Quarterly Journal of the Geological Society of London/Volume 33/On certain Ancient Devitrified Pitchstones and Perlites from the Lower Silurian District of Shropshire
[Plate XX.]
I.
The principal object of the present communication is twofold:—in the first place, to bring under the notice of the Society the occurrence in Shropshire of an extremely interesting series of ancient vitreous and semivitreous lavas, with their associated agglomerates and ashes; and in the second, to show, from an examination of their structure and composition, that originally they were absolutely identical with some of the glassy volcanic rocks ejected during the most recent geological periods.
In a previous paper (published in vol. xxx. pp. 529–567, of the Quarterly Journal) I arrived at the same result as to the identity of ancient and recent volcanic products, from an investigation restricted to the basic group of rocks; and I am now enabled to show that those of the acid type afford equally strong evidence in the same direction.
The discovery of several highly characteristic varieties of glassy rocks and volcanic ashes of Palæozoic age is, I think, a matter of considerable interest from a petrological point of view, more especially as their mode of occurrence and their relations to the surrounding strata afford the clearest evidence that the geological structure of a part of the district in which they occur has hitherto been misunderstood.
On Sheet 61 of the Geological-Survey map a band of "greenstone" is represented as constituting the axis of the somewhat irregular narrow ridge of which Ercal Hill and the Wrekin form the greater portion. It commences half a mile south of Wellington, and extends in a south-westerly direction to a distance of two miles and three quarters. At two points not far from each other the ridge is cut through by deep and narrow gorges, which traverse it from south-east to north-west, or at right angles to its general direction. The isolated hill thus formed by the two ravines is locally known as Lawrence Hill, and is indicated though not named on the map.
At a short distance to the west there is laid down a still larger mass of "greenstone," which extends from the vicinity of Wrockwardine as far as Uppington, and forms a low ridge parallel to that of the Wrekin.
With the exception of some thick beds of indurated ashes which occur in the Wrekin and Lawrence Hill, the principal rock-masses are of similar character in both ridges; and a slight examination with a lens would suffice to show any petrologist that the term greenstone is the most inappropriate that could possibly be applied to them. They all belong to a highly acid type, and have not the slightest resemblance whatever to greenstone or any other rock of the basic series. In a large quarry at the south end of Lawrence Hill thick beds of volcanic ash are seen dipping in a northerly direction, at an angle of 55°. Like all the rocks of the range, they have been greatly disturbed and fractured; but the strike of the beds is clearly across the ridge. They here consist of several alternations of coarse and fine material, the whole of which have been highly indurated and otherwise altered. These beds are covered by masses of altered pitchstones and felsites, which are well exposed on the south slope of the hill, on its steep northern face, and also on the opposite crags of Ercal Hill. Returning southwards to the narrow ravine which separates Lawrence Hill from the Wrekin proper, the ash-beds are again exposed in the precipitous face of the latter; the bedding is here massive or obliterated, although the fragmental character of the rock is still perfectly distinct. Further southwards, along the summit of the ridge, beds of the coarser ash are again seen, and may be traced at intervals for a distance of more than six hundred yards. From this point to the south-western termination of the ridge there are comparatively few exposures of rock; and these consist of compact reddish-brown felsite or altered pitchstone—the compact felspar of Murchison and others.
On the summit of the ridge, at a short distance to the north of the wood, there is a slight rounded elevation formed by a mass of altered dolerite, which appears to be intrusive; and in the quarry at the south end of Lawrence Hill there are two dykes: one, on the east side, is 12 ft. wide; the other is 14 ft. at bottom, cuts through the lower ash-beds, then bifurcates, and the two diverging branches rise through the upper beds to the surface. The rock forming both dykes is a highly altered basalt.
It is evident, therefore, from an examination of this part of the ridge, that the axis is not formed by a continuous band of greenstone, as hitherto represented, but that in reality it here consists of an extensive series of regularly stratified agglomerates and ashes alternating with amorphous masses of altered pitchstones or felsites.
