feet in another. Such extremes not infrequently occur within a few fathoms of each other."[1] Other veins preserve an almost unvarying breadth and freedom from these perplexing contractions; and we believe these differences of character may be distinctly referred to the natural structure of the rocks, and the movements to which they have been subjected.
Veins, in their descent through the rocks, approach more or less to a vertical position; their deviation from it seldom exceeds 10 degrees in the mining countries of the north of England; but in Cornwall, so rich in complicated phenomena, the underlie, or deviation from the vertical, is supposed by Mr. Fox to average 20 degrees, but seldom to exceed 45. The mechanical theory of these inclinations of veins is yet altogether imperfect; we do not know in what degree these peculiarities depend on original jointed structure of the rocks, nor how to refer their various directions to sudden fractures or gradual pressures, such as Werner pictured to himself. Nor shall we escape from this ignorance, until the directions taken by the veins, or, to speak more accurately, the planes of their fissures, are compared geometrically with the planes of the joints, the planes of stratification, and the local axes of elevation and depression. In the lead mining districts of the North of England, a notion exists that the greater number of veins are at right angles to the planes of stratification: this idea is put as a general assertion by Williams (Mineral Kingdom, vol. i. p. 317.), a writer whose extensive experience in mining renders even a dogma of this nature worth recording. His words are, "rake veins have a greater or lesser hade or slope in proportion to the declivity of the strata, as the mineral fissure, or vein, is a transverse section cut at right angles to the lay or bed of the strata;"—"whatever be the slope of the strata one way, the hade or slope of the vein is as much from the perpendicular the other way." And he then confines this remark to veins which range with the bearing of the strata; distinguishing them from others which "cut right across the strata," and a third group cutting them diagonally, which he rightly terms "oblique veins." The reader who compares this description of the ordinary relation of the deviations and dip of veins, with Mr. Murchison's notices of the prevalent character of the joint planes in the silurian rocks, will not fail to perceive the conformity of two independent sets of observations, and gather in consequence a useful notion of the affinity of vein fissures, and the divisional planes which constitute a part of the structure of all stratified rocks. It is much to be wished that the triple co-ordination recommended above, as necessary to a just view of the origin of vein fissures, should be carefully executed on many of the complicated phenomena of the Cornish mines. The cleavage planes of the slaty rocks, which inclose mineral veins, should also be included in the survey.
Some veins, like rock dykes, occupy one "clean" fissure of the rocks; others branch off into strings, or become divided into forks, which continue for a longer or shorter space till they are lost in clefts of the rocks, or turn to re-unite themselves with the main trunk. Such "strings," or "feeders," as they are called in Cornwall, appear under very various circumstances, both on the horizontal and vertical sections. Occasionally a poor vein is worth following for its rich lateral strings; and it is a common notion of miners that such appendages are influential on the productiveness of a vein.
One of the most curious accidents which affect a vein fissure, is its bending or expanding against particular layers of rock, so as to constitute what, in the mining country of Aldstone Moor, are called "flats," or lateral extensions parallel to the stratification. These are often cavernous in the middle, and yield beautiful crystallisations.
Veins sometimes appear as one united mass, due to one single or uninterrupted deposition of mineral substances; in other cases there are divisions in the veins, or by the side of them, which contain clay or quartz ribs, or in some other way give indications of successive rents in the same general direction. Such appearances have been often noticed (as by Werner, Carne, Fox, &c.), and considered as capable of explaining, in some instances, the curious and very common accident of portions of the neighbouring rocks, enveloped in the mass of the veins, always near to and even opposite to the parts whence they were disjoined. Such portions of the neighbouring rocks are called "rider" and being frequently traversed and impregnated by the vein substances, acquire a characteristic aspect; which being found again not infrequently in the rock on the sides of the vein, especially where "strings" pass off from the mass of the vein, such bounding rocks are said to be "ridered."
In this manner, by (successive?) nearly parallel rifts in the rocks, which all received mineral depositions, a "strong vein" becomes of almost indefinite width, even 30, 40, or more feet across, and often bewilders the miner, unable to interpret or follow the seemingly capricious manner of the mineral aggregation.
The rocky boundaries of the veins are often somewhat peculiar in character near the vein: sometimes, as in the case of rock dykes, they appear harder than the rest of the rock; at other times some difference of mineral impregnation, pyritous, or serpentinous admixture, appears, which distinguishes the so-called "walls" of a vein. But this term is apt to mislead a geologist into the notion that some definite parallel band always insulates the vein from the in closing rock; which is, in general, not the fact. In Cornwall generally, it is thought by Mr. Fox that the rocks diminish in hardness near a vein; and similar facts are mentioned by Werner.
A curious circumstance is noticed by Mr. Fox and others, regarding the arrangement of the quartz in the cross courses of Cornwall. This mineral does not in such cases appear in its usual pyramidal or prismatic crystallisation, but is of a fibrous structure, the axes of the fibres lying across the vein, exactly as we may see in hundreds of examples in thin quartz veins which divide argillaceous slate, and other rocks. There are in some cases several parallel plates of this fibrous quartz, marking successive small rents.
In the cross courses of Cornwall, which contain
quartz, clay, and other substances, these are very commonly
arranged in alternate layers parallel to the walls.
(Mr. Fox.) The same thing obtains very generally,
though not universally, in veins of all ages and contents;
as the small specimens commonly sold in Derbyshire
very prettily illustrate. It is generally to be observed
in such cases, that the crystallisations are so arranged
that the terminal faces point inwards each way from
the walls of the vein, and that those bands of crystallisation
which are nearest to the walls, have themselves
served as surfaces of attachment for the next layer,
which is usually moulded on the other as if that had
been deposited first. This appearance has suggested
successive irruptions of melted matter, successive secretions
from solution, successive accumulations from sublimation,
and successive depositions by electrical currents,
to persons whose views led them thus diversely; but a
succession of operations is commonly (not universally)
admitted to explain these appearances.
Another peculiar appearance in mineral veins, noticed by Williams, Fox, Henwood, and others—and which from personal inspection the author knows to be frequent both in primary and secondary mining tracts is the segregation of the metallic contents of a vein into portions inclined at various angles in different veins, but nearly parallel in the same vein. These are called "pipes" or "shoots;" and their occurrence is of such importance, as to mark, in a long vein, a series of parallel spaces more than usually metalliferous. The relation of these pipes of ore to the natural structures of the neighbouring rocks is a subject of research strongly to be recommended to intelligent mine agents, both for its practical and scientific value. Mr. Fox observes, from the information of Mr. R. Tregaskis, that when veins are nearly at right angles to the beds of killas, the masses of ore which they contain are generally conformable, in their underlie, to the direction or dip of such beds; in other words, they usually take an oblique direction in. the veins, and form what the miners call "shoots" of ore: and when the directions of the beds and veins are nearly parallel to each other, the ore has not usually any independent dip or shoot in a lode; it is then termed a "pipe" of ore.