The general strike of the surrounding strata is north-east; and the central ridge is flanked by masses of quartzite, which are laid down on the map as Caradoc Sandstone, altered by the supposed intrusive greenstone. Whether these rocks be altered Caradoc strata or not, they are clearly unconformable to the stratified ash-beds of the ridge; and I think there is some reason to believe that the latter belong to the older contemporaneous volcanic series so extensively developed in the Lower-Silurian district of Salop and Radnor.
The mass of trap lying to the west of Wellington and the Wrekin is a hard rock which has suffered less from denudation than the soft Triassic sandstones by which it is surrounded; it forms a low hilly tract, usually presenting rounded or flat surfaces, on which several large boulders of granite and felstone have been stranded. These erratics appear to be quite similar in character to those forming the well-known and far more numerous group just north of Wolverhampton. With the exception of some portions at the southern end of the mass, the rock appears to present the same general characters throughout, specimens from the northern end being quite similar to some of those collected at the opposite extremity. It is for the most part a hard compact rock of dark red or brown colour, and is the compact felspar and hornstone of Murchison, who also refers to one variety as a porphyritic clinkstone.
In order to show clearly the true character and structure of these rocks, it will be necessary, in the first place, to give a short account of their recent analogues; as rocks of this peculiar type are by no means of wide distribution, and some of them have not been previously observed in these islands.
II. Structure of Modern Perlites.
Perlite, spherulitic perlite, perlitic pitchstone, and perlitic obsidian belong, as is well known, to the glassy group of acid rocks, their average amount of silica being at least 70 per cent. In typical unaltered specimens the mass consists of a true glass which has no action on polarized light.
The spheroidal and ellipsoidal balls by which perlite is chiefly characterized have been described by Zirkel[1], Rosenbusch[2], and Lassaulx[3] as consisting of concentric laminæ arranged like the coats of an onion—a comparison which may, I think, possibly lead to erroneous ideas as to their real character; for however close may be the resemblance, as seen in thin sections, there is no real analogy between the structure of these perlitic spheroids and that of a tunicated bulb built up of broad scales which surround each other in a concentric manner.
An examination of typical specimens from the old volcanic districts of Schemnitz in Hungary, Meissen in Saxony, and Cabo de Gata on the south coast of Spain shows that the spheroids have not been formed by the superposition of successive laminæ: they are not concretions in any sense of the term; nor is there any thing about them suggestive of any process of progressive construction. When a rather thick section is examined under a low power of the microscope, it is at once seen that the little spheroidal balls are merely portions of the homogeneous glass which have been partially separated from the general mass by the formation of a number of small curved planes of fracture; these are more or less concentric with each other, but vary greatly in size, and are irregularly disposed in various directions round the centre. As these curved planes lie at various depths in the section, some of them appear with a convex or concave surface, according as the slice happens to cut through the upper or lower half of a spheroid. Such being the general arrangement, the lines seldom form closed curves when seen in thin sections. That these lines and curved planes are really fine cracks is clearly shown by the way in which they are frequently filled more or less completely by the infiltration of foreign matter in solution; and that they were caused by the strain produced during contraction of the brittle glass is rendered evident by such examples as the following, in which may be traced nearly every gradation between long rectilinear fissures or joints and those forming the typical perlitic structure. Fig. 1 (Pl. XX.) represents the arrangement observed in a section of perlitic pitchstone from near Meissen. Several roughly parallel joints divide the glass into small columns; and these are again traversed by cross joints, which thus form minute rectangular blocks. In the central one (a) a spheroid is formed by four curved lines, which clearly branch off from the lateral joints, and round off the corners of the rectangle. In the upper compartment there are three distinct spheroids; and in the lower one there are two of irregular shape. Fig. 2 is from another part of the same slice. In fig. 3 the formation differs in this respect, that in both columns several spheroids are piled one on the other without any intervening cross joints. In a typical perlite from Cabo de Gata in Granada, there are numerous large spheroids, in which many of smaller dimensions are enclosed (see fig. 4). In nearly all the specimens examined the glass is divided into areas of various sizes and forms by a number of parallel or diverging straight lines (joints), the intervening spaces being frequently crowded with spheroids, many of which are flattened against each other (see fig. 5, Pl. XX.).
An examination of all the facts leads to the conclusion that the perlitic texture is purely a phenomenon of contraction; and I quite agree with Mr. Rutley[4] that the explanation of the spheroidal structure in basalt recently laid before the Society by the Rev. T. G. Bonney[5] is a closely parallel case. There is, however, this difference—that, in the case of basalt, the comparatively tough texture produced by the interlacing of its crystalline constituents would enable it to resist the actual fracture so frequently exhibited by the more brittle perlites.
The perlites and other vitreous rocks usually contain numerous minute microliths, which have been described by Zirkel under the names of belonites and trichites; their mode of occurrence and relation to the superinduced spheroidal (perlitic) structure deserve special attention.
The belonites are minute translucent prisms, either colourless or of pale yellow or greenish shades; they occur in immense numbers, and are frequently crowded together in stream-like bands, with their long axes lying in one general direction. Whenever a stream encounters any small crystal of felspar, quartz, or mica imbedded in the mass, the belonites are invariably diverted from their course and bend round it, their axes lying parallel with, its sides. Their relation to the perlitic spheroids, however, is totally different; for instead of winding round them, they continue their course uninterruptedly through and across them (see fig. 8). It becomes evident, therefore, that this so-called fluidal structure may be explained as the result of movements imparted to a mass composed of solid crystals and a viscid base—and, on the other hand, that the perlitic spheroids must have been formed subsequently to the solidification of the entire mass.
The trichites are extremely minute and slender hair-like crystals, either straight or bent and twisted into most irregular curved and even zigzag forms. They are usually black and opaque, but when partially decomposed appear of a reddish-brown colour.
Spherulites also frequently occur in glassy rocks. They are globular in form, though quite distinct in character from the perlitic balls or spheroids, with which, however, they are not unfrequently associated. They are seldom very translucent, even in quite thin sections, but invariably polarize light, and exhibit a fine fibrous radial structure. They will be more fully described in the sequel.
These semicrystalline bodies were evidently the last substances formed prior to the solidification of the mass; for not only are the more perfectly crystallized constituents (felspar, mica, &c.) enclosed in them, but even the streams of microliths also occasionally pass straight through them—a fact which appears to have escaped the notice of previous observers. As regards the general order of formation, the evidence seems therefore to indicate that crystals of felspar, mica, quartz, &c. were enclosed in a viscous glassy magma, which was also crowded with innumerable microliths; before the mass solidified, the fluidal structure was imparted to it; and subsequently, during solidification, the radiating spherulites were formed, without disturbing the previous arrangement of the microliths[6].
The characters just described are extremely well shown by a single group of minerals in a section of one of the Kremnitz perlites (see fig. 7). Fibrous spherulites are here seen to be traversed by streams of microliths, and also to be penetrated by plates of mica and crystals of felspar. In the figure, one end of a plate of brown mica is enclosed in a spherulite, while the opposite end penetrates a crystal of orthoclase; the latter interferes with an adjacent spherulite, which is partly formed round it on the left side, and a partially included plate of mica projects from it on the right. It will also be seen that the streams of microliths invariably flow round all the crystals, whether large or small.
Having sketched the most prominent features of comparatively recent perlitic and spherulitic rocks, I will now describe a few of their ancient prototypes.
III. Structure of ancient Spherulitic Pitchstones and Perlites.
As previously observed, the principal rock-masses in the two parallel ridges near Wellington present the same general characters: the one to the west of the Wrekin, however, is the more important for my purpose, as it affords the most interesting and typical varieties, and also supplies the best specimens for examination.