According to Mr. Kenwood (Mining Review), the "shoots" usually dip from the granite, and towards the slate, whichever of them may be the containing rock.
The reality of the dependence of the distribution of metallic ores, in a continuous vein, upon some qualities of the surrounding rocks, is very perfectly demonstrated by facts known in the north of England. The mining districts of Aldstone Moor, Teesdale, Swaledale, &c. consist of shales, grits, and limestones, traversed by east and west and north and south veins, which variously dislocate the strata. In the course of these unequal dislocations, coupled with unequal thicknesses of the strata, various oppositions of the argillaceous, arenaceous, and calcareous rocks happen; and there are simple rules which seldom fail in determining what parts of a vein may be found productive. First, it is chiefly in the limestone district that the veins are productive, though the fissures traverse a vast thickness of superincumbent shales, grits, and coal. Secondly, in a series of limestones, gritstones, and shales, which margin a vein, it will happen that, when inclosed between walls or cheeks which are both argillaceous, the vein will be unproductive, and generally "nipped," or reduced in width; with argillaceous beds on one side, and gritstones or limestones on the other, the same effects appear, but in an inferior degree; gritstone opposing gritstone yields irregular results, according to the mass and quality of the gritstone, so that in several districts (Grassington, Allenhead, &c.) much lead ore has been found in such situations; but when limestone is opposite to limestone, the vein is always most productive. Now, if we consider that, in the many displacements of veins, a thick limestone rock will be less frequently carried altogether away from its fellow beds than a thinner one, we see at once a reason why the "main limestone" of Swaledale (or "twelve fathom" limestone of Aldstone) is by far the most productive among the "bearing beds" of those counties; for it is the thickest limestone there known. There may be other reasons in addition; but this is obvious and important, and agrees with an opinion of those countries, which affirms that veins of small amount of dislocation (or "throw" as it is called) are, on the whole, more regularly productive than those attended by enormous displacement. (See Forster and Sopwith on the Veins of Aldstone Moor; and Geology of Yorkshire, vol. ii.)
In Cornwall, some veins bear tin or copper both in granite and killas; others yield more in one of these rocks; the veins are also very unequal in their produce in relation to depth from the surface; yet, as a general result, it seems to be admitted by all writers, that the contents of the veins undergo real and decided variations wherever the bounding rocks (or "country," as the miners term the mass of rocks adjoining a vein) experience changes of their nature or structure. (See the papers of Mr. Carne, Fox, &c. in Trans, of Geol. Soc. of Cornwall; Mr. Taylor's Report; Kenwood's Survey, &c.)
The same truth of the dependence of the contents of mineral veins upon the containing rocks is put in a strong light by Von Dechen, in his translation of De la Beche's Geological Manual. He notices the mechanical dependence of the width of the vein upon the solidity of the neighbouring strata, and points out other phenomena analogous to what have been mentioned above. "The veins of Kupferberg, in Silesia, bear ore only in hornblende schist, and become impoverished in mica schist." "At Stadtberg, veins which divide zechstein, kupferschiefer, and the subjacent clay slate and flinty slate, never bear ore above the kupferschiefer." At Bieber, cobalt veins traverse the kupferschiefer, and are unproductive in the subjacent red mica schist."
It has been generally thought that depth below the surface of the earth was influential on the quantity and quality of ore contained in a vein. Pryce, writing in 1778, says,—"The richest strata for copper is between 40 and 80 fathoms deep; and for tin between 20 and 60; and though a great quantity may be raised of either at fourscore or 100 fathoms, yet the quality is often decayed, or dry of metal"[2] This does not appear confirmed by recent experience, which has in some instances (Dolcoath mine) gone to the depth of 260 fathoms without exhausting the supply. That copper, upon the whole, occupies greater depths than tin, is a common opinion in Cornwall. Mr. W. Phillips observes, "At about 80 or 100 feet under the surface, the first traces of copper or tin are usually found; rarely nearer to it than 80 feet. If tin be first discovered even without a trace of copper, it is not unusual that, in the course of sinking 80 or 100 feet or more, all trace of it is lost, and copper only is found; but if, instead of tin, copper be first discovered at a depth of 80 or 100 feet, it seldom or never happens that tin is found below it in the same vein." Mr. Fox adds,—"There are, however, many instances of tin ore accompanying copper ore to a great depth; and in Dolcoath mine it is found in a copper lode more than 200 fathoms below the surface, and even under the copper." Mr. Carne observes,—"In general an ochreous oxide of iron (gossan) is found in the upper part of the copper veins, to which sulphuret of iron ('mundic') frequently succeeds, below which the miners confidently expect to obtain copper ore."
Relation of Veins to each other.
Adopting the opinion of Werner, that veins which cross and cut through others are of newer formation, we shall find great interest in the description given by Mr. Carne of the principal vein systems of Cornwall[3], and Werner's earlier classification of the veins of Freyberg.
Mr. Carne, distinguishing between contemporaneous veins and those which he considers as "true veins[4]," arranges the latter according to the difference of their antiquity, as inferred from their observed intersections, in eight classes.
The first Class includes the oldest tin veins. The underlie of these oldest tin veins is to the north; they are traversed by those of the second class. They form a very large majority of the whole.
The Second Class includes the more recent tin lodes. There are few veins of this class; they underlie to the south. The tin veins are generally east and west veins[5], ranging from 5° to 15° south of east and north of west; in some cases due east and west; and less frequently north of east and south of west. In St. Just, nearly S.E. and N.W. In Polgorth one is north and south.
The veinstones of tin lodes are quartz, chlorite, capel (quartz and schorl, or quartz and mica, or quartz, schorl, and chlorite), and rarely schorl, or fluor. The width of tin lodes varies from 36 feet to a mere string; the average being from 1 to 4 feet. The average underlie is about 2 feet in a fathom: extreme cases give 10 feet; or, in contact with copper lodes, 16 feet. Most of the productive tin lodes have been found in a slaty country.
To the Third Class belong the oldest east and west copper lodes. These form the great majority of the copper lodes of Cornwall. Their veinstone is generally quartz; sometimes fluor, quartz and fluor, capel, chlorite, hornstone and porphyry, or chalcedony. The average width is not more than 3 feet.
The direction is mostly south of east, and north of west, about 10° upon an average; sometimes E. and W.; or north of east and south of west. The underlie is various, but generally northwards; in a particular tract mostly southwards; in some cases the same vein changes its underlie from north to south. The average amount of underlie is 2 feet per fathom, the greatest 8. These copper lodes always traverse tin lodes. They are usually accompanied by small veins or partings of clay, called by miners "flukan."
The Fourth Class is composed of the contra[6] copper lodes. These are similar to the third class, excepting in their direction, their greater width, and their having more flukan in their composition. The average width may be stated at 4 feet.
Their direction is in general from 30 to 45 degrees south of east and north of west; some, however, run in an opposite direction, namely, north-east and south-west.
Their underlie is much the same as that of the other
copper lodes, to which they are much inferior in number,
The Fifth Class includes the "cross courses:" these are sometimes composed wholly of quartz, but they usually contain, besides quartz, a large portion of flukan, and sometimes of gossan.
Their width is usually greater than that of the veins previously mentioned, averaging at least 6 feet.
Their direction is usually west of north and east of south, but sometimes north and south, or east of north and west of south.
Their underlie is various: most of those which point east of north, underlie towards the west; and on the contrary, those which point west of north, underlie towards the east.
Cross courses have been traced for several miles: they rarely yield tin or copper; lead is the principal metal found in them.
In the Sixth Class, the more recent copper lodes, which are not numerous, nor in their size, direction, or underlie, materially different from older veins of this metal which have been described. They have more clay in them than is usually seen in the cross courses.
The Seventh Class contains the cross flukans, or cross courses which are composed wholly of clay; they are seldom more than one foot wide, but no water passes through them.
Their general direction is nearly north and south their underlie is much the same as that of the cross courses, generally towards the east.
In the Eighth Class are ranked the slides, which are composed wholly of slimy clay, and appear like natural partings in the rock.
They run in all directions, but in general are nearly parallel to the tin and copper lodes, which they throw up or down. They are narrow, and underlie very fast.
It has been observed by Mr. Carne, as a result of the preceding investigations, that "veins which contain the greatest quantity of flukan or clay, are generally found to traverse those which contain a less or none at all of that substance;" and this generalisation is confirmed by several facts communicated to Mr. Fox by the intelligent mine agents of Cornwall.
Werner used the same method of classification as that employed by Mr. Carne, for the phenomena which attend the mineral veins in a district as rich in metallic treasures as Cornwall; and the examination is the more valuable in comparison, because the treasures are generally different, and lie in different strata. Gneiss is the great repository of metallic veins in the Freyberg district, and aigentiferous lead ore the principal product. The ancient mining district in question is only about two German miles long, and one broad; yet, within these limits, Werner observed at least eight principal deposits of metallic veins, perfectly distinct from one another, and for the most part containing different metals. Of the veins which are thus distinguished, the first four intersect one another, so as to give a definite scale of antiquity, but the last four are obscurely characterised in this respect from other considerations.
The first, and decidedly the most ancient, of these de. posits, which yields argentiferous lead glance (galena), is one of the most important of the whole district, having constantly yielded, since the earliest period of working the mines of Freyberg, a large quantity of lead and silver, and a smaller of copper. It consists of coarse granular lead ore, with silver in the proportion of 1½ to 2½ oz. in the quintal; common arsenical pyrites; black blende in large grains; common iron and liver pyrites; a little copper pyrites; a little sparry ironstone. The veinstones are quartz; and sometimes a little brown spar, and calc spar. The various substances here named are not believed by Werner to be all of the same antiquity, but to have been formed successively in the vein, the oldest being nearest the sides.
These veins are from 2½ to 6 feet across, and are chiefly northern veins.
The second metalliferous deposit yields lead mixed with a larger proportion of silver than any other. It contains lead glance, very rich in silver; black blende, small granular; common iron and liver pyrites; a little arsenical pyrites. Dark red silver ore, brittle silver ore, white silver glance, and plumose antimony ore also occur. The veinstones are principally quartz, much brown spar, and calc spar. There is a difference of situation in the rein, characteristic of these substances; quartz is generally on the outside. The veins are from 2 feet to 10 inches wide, and are south and south-west veins.
The third deposit yields lead glance, with but little silver. Its contents are lead glance, with nearly an ounce of silver to the quintal; much iron pyrites; some black blende; a little red iron ochre. The veinsstones are quartz; sometimes also chlorite, mixed and surrounded with clay. These are all northern veins.
The fourth deposit is also composed of lead glance, with but little silver (from a quarter to three quarters of an ounce of silver to the quintal). Besides the lead ore, there is radiated pyrites, and sometimes a small quantity of brown blende. The veinstones are very distinct, and consist of heavy spar, fluor spar, a little quartz, and rarely calcareous spar. The veins are from 1 foot to a fathom in width, and have generally a western direction.
(To this vein system, Werner refers many deposits beyond the Saxon districts, not hesitating to include the Derbyshire mines, which certainly offer several interesting analogies as to the veinstones, the direction, and contents of the veins.)
The fifth deposit contains native silver, silver glance, and glance cobalt, besides a small portion of grey copper ore; lead glance rich in silver; a little brown blende; and sparry ironstone. The veinstones are disintegrated heavy spar, and blue fluor. It always occurs at the intersection of the southern and western veins (or first and fourth vein systems here described), or in the middle of the western veins.
The sixth deposit consists of native arsenic and red silver ore, with sometimes a little orpiment; and rarely a little copper nickel, glance cobalt, native silver, lead glance, iron pyrites, and sparry ironstone. The veinstones are heavy spar, green fluor, calcareous spar, and a little brown spar. Occurs in the intersections or in the middle of veins.
(The distinction of age between this and the last system is obscure.)
The seventh deposit consists of red ironstone, containing also a little iron glance, quartz, and heavy spar. Occurs in the upper parts of veins.
The eighth deposit contains copper pyrites, mountain green, malachite, red and brown iron ochre; with veinstones of quartz and fluor. It is of small importance.
In the valuable lead mines of Aldstone Moor, cases of intersection so complicated as those of Cornwall and other tracts of primary strata seldom or never occur. The main facts are the general east and west direction (by compass) of the lead veins, and the intersection of these by cross courses which range, like these in Cornwall, mostly west of north and south of east. Their "throw" is sometimes very great. The underlie of the veins is seldom considerable; and being mostly in the same direction in each mining field, intersections of the veins are not commonly met with. The cross courses are, as in Cornwall, commonly wider than the veins, and seldom produce any thing valuable. The veinstones are quartz, fluor, carbonate of lime, sulphate of barytes, &c.
That veins are enriched near the places where they are intersected by cross courses, is an opinion common in Cornwall, and for which good evidence appears: sometimes this happens only on one side of the cross course, as at Huel Creber mine, near Tavistock. Reciprocally, the cross courses are productive near the places where they cut the veins. When veins cross one another, it is supposed that the intersections are seldom enriched if the veins differ much in underlie.
Slides often contain ore, in the part between the separated portions of the veins which they divide and dislocate.
Theory of Mineral Veins.