At "Lea Hock," near the south-western termination of the ridge, there is a large quarry near the Shrewsbury road, in which the rock is very well exposed. In one part it is intersected in all directions by numerous joints and cross joints, which cause it to break into small fragments; so that fresh surfaces are difficult to procure. The jointage-planes are generally smooth, and coated with peroxide of iron, and frequently exhibit on their slightly weathered faces numerous fine parallel lines, which are either straight or tortuous, and even exhibit a complicated folding and crumpling, like that seen in crystalline schists. Sometimes, however, they widen out into distinct bands, and then produce a striped or laminated appearance. In some cases the parallel stripes are so distinctly marked that they closely resemble laminæ of deposition, or lines of foliation, and have in fact been regarded as evidence of the original stratification of the rocks here described. An examination of their internal structure shows, however, that they invariably indicate the presence of those remarkable streams of microliths previously described in the Hungarian perlites (p. 453, Pl. XX. fig. 7)[7]. These finely banded rocks also occur in the Wrekin.
Among the most interesting examples collected in the quarry just mentioned are several varieties of a remarkable spherulitic rock. These exhibit the closest analogy with the comparatively rare though well-known group of volcanic vitreous rocks already referred to, and may. I think, be appropriately described as
Ancient Spherulitic Pitchstones and Perlites.
One beautiful variety of this rock consists of numerous bright-red spherulites set in a grey or yellowish-green matrix. Sometimes they occur singly, and are irregularly scattered throughout the mass; or, as frequently happens, several are crowded together so as to interfere with the development of their regular spherical form; while in other specimens they are arranged in rows, like strings of coral beads, and thus form parallel layers. This is a well-marked feature even in hand specimens; and when the spherulites are closely pressed together, a thin slice exhibits a series of continuous red bands, with undulating outlines formed by the mutual interference of successive contiguous spheres (see fig. 8). This is an extremely hard rock of a bright red colour, and closely resembles some varieties of jasper.
As seen in thin slices, the spherulites (fig. 9, Pl. XX.) usually consist of a circular central disk of bright red surrounded by a colourless ring (distinguished by two shades in the figure); the latter varies greatly in width, and is perfectly continuous with the red portion, of which it is merely the unstained border; and then there is an outer zone of transparent glass (unshaded in the figure).
When examined under the microscope in polarized light, with crossed prisms, the central red spot and its colourless border exhibit a perfectly distinct fibrous radial structure, the central disk still retains its bright red tint, and the colourless border appears of a pale grey, except where obscured by the arms of the black cross. Frequently, however, the red stain extends quite to the edge; and a fibrous red spherulite is then surrounded by a zone of homogeneous glass. Although the fibrous crystals usually radiate from a central point, there are not a few spherulites which exhibit two distinct modifications of this arrangement. In one the fibres are seen to radiate from several points surrounding a felsitic mass of irregular shape; the rays forming the different groups meet each other along diverging straight lines; and the whole is surrounded by a glassy ring. In other cases the spherulites are ellipsoidal, and the fibres usually radiate from a point near one extremity of the axis. Small crystals of felspar are frequently enclosed in the spherulites; but, precisely as in the Hungarian perlite previously mentioned (p. 453 and fig. 7), their position has no relation whatever to the radial crystallization of the substance by which they are surrounded; this is clearly seen in fig. 10, which shows two felspar crystals enclosed in a spherulite.
Another striking resemblance between the ancient and more recent examples is found in the fact that the transparent matrix in which the spherulites are enclosed frequently exhibits a perfectly distinct perlitic texture, as shown here and there in fig. 8, and is also crowded with streams of microliths, which pass straight through the spherulites, precisely as in the Kremnitz rock represented in fig. 7.
The microliths closely resemble the more recent belonites in size and shape; and even the singular and unmistakable trichites, with the same twisted and knotted forms, are abundant in some of my sections. One kind, consisting of strings of minute dots, are the most prevalent, and are precisely similar to those observed in some specimens of spherulitic pitchstone from Schemnitz.
Besides the felspar crystals just mentioned, others are scattered here and there through the matrix, and cause streams of belonites to bend round them. Orthoclase and plagioclase are both present, and are remarkably well preserved; the latter appears to predominate; and the crystals are often beautifully striated.