There is, perhaps, no portion of geological science
less satisfactory than the variety of opinions, and conjectures,
which, till within a few years, constituted what
was called the "Theory" of mineral veins. In no department
of geology is it so difficult to observe accurately
the phenomena which form the basis of reasoning,
or to obtain from experience the data which ought to
limit and direct speculation. A short inspection of a
mine, with the disadvantage of confused lights and
noises, and explanations hid in a phraseology of very
difficult interpretation, leaves on the mind a feeling of
disquieting disappointment. The important facts of
the intersection of veins are not seen; the segregations
of ore in a vein, the change of the contents with the
change of ground, with the depth, the underlie, and
other influential conditions, must all be taken on the
affirmation of the agent, in whose office the stranger
expects in vain to find a complete record of the subterranean
operations, with all the scientific data which they
have revealed. Dr. Boase was so impressed with
these difficulties, that in his examination of the veins of
Cornwall, with a view to understand their formation, he
declined to enter the mines at all, preferring to trust his
reasonings on the few phenomena in the sea cliffs, which
he could accurately examine, than on the almost innumerable
facts which the mining art has disclosed, only
to be, in many cases, lost for ever to science. The
want of a national system of mining records is now
acknowledged, and ought to be remedied.[7] Werner's
views on this subject are not unworthy of his high reputation.
(See his work "On the Origin of Veins.")
Even under these extreme disadvantages with respect to the facts, the theory of mineral veins might have been more rapidly advanced, had a right method been followed in the interpretation of them; but this subject fell under the general misfortune of geology, and was considered rather as a boundless arena for Neptunists and Plutonists, for Wernerian and Huttonian controversy, than as a storehouse of more curious truths than those contained in the rude notions of injection by heat, or solution by water.
In the unfortunate dissociation of reasoners and observers, which is not even yet remedied, the imperfections of the closest speculations were too apparent to the miner to leave him the slightest confidence in the explanations proposed; and when, moreover, to every general rule regarding the position and contents of veins, gathered from observation, and seemingly established, further experience brought exceptions, how can we wonder that practical men gave up the problems as desperate, rejected mechanical and chemical causes altogether, and, resolute in ignorance, believed the veins to be contemporaneous with and an essential part of the stratified rocks, in whose history they felt no interest? This was the "vulgar notion" in the time of Agricola (1556), but it has been revived among men of science in the 19th century.
This, in fact, is the fundamental question in the theory of mineral veins; and though the state of knowledge on the point is so much advanced since the days of Werner and Playfair, that Macculloch thought, and most geologists feel, the question to be completely decided, we do not think it unnecessary to substantiate the truths which they have rather assumed than proved, and examine the objections which they neglected.
Veins are posterior to the Rocks which they traverse.
Werner, in his definition already given, assumes as a truth, that veins are of posterior date to the rocks which they traverse, because they fill fissures in them, but he was aware of the opinion which had, and still has, supporters, that veins were formed at the same time, and are of the same age, as the rocks in which they occur. He takes the trouble to examine this point, and to establish the origin of veins by the filling up of originally open fissures as a fundamental point of theoretical and practical importance. He offers nine proofs in support of this unequivocal statement, hoping to "remove all doubt of its truth from the mind of every intelligent and unprejudiced geognost and miner." These proofs, though not very skilfully managed, appear sufficient to establish the conclusion as far as regards the phenomena described by Werner, and commonly met with in mining experience. Practical miners, in all but a few districts, seldom express the slightest doubt of the truth of the Wernerian postulate, from which we have here retrenched the part which affirms that the veins were open in the upper parts.
Those who in modern times reject this origin of veins, and revive the notion that they are contemporaneous with, and a part of the rock formation, in which they lie, are influenced in their views, first, by the difficulty of explaining, according to simple mechanical laws, the displacements which, on the Wernerian supposition, the fissured rocks must have experienced; secondly, by the admitted fact, that there is some general, and often some special, affinity between the contents of the vein and the nature of the including rock; thirdly, there are cases in which substances of the same nature as those in veins, and combined in the same manner, are found in cavities which are unconnected with veins.
These circumstances have been regarded as of much importance, especially in Cornwall, where numerous veins, occurring under various circumstances, and in closing a vast variety of minerals, have been worked extensively to unusual depths, by men of great experience. If, then, in a country so favourably circumstanced, we find the theory of veins halting at the first step, we must admit that the general argument by which this step is fixed, is far from clear, or be prepared to encounter peculiar difficulties in the application of it to Cornwall. That the general argument is not really defective, we shall endeavour to shew, by examining the three classes of objections which have been referred to.
1. The mechanical difficulty of explaining the movements of the masses of rock in which the veins lie, is more considerable in Cornwall than in any other mining country yet investigated. In Vol. I. p. 40. we have given a sketch of the usual relation (a, b, d, e) of the planes of displacement to those of stratification, and an example of the contrary (c). Now this latter case, so rare in general, is not infrequent in Cornwall. Another cause of difficulty is the excessive abundance of the veins, and the variety of direction, inclination, and inequality of apparent displacement, which they manifest.
The accompanying plan and section of Huel Peever mine will explain many points peculiar to the Cornish veins.
On the ground plan it will be seen that six parallel courses (a tin vein, two copper veins, an elvan course, and two "slides") are "shifted" to the south by the cross course y, and again still further to the south by the cross course x, each through the same horizontal space.
In the vertical transverse section (taken from north to south), it is seen that the two "slides" c and d pass through and interrupt, in their inclined courses, both the copper vein b, which is inclined in the same way (to the north) as the slides, and the tin vein a, which is inclined the contrary way (see the points marked A, B, C, D, G): also, it is seen that the copper vein b passes through and displaces the tin vein a (compare the points F and E); moreover, it appears that, excepting the displacements from A to C, B to D, F to E, and at G, there is no irregularity, the divided parts of the vein
being respectively parallel. 
| GROUND FLAN OF HUEL PEEVER. | |
| a, a′, a″. Tin vein worked. | d. South ditto. |
| b, b″. Copper vein, called "John's Gossan." | e. Copper vein. |
| c. North "slide." | f. Vein of clay. (Elvan.) |
| x, y, z. Cross courses. | |
The ordinary explanation is that the tin vein, now
appearing in four parts, a, a″, a″, a‴, is the oldest
vein, and was formed in one straight line; after its
formation the copper vein b b″ was formed by filling a
straight continuous fissure, which was made by violent
fracture of the mass of the rocks across the tin vein.
This was accompanied by a dislocation of the rocks in closing
TRANSVERSE SECTION OF HUEL PEEVER.
the parts separated by the distance F E. At some later period the slide c was formed by a similar fracture and displacement, crossing both the copper vein and the tin vein, and shifting the parts of them both, so that the copper vein was divided into two parts, b and b′, separated by the interval A C; and the tin vein again divided and its parts a and a′ separated by the interval B D (which is equal to A C). At the same or some other time, the slide d produced a slighter effect on the tin vein a at G. What other effects may have accompanied the other intersections, which are indicated as possible, viz. c and d, b and d, the locality does not shew.
Finally, after all these fractures, three fissures in a north and south direction, x, y, z in the ground plan (not seen in the vertical section), have been formed across a, b, c, d, e, f, and have been accompanied by dislocation in a horizontal direction along nearly vertical planes. (These drawings are from Mr. William's paper on Huel Peever Mine, in the Geol. Transactions, vol. iv. plate 7.)
The mine in question was supposed to present an unusual complication of phenomena; and, in fact, the practical men were baffled by the "accidents" to which the veins were found subject in the course of the workings. It will be seen that the horizontal displacements indicated on the plan follow, in this plane, the general law given in Vol. I. p. 40. for a vertical plane, thus bringing the Cornish veins in this respect into analogy with those of other districts, as, for example, Aldstone Moor, in Cumberland. There is no difficulty in this respect.
On turning to the vertical section across the veins from north to south, we find three apparent displacements: one to a small extent, at the intersection of b and a, which is contrary to the common law above referred to; a second, of twice the extent, at the intersection of c and b, and c and a, which agrees with that law; and a third, of small extent, where d and a meet, which is again exceptional. Now, that the movements supposed are possible, without inconsistency, in this case, any one can satisfy himself by a model; and that the result, i.e. the new position of all the masses, is perfectly explained by such movements, is obvious from the following facts: first, the displacement of each of the veins b and a, on the line of fissure c e is equal; in the next place, the divided parts retain their parallelism; and, which is not of least importance, they agree in their characteristic contents.
Such cases do not oppose, but strongly confirm, the opinion that veins are posterior to the rocks which they traverse, and of unequal antiquity as compared with one another. But it must not be thought that the Cornish geologists, who have revived the opinion of Stahl, that the veins are contemporaneous with the rocks, have no stronger case than that of Huel Peever. Mr. Kenwood, in his communication to the Geological Society (Nov. 1832), mentions several instances of remarkable intersections, some of which are, and others are not, easily explicable by the supposition of real movements in right lines.
Thus, if, "in Weeth mine, two cross courses are traversed by the same east and west lode, and one is heaved to the left, and the other to the right" (in a horizontal plane,) this would necessarily happen if the cross courses dipped in contrary directions, and the movement on the plane of dislocation were vertical. In all such cases, precise and complete measures are necessary, to enable a candid inquirer to form a satisfactory opinion as to the mechanical solution of the problem of displacements involved in the data; and such a case Mr. Kenwood presented to the Section of the British Association at Liverpool. Most of the phenomena described in that communication were capable of explanation by simple movements in right lines, but some were not; particularly the case of two veins, dipping in opposite directions, and yet heaved the same way, contrary to the mechanical necessity of the case, had the movement been real. In such cases, angular movements of the masses, which are known, by examples of common faults, to be real causes, may be appealed to.
It is impossible now to enter into a minute examination of this and other such cases of embarrassment, which change their aspect when a whole district of related veins is submitted to consideration; but having examined many of the published examples of intersection of veins in Cornwall, it is our opinion at present that much of the difficulty has arisen from the incomplete description of the phenomena, and the division of the general problems belonging to a considerable extent of displaced ground, into a multitude of minor cases, the key to which is in their connection. There can be no doubt that the great mass of these phenomena are perfectly reconcileable with the hypothesis of real displacement of the masses of rock, and it appears to us that little is wanting to reduce the whole to understood laws, except a greater attention to the influence which the jointed structure of the rocks must be admitted to have exerted in modifying the result of mechanical movements.
In the plan and section of Tin Croft mine, given by William Phillips in his Outlines of Mineralogy and Geology, p. 165., one of two parallel copper veins is represented as sending off two branches on one side, probably into joints of the rocks. Had the veins there represented been traversed by a "slide" underlying to the south, the phenomena, now so clear, might have been rendered very difficult to comprehend. We have made models of some of the possible cases of real movement, the complexity of which in the case of the Cornish veins appears to us greater than any thing yet found in mining.
Turning from this district to others of less complexity, we shall see immediately the impossibility of a reasonable doubt as to the fact of veins now occupying what were fissures in the rock. In the mining districts of Wales, Derbyshire, Yorkshire, Cumberland, where sandstones, shales, coal, basalt, and limestones, alternate in one or more successions, and are all divided by the same vein, to which of these strata of unequal antiquity is the vein contemporaneous? When, on the opposite sides of such a vein, are seen the separated parts of large corals, and in innumerable cases of the small strings passing off from a vein, the division of shells like Producta, Euomphalus, &c.; all further discussion is useless, and the facts thus proved in cases free from complexity, are with justice employed to interpret, in other districts, results which are marked by additional influences.[8]
2. The affinity between certain rocks and the veins in them is real, and sometimes leads to an intimate union of their substance by mutual penetration. To this, considered as an objection to his theory of veins, Werner makes the following reply.
"The union between a vein and a rock, on some occasions, is so intimate as to give the appearance of their having been melted together, if I may so express myself." "In places where this peculiarity occurs, the rock has had a strong attraction for the substance of the vein introduced into the rent, and has become so intimately mixed with it, that they now appear to be one and the same substance; at least, it is not easy to mark a line of separation between the rock and the vein. This is particularly the case with veins of quartz and hornblende, when they occur in newer gneiss of a quartzy nature; but veins of pyrites in this rock do not present this appearance, which is, upon the whole, a rare occurrence. In general, the vein and rock are very distinctly separated from each other; and there are sometimes interspersed between them thin layers of an earthy matter called besteg. A vein is very seldom united to the rock so as to adhere intimately with it through its whole course; but this only takes place in certain parts." (Werner on Veins, p. 90.)
To this it seems only necessary to add, that in whatever manner the ingredients of mineral veins were placed in their present situations, it is not possible to doubt that the specific relations alluded to must have been manifested. Were all the mineral masses injected by fusion, as Hutton thought, there would be segregations, and peculiar arrangements, produced by the conditions of cooling, the conducting power of the rocks, and their inherent molecular forces. Were they introduced by solution, as Werner believed, what menstruum capable of dissolving such a heterogeneous mixture could be without power on the walls of the fissure, or some part of them? Were the elementary parts of the substance of veins raised by sublimation, molecular attractions would be exerted unequally by the different parts of the sides of the fissure; and if electrical currents were the agents of transferring the metallic substances to their peculiar repositories, the conditions of the rocks as to conduction of heat and electricity become of paramount influence. The specific affinities which the contents of one vein display to the different rocks which bound it (as in the lead mining districts of the north of England), when rightly viewed, offer a most convincing proof that the substance of veins was introduced among these rocks after they had acquired such conditions of hardness, position, &c. as to exert unequal powers in determining the arrangement of the substances presented to their influence.