The rock just described appears to pass gradually into a variety from which the spherulites are absent, but which presents most excellent examples of perlitic structure.
Devitrified Perlite.
The examples in my possession are of a rather dull yellowish-brown colour, and are slightly fissile in one direction. When examined with a lens, a freshly broken surface exhibits numerous small convex and concave faces; and when a thin slice is placed under the microscope a true perlitic structure is at once seen to be as distinct and unquestionable as in any of the more recent glassy- rocks. Fig. 6 represents a portion of a thin slice of one of these ancient perlites; and beside it I have placed an equally careful drawing of a typical perlite from Schemnitz (sec fig. 5). The older rock has undergone a considerable amount of alteration ; and that constitutes the only perceptible difference between them. In some of the Meissen pitchstones the perlitic structure is nearly or quite absent, while in others it is well developed; and it fortunately happens that specimens occur in various stages of alteration.
In one of the red varieties the colour of the mass is due to the in- filtration of bright red ferric oxide, which has followed the lines of fissure, and has also stained the glass for a short distance on each side, as represented by the light shade in fig. 1. In the same specimen, however, a yellowish-brown substance here and there takes the place of the red oxide, and shows a marked tendency to aggregation in little spherical nodules. In a brown variety from the same locality, a pale brown flocculent substance is alone present, and has invaded the glass in the same manner but to a far greater extent than in the preceding examples; the parts permeated by it have a distinct action on polarized light; and it is quite evident that a further extension of the process of alteration would impart to the mass a pseudo-felsitic aspect.
The kind of alteration here described has clearly been produced by chemical action; and it has followed precisely the same course as that which has so frequently converted fractured crystals of olivine into serpentinous pseudomorphs.
In the ancient perlite a similar process has been in operation, and has produced the appearance represented in fig. 6. The shaded parts indicate the presence of a yellowish-green substance, which accompanies the lines of fissure and has invaded the glass on each side.
Devitrification of the glassy magma.—In addition to the chemical alteration just described, the original glassy base of these old rocks has also undergone certain molecular changes which it is highly important to notice. A slight examination of the two thin slices represented in figs. 5 & 6 suffices to show the identity of their general character as seen in ordinary light; but when placed under the polarizing microscope between crossed prisms, it is at once seen that the matrix of the Schemnitz rock remains dark, while that of the older one transmits light in many places, and the field of view exhibits an irregular mosaic of light and dark grains.
On rotating the object, some of the dark grains may then be seen to transmit light in certain positions; but the greater number always remain dark; and it becomes evident that the mass consists of a homogeneous glass with numerous doubly refracting patches disseminated through it. The extent to which the two substances prevail varies considerably in different parts of the rock: in some of the highly altered spherulitic varieties there may be seen a perfect mosaic of varying pale shades of colour, while in others the glassy substance predominates. The peculiar character of the doubly refracting portion of the base is extremely well shown when the axes of the Nicols are inclined to each other; by a slight rotation of the object alternately to right and left, the previously well-defined granules gradually assume shadowy indefinite forms of variable dimensions, alternately appearing and disappearing; while the perlitic curved lines are seen to pass continuously through them, and even the minute belonites and trichites appear with opposite ends enclosed in two of the adjacent pseudo-granules.
When the devitrification appears to be complete, and a granular mosaic structure the most distinct, there is not the slightest appearance of any such fragmental character when examined in ordinary light. Under all powers of the microscope the matrix then seems to be a perfect glass traversed by straight or tortuous streams of microliths and intersected by the perlitic fissures already described.
Although these investigations show conclusively that certain molecular changes have taken place, it should not be overlooked that the structure resulting from devitrification differs in character from that of a true felsite; and I think the rocks here described afford no evidence in favour of the view held by Vogelsang and others, that the base of the so-called quartz-porphyries may have been originally of a glassy nature.