3. Strings and branches of metallic and sparry substances, like those which occur in veins, but inclosed on all sides in rock, are of sufficiently frequent occurrence to demonstrate that not all mineral repositories have been open fissures, filled by depositions from above, as Werner taught, or by injection from below, as Hutton contended, or by mere sublimation, as other writers besides M. Necker have advanced on good though limited evidence. We have shewn, while treating of the "forms of igneous rocks," that such "contemporaneous veins," as Jameson properly calls them, have arisen from the same play of affinities as the spherical arrangements of the orbicular greenstone of Corsica; they are "segregations" of parts of a fluid compound, depending on circumstances which affect its transition to a solid state. Such results may be admitted to have happened with metallic veins, whenever the evidence is equally clear. They are admitted by some writers for some of the veins in Cornwall.
But yet, a general contemplation of insulated metallic and sparry masses, which fill cracks and other cavities in rock, will not allow us to adopt this as a general explanation. These cracks and cavities have existed as such in the limestones of the north of England, before the introduction of their crystallised contents. For some of these cavities are the inner hollows of bivalve shells, which shut close and have no opening (Producta); others are the closed chambers of cephalopodous shells (Orthoceras, &c.). Nor is it doubtful that many, if not all, the cracks and joints which, near a metallic vein, hold sulphuret of lead, or carbonate of copper, have been produced during the condensation of the stone, since we not uncommonly find them crossing and dividing the substance of shells and corals (Wensleydale) and fishes (Whitley quarry, near Cullercoats).
Upon the whole, therefore, whether the mineral substances occur in distinct regular fissures, occupy plane joints, lie in irregular cracks or holes of rock, or line secret hollows in shells—in all of these cases the existence of a cavity to receive the crystallised substances is demonstrated, as the most ordinary antecedent to the production of the mineral mass. It follows as a consequence, that ordinarily, when veins cross, and one passes through and divides the other, the "cross vein" is of later origin than that which is cut through. But as to the vein fissures having been originally open above or below, and as to the manner of their being filled, these points remain for further consideration.
Origin of Vein Fissures.
The theory of the origin of veins being thus to a certain degree insulated from that of the rocks in which they lie, the next thing to be determined is the origin of the fissures in which the metallic and other mineral combinations have been effected. The fundamental facts for this inference are the prevalent parallelism of directions of the several systems of veins which, in a given district, belong to successive periods of formation; the penetration of these fissures through a great variety of rocks; their length on the surface (some extending even several miles); their depth, which in large veins exceeds the range of mining enterprise; the displacements of the rocks which they divide; their various intersections and mutual relations. It is obvious that the inferences to be adopted from these data, will be trustworthy in proportion to the variety of sources from which they are gathered, and especially if the seemingly peculiar phenomena of vein fissures can be referred to general laws which extend beyond the mining districts.
Now that this reference to general laws can be effected, will appear evident from the consideration that similar parallelism of structural fissures, passing through various rocks for greater length than mineral veins, to unknown depths, with the same variety of mutual relations, have been found in other than mining countries, by the observation of rock dykes, and the symmetrical structures of rocks called joints, and cleavage. The most prevalent direction of the Cornish veins (east by north), is that of certain characteristic joints in a considerable portion of England, beyond the region whence the results contained in Vol. I. p. 65. were derived; and the lines of the great cross courses of the Penine chain, Flintshire, and Cornwall (north-north-west), are also coincident with a very general divisional structure of the rocks in most parts of Great Britain and several other parts of Europe. Mr. Kenwood and Dr. Boase expressly state, that the cross courses and principal veins more or less "coincide with the lines of symmetrical structures by which all the rocks of Cornwall are divided." (Kenwood, in Mining Review.)
The symmetrical structures of rocks, are, however, different from the fissures now filled by veins and rock dykes; for they are seldom so continuous, either in length or depth; they are almost universally unaccompanied by displacement of the side; and they often change their width, frequency, and other characters, according to the nature of the rocks. It is obvious, therefore, that it is not merely by the filling of joints of the rock that veins and dykes were produced; the rocks have been disturbed in position, opened to a greater extent than the original divisional structures, or else these last are only to be regarded as minor effects of great disturbing forces which broke the strata along the lines of vein fissures and rock dykes. The following remarks are intended to show that symmetrical divisional planes, such as joints and cleavage, are due to other causes than disruption of the strata.
1. It is a fact, that from divisional planes ranging for many yards or even hundreds of yards, and separated by wide intervals, to the fine parallel, almost invisible, cleavage of coal (called "cleat."), and of clay slate, there is an almost perfect gradation of structures, which have a definite relation to the different nature of rocks, while subject to the same mechanical pressures and movements. In coal, shale, clay slate, and laminated limestone, it is in vain to attribute these regular divisions to any thing but the molecular arrangement which explains the structure of basalt.
2. In different beds of rock, as shale, limestone, and gritstone, which alternate, it is not uncommon to find the slopes or inclinations of the joint planes to vary, nearly as in different beds of slate the planes of cleavage will deviate from parallelism.
3. The joints are, for the most part, not continuous through all these alternating strata, but in each rock are characteristic divisions which enter no other.
4. In symmetry, extent, and frequency, joints are not at all less, but rather more, remarkable at points far removed from axes and centres of disturbance of the rocks.
5. Near such axes of movement, many irregular fractures of the rocks occur, and predominate over the natural joints, which appear not uncommonly to have been obscured, closed up, or complicated by irregular pressures and cracks in such situations.
It follows from these considerations, that whatever analogy of direction may appear between the lines of mineral veins and those of the natural structures of rocks, this only indicates the influence which such lines of weakness would necessarily exert on the direction of fractures produced by mechanical pressure. Now, as, in addition to joints, many other circumstances, as the unequal loading of the parts broken, and the varying thickness of unequally resisting masses, &c., must have contributed to the weakening of parts of the crust of the earth, the want of perfect accordance between the joints and all the lines of vein fissures, is no sufficient argument against the anteriority and real influence of the former over the latter.
The curious circumstance, not uncommonly seen in the mining district of Aldstone Moor, of the change of the "hade," or inclination of the vein, in its passage through different rocks, is perhaps explained by this admission of the relation of vein fissures and joints. The veins which pass perpendicularly through limestone beds, acquire an inclination in the alternating shales, and they are usually wider in the limestone than in the shale. Now, in both of these circumstances, the vein fissures resemble common joints, which not uncommonly are more inclined and much narrower in shales, than in the limestone strata of the same district.