The thin sections are frequently traversed by narrow veins, which pass through all the constituents, as shown in fig. 10. A fine fissure has here cut through the matrix, a spherulite, and an enclosed crystal of felspar. In all cases these veins appear to consist of the same substance as the devitrified matrix; and when the latter is penetrated by one of them, it is impossible to distinguish the one from the other except in certain positions of the prisms, when the continuity of the vein may still be detected.
Quartz-veins are not uncommon; and in some parts of the spherulitic rock chalcedonic nodules are rather abundant.
Although the microscopical structure of these rocks is in itself decisive as to their origin, I may add that their chemical composition is also in perfect accordance with their other characters. Mr. J. A. Phillips kindly made for me an analysis in duplicate of a specimen of the devitrified perlitic pitchstone, with the results shown in the following Table, in which I have also included analyses of two Miocene perlites:—
| Spec. grav. 2⋅62. | I. | II. | III. | IV. |
Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1⋅47 | 1⋅46 | 3⋅50 | 2⋅90 |
Silica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
72⋅18 | 72⋅19 | 72⋅52 | 73⋅00 |
Alumina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
14⋅46 | 14⋅44 | 13⋅72 | 12⋅31 |
Ferric oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1⋅78 | 1⋅59 | 2⋅08 | 2⋅05 |
Ferrous oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
0⋅91 | 0⋅91 | ||
Oxide of manganese . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
trace. | trace. | ||
Lime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
0⋅92 | 0⋅93 | 1⋅15 | 1⋅20 |
Magnesia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
trace. | trace. | 0⋅45 | 1⋅47 |
Potash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
6⋅10 | 6⋅14 | 5⋅68 | 5⋅96 |
Soda . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1⋅92 | 1⋅96 | 1⋅15 | 1⋅36 |
| 99⋅74 | 99⋅62 | 100⋅25 | 100⋅25 |
III. Perlite, from Hlinyik, near Schemnitz; analyzed by Von Sommaruga, quoted by J. Roth[8].
IV. Pearlstone (Hungary); analyzed by Rammelsberg[9].
An inspection of a list of analyses shows that there is quite as close an agreement between these examples of ancient and Tertiary perlites as can be found in a series of the latter only; and as this similarity of chemical composition exists notwithstanding the alteration to which the older rocks have been subjected, it may be inferred that in this instance, as in others, the changes have been almost entirely molecular, little or nothing having been taken from or added to the mass.
Intimately associated with the characteristic devitrified pitchstone there occurs a hard compact variety of a dark red colour and semi-vitreous aspect, with a subconchoidal fracture and sharp cutting-edges. Some specimens show a banded structure, even to the naked eye; while others, apparently amorphous, exhibit under the microscope most interesting examples of fluidal structure, but with no indications of perlitic or spherulitic formations. These also appear to consist of devitrified glass, and are found to pass into masses of ordinary "hornstone" or "compact felspar," which were probably never in a vitreous condition.
A close examination of very thin slices shows that the red colour of these rocks is entirely due to the diffusion of hydrous ferric oxide through the mass; a magnifying power of 800 enables one to perceive that the matrix consists of a colourless glassy substance crowded in parts with minute yellowish-red specks, which have the appearance of a fine dust even under the highest powers. In many cases the colouring-matter clearly has its origin in minute ochreous patches presenting crystalline forms with ragged outlines, from which the red stain has spread in all directions.
Indurated Volcanic Agglomerates, and Ashes.
An examination of the stratified fragmental rocks of Lawrence Hill and the Wrekin leaves no room for doubt as to their real character.
The various beds consist for the most part of a breccia composed of small angular and slightly rounded fragments of compact red felsite and altered pitchstone, quite similar to those forming the masses with which they are associated; these, together with other materials, have been cemented together, and now form an extremely hard rock, which frequently exhibits manifest signs of subsequent alteration. Fragments of larger size, however, are not uncommon; and in one of the coarser ash-beds a block of beautiful spherulitic pitchstone, 8 inches in diameter, was found imbedded in the mass. A vertical slice of one of the finer ash-beds exhibits under the microscope a series of thin layers composed of angular fragments of pitchstone, in some of which small crystals of orthoclase and plagioclase are surrounded by streams of microliths, while others have a felsitic or crypto- crystalline texture and are crowded with minute crystals of felspar. A few fragments of spherulites have been detected; and there are also great numbers of broken felspar- crystals scattered through the mass; some of the thin layers are, in fact, almost wholly composed of abraded felspar crystals, with small fragments of microcrystalline trap disseminated among them.