Another curious fact, noticed in Cornwall, appears intelligible by considering the disturbing force as having opened at once two parallel discontinuous natural joints; so that opposite the point where one fissure ended, the other became open enough to receive substances of the same kind, and thus, as the miners say, to "splice " the vein.
All the principal circumstances which attend the dislocations of the strata along the planes of mineral veins, are equally witnessed in the cases of common rock dykes, and faults; the same general laws as to the relation of planes of strata and planes of dislocation apply, with similar exceptions; nor are there wanting in all these cases, proofs of the fact that some of the fissures have been subject to more than one movement. In mineral veins this is manifested by the striated surfaces of rock and veinstones ("slickenside"); it equally appears on the lines of disturbed strata (coal shales, carboniferous limestones), and with equal variation and confusion of direction, so as in many cases to suggest the probability that the great movements were, as indeed could hardly be otherwise, complicated with many displacements of small masses in different directions. In some instances, as already explained (Vol. I. p. 42.), the striation is in one only direction, marking a great simplicity of movement: this is also the most common case of mineral veins.
Whatever difficulties these phenomena may be thought to present, they are common to all cases of displaced strata, and must be parts of one general investigation. In this sufficient progress has already been made, to assure us that, when the data and measures necessary to form a right conception of the conditions are furnished, the mechanical problems of displacement are not beyond solution.
Filling of the Fissures.
We are thus conducted to that point in the history of veins, which was reached by Von Oppel (in 1769), and are stopped by the same impediment. In his Essay on the Working of Veins (quoted by Werner), he says: —"The natural structure of the globe seems to show us, that after the formation of the primitive and principal secondary mountains, they had suffered great desiccation, and been exposed to violent shocks. In consequence of these changes, the rocks and mountains, which formerly composed one continuous mass, were split asunder; whilst this took place, it might easily happen that one of the rocks split from the other without ceasing to touch it; or these parts might be separated from each other, leaving between them open spaces, which were afterwards filled up, in part at least, with different mineral substances. The greater part of these grand events belong to that part of subterranean natural history, which can only be elucidated by a consideration of the facts which the earth presents to our view; for all these great revolutions took place at a period long before the globe became habitable to the human species. But whether fissures and veins were actually formed in the manner we have described, or not, it is no less true that this manner of representing their mode of formation, and the relative situation which they bear to one another in the mountain, is the most simple way of accounting for them. It explains the uniform law of their formation both in a general and more particular manner, and, consequently, we shall admit it as the real one. This hypothesis would be still more satisfactory to the naturalist, if it were equally easy for him to conceive how a new mineral substance could be formed in these fissures, of a nature different from the rocks in which the veins occur."
One of the early attempts to conquer this difficulty is that of Lehman, who deserved more attention than Werner's somewhat contemptuous notice.
"What is called a rent, is an open fissure in a mountain, which has been produced by a division of the rocks; and veins are, in my opinion, nothing but fissures which have been filled by nature with stones, minerals, metals, and clay—in short, which are of a very different nature from the rock itself." Farther on he says,—"The veins which we find in mines, appear to be only the branches and shoots of an immense trunk, which is placed at a prodigious depth in the bowels of the earth; but, in consequence of its great depth, we have not yet been able to reach the trunk. The large veins are its principal branches, and the slender ones its inferior twigs. What I have said, will not appear incredible, when we consider, that in the bowels of the earth, according to every observation, is the workhouse where nature carries on the manufacture of the metals; that from time immemorial she has been working at, and elaborating their primitive particles; that these particles issue forth, in the form of vapours and exhalations, to the very surface of the globe, through the rents, in the same manner as the sap rises and circulates through vegetables, by means of the vessels and fibres of which they are composed."[9]
Another effort to penetrate the mystery of metallic depositions, was that of Werner, who, in 1791, gave what he considered a "New Theory of Mineral Veins," of which the principal points of novelty are, the application of the phenomena of intersections to determine the ages of veins, and the hypothesis of aqueous solution for the filling of the fissures. In proof that the fissures of veins were filled from above, Werner mentions the occurrence of rounded pebbles at the depth of 180 fathoms in the vein Elias in Danielstollen at Joachimsthal, and similar instances in the Stoll refier near Riegelsdorf in Hessia, and in Dauphiné.
His notion of the manner in which veins were filled, partakes of the errors which belong to all the Wernerian hypotheses of the origin of mineral masses. He says,—"The same precipitation which in the humid way formed the strata and beds of rocks (also the minerals contained in these rocks), furnished and produced the substance of veins; this took place during the time when the solution from which the precipitate was formed, covered the already existing rents, and which were as yet wholly or in part empty, and open in their upper part."[10]
The Huttonian hypothesis of the earth's construction, opposed in almost every point to that of Werner, conducted naturally to a different interpretation of the same facts. The fissures were produced by forces depending on subterranean heat, and were filled by injection like rock dykes; and the parallel bands in the vein, which Werner ascribed to successive aqueous deposition, were referred by Hutton and Playfair to successive igneous injection. In support of this explanation, the acknowledged impossibility of solution in water of native, sulphuretted, and oxidised, metals, and many of the veinstones, was alleged, as fatal to the Wernerian but favourable to the Huttonian view.
The complicated phenomena of veins led some English writers, who admitted the posteriority of veins to the rocks which inclose them, to suppose their contents to have been collected from the neighbouring strata, by some peculiar process of segregation, depending on electrical currents. Thus it was supposed the successive depositions, and peculiar positions of the various substances which occur in veins, might be accounted for.
Lastly, the vague suggestion of electrical agency, in depositing the materials of mineral veins, has been reduced to a regular system by Mr. Fox, who, uniting the knowledge of veins to a zeal in conducting ingenious experiments which has led to most valuable results, has successively matured his views and advanced his experiments, till they have attracted very general attention. Perhaps the most complete account of his hypothesis is that which appeared in connection with a valuable collection of facts regarding mines, in the Report of the Polytechnic Society for 1837.
In this paper, Mr. Fox assumes as sufficiently proved, the origin of fissures, from various causes, and at various intervals, and the enlargement of them from time to time; the progressive filling up of these fissures; and their penetration to great depths and regions of high temperature. In such fissures, he shows the probability of the circulation of heated water by ascent and descent; and the deposition of quartz and other earthy substances in cool parts, which had been dissolved by the water in hotter parts. In such fissures, filled with metallic and earthy solutions, the different sorts of matter on the sides must necessarily produce electrical action, which might be exalted by the peculiar distribution of temperature. The currents of electricity generated would pass more easily in the fissures than through the rocks, and in directions conformable to the general magnetical currents of the district. These would be east and west, or somewhat north or south of these points, according to the position of the magnetical poles at the period when the process was going on. Electrical currents thus circumstanced, would deposit the bases of the decomposed earthy and metallic salts on different parts of the rocky boundaries of the veins, according to the momentary electrical state and intensity of the points; in which conditions the nature and position of the rocks would be influential. When by such processes particular arrangements had happened, new actions might arise, and secondary phenomena, such as the transformation of ores, without change of form, which are otherwise very difficult to understand; lateral rents might also be filled by virtue of these new actions, even though they were not in the most favourable lines of electrical circulation.
The general hypothesis being admitted, it appears to follow, that the observed influence of cross courses on the quality and abundance of particular accumulations of ore in the veins which they divide, affords strong ground to believe that, in such cases, the depositions of these ores was subsequent to the displacement of the vein fissure by the cross course. It appears to be Mr. Fox's opinion, that the clays in the flukans and cross courses were introduced mechanically, and that they affected, in a particular manner, the metallic distributions.
Not the least striking among the arguments in favour of Mr. Fox's electrical theory of mineral veins, is the fact, that he has formed experimentally many well defined metalliferous veins by voltaic currents, operating under circumstances expressly arranged in imitation of those which really occur in Cornwall. (See Reports of the Newcastle Meeting of the British Association, 1838.)
Recapitulation.
In considering these various views of the repletion of mineral veins, it must appear evident that some things at least are very probably established; the successive enlargement of some veins, the progressive repletion of most of them, and the influence of general polarities in the distribution of, at least, the crystallised materials. The more closely the investigation is made, the less certain appears the connection in time between the production of the fissure and its repletion. If the relative ages of vein fissures may be known by their intersections, this does not so clearly apply to their contents; and thus we find it quite possible that no long geological period, such as Werner contemplated, may have intervened between the older and the younger vein-fissures of a given district.
It certainly appears at present unsafe to adopt any one of the views here noticed exclusively. Sublimation and re-crystallisation of metallic matters (whether pure metals, sulphuret, or oxides) are common phenomena; and the passage of veins downwards to heated regions is too probable to render it doubtful that such operations have sometimes contributed to fill the fissures of rocks. Mr. Patterson's experiment of the influence of steam in causing the sublimation of galena in an earthen tube heated in the middle (Phil. Journal, 1829), is an important illustration.
The deposition of blende, sulphuret of iron, carbonate of lime, sulphate of barytes, quartz, &c. in cavities of organic bodies, and in other situations, by the agency of water, must exempt Werner from the charge of absurdity in attributing to aqueous solution some of the phenomena of the repletion of mineral veins; but, as a general explanation, his system is of no value.
Nor does it appear, at present, just to attribute a much larger measure of success to Playfair's application of the Huttonian hypothesis. It is, indeed, certain, in many instances, that metallic impregnations are mixed with rock dykes, or lie in veins by the side of them. Some veins may have been filled by injection, especially such as appear very simple in their structure, uniform in their composition, and wholly independent of the neighbouring rocks in the distribution of their contents. Such veins there are; but this speculation does no well meet the cases of many parallel bands in a vein, segregations in lines of particular rocks, and in closed cavities of rocks, the mixture of fusible and in fusible substances, and the variation of the contents of veins according to their directions, and other characteristic facts.
All of these excepted facts, indeed, appear indicative of other agencies and polarities accompanying and governing the deposition of metallic ores. It is difficult to doubt the truth of the views which ascribe these peculiar and characteristic arrangements to electrical action, and perhaps the principal problem now remaining, is to determine whether, as Mr. Fox believes, the electrical currents were voltaic, generated by the chemical action of particular solutions on particular substances, or thermo-electric, depending on the application and conduction of heat. As far as experimental research goes, the labours of Becquerel, Crosse, Fox, and Bird appear at present to give the advantage to voltaic electricity as the agent of arrangement in metallic deposits. The other source of electrical power has been less inquired into in this respect; and yet, when we consider the facts of the communication established by metallic veins of different conducting power, from the cold surface to the hot interior of the globe, and recollect that permanent differences of subterranean temperature are commonly observed among contiguous rocks (as the killas and granite of Cornwall, which differ 3°), it is difficult to check the belief that thermo-electric currents, however weak in intensity, are now important in their agency, and may formerly have been much more so.
In these remarks we have chiefly in view the arrangement of the substances in a vein; the accumulation of these may be due to quite different causes. In some cases it really appears that a complete account of the accumulation of the substances is very difficult to collect, except we call in successively the solvent powers of water and heat. The formation of sulphuret is obviously one of the most important of all the facts requiring explanation in mineral veins, because a very large proportion of metallic ores (tin is the principal exception) appears in this state. Heat, by sublimation, sulphuretted hydrogen, by decomposition of metallic salts, may give us the sulphuret; but in the latter case, from what prior condition is the sulphuretted hydrogen derived? Mr. Fox proposes the decomposition of other sulphuret, by electrical action. Thus we make no advance, and again turn to the simple action of heat, which, in like manner, stops at the origin of these sulphuret, and only accounts for their transfer from the deeper parts of the earth. This, perhaps, measures our possible knowledge as to the origin of the metallic ores. They have been transferred from the interior of the earth toward its surface, principally along the fissures opened by violent movements.
But this conclusion does not necessarily apply to the sparry contents of the veins. Aqueous solution of most of these is possible, but of some it gives no sufficient account. Some, as salts of lime, abounding in a limestone country, may reasonably be attributed to the action of water passing through the rocks; others, as quartz, may be thought to require much heat for their solution; the clays and rolled fragments mark mechanical action of water; and thus, finally, it appears that the present aspect of mineral veins is the result of many secondary chemical, electrical, and mechanical actions, the general antecedent to which is the influence of a high temperature below the surface of the earth.
- ↑ Fox, in the Report of the Polytechnic Society.
- ↑ Mineralogia Cornubiensis, p. 79.
- ↑ In the Trans, of the Geol. Soc. of Cornwall, vol. ii.
- ↑ In Cornwall, metalliferous veins are called "lodes."
- ↑ The directions are by compass, whose westerly variation is in Cornwall
- ↑ Veins which range from 30 to 60 degrees north or south of the east and west points are called contras.
- ↑ This subject has attracted the attention of the British Association for the Advancement of Science, who directed a representation to be submitted to the government. The result is a Mining Record Office.
- ↑ But it is not now necessary to appeal to such evidence in districts distant from Cornwall, since Mr. de la Beche has discovered encrinites and other organic remains imbedded in killas (grauwacke), close to the walls of Great Crinnis copper and tin lode, says,—Mr. Fox, in his Summary of phenomena in the Veins of Cornwall, p. 25.—Report of Polytechnic Society, 1836.
- ↑ Lehman, Abhandlung von den Metahnüttern und der Erzeugung der Metalle. 1753, quoted by Werner.
- ↑ On Veins, p. 50.—See also p. 110. for a further development of this very crude notion, mixed with some very ingenious suggestions, and important views of the relations of geology and mining.