At one time some of these ashes must have been slightly vesicular; for they now contain many cavities whose walls are coated with chlorite, and the interior filled with crystalline quartz. In a few cases calcite and quartz are associated together. There are also some interesting examples of microscopic cavities filled with calcite and epidote; bright yellow prisms of the latter project from the walls and cross each other in various directions, the intervening spaces being filled in with the calcite. Pale lemon-coloured epidote is by no means uncommon in the coarser ash-beds; it usually occurs in fan-shaped groups of flat prisms, which exhibit delicate bright tints in polarized light. In all the descriptions of the optical characters of this mineral which have come under my notice, it is said to be strongly dichroic; this, however, is certainly not the case with any of the numerous examples of pale yellow epidote which I have observed in these rocks, and in the altered syenites of Leicestershire and other localities.
The masses of rock here described represent a portion only of a series of similar products which have been erupted along an old line of volcanic action; porphyrinic and other varieties occur at Charlton Hill and Caer Caradoc near Church Stretton: while in the hilly district to the west there are still traces of old volcanic vents, accompanied by a most interesting variety of basic and acid lavas, which I hope to describe on a future occasion.
Conclusion.
The results arrived at in the present memoir may be briefly summed up as follows:—
1. The highly characteristic internal structure of some of the rocks affords the clearest proof of their original vitreous condition; for the peculiar perlitic and spherulitic formations, with their associated microliths, have never been observed except in connexion with the obsidian or pitchstone varieties of volcanic glass.
2. It appears also that, in the older as in the younger series, there is the same gradation between the vitreous and stony varieties; and as the perlitic and other glassy rocks are well known to be subaerial volcanic products, the rocks here described afford strong evidence that during the earlier geological periods volcanic action was of the same kind and produced the same results as in more recent times. I will only add, in conclusion, that probably no one who exa- mines a good series of the Schemnitz rocks, or the beautiful rhyolites of the Euganean Hills, will fail to recognize among their numerous varieties the precise analogues of the ancient volcanic rocks of Shropshire.
EXPLANATION OF PLATE XX.
2. Another portion of the same slice.
3. Part of a thin slice of perlite from Schemnitz.
4. Perlitic spheroid from Cabo de Gata. see p. 452.
5. Section of perlite from Schemnitz, showing parallel and cross joints, with general grouping of the perlitic spheroids, see p. 452.
6. Ancient perlitic pitchstone. "Lea Rock." near Wellington, showing a similar original structure. The shaded pans represent a green-coloured product of alteration, see p. 456.
7. Section of spherulitic perlite from Kremnitz, described p. 453.
8. Section of spherulitic pitchstone from Lea Rock, containing bands of spherulites and streams of mioroliths in a transparent perlitic matrix.
9. Diagram of spherulite, described p. 454.
10. Spherulite in ancient pitchstone, Lea Rock, containing two crystals of felspar, and traversed by a vein filled with a substance having similar optical characters to the devitrified matrix, see p. 457.
The figures faithfully represent the originals as seen with the magnifying powers indicated, with the exception of the fibrous crystallization of the spherulites, which is, of course, greatly exaggerated.
Discussion.
Mr. Warington W. Smyth thought that, with regard to the rocks of the Lizard Point. Mr. Bonney's paper would carry conviction to the minds of many who had been led to different views by the works of previous writers. The facts brought forward seemed to correlate these rocks with others which have been examined in Sweden and Norway, in the south of Spain, and in Elba.
Prof. Judd. referring to Mr. Allport's paper, said that the similarity of these ancient rocks to those of Tertiary times extended to the most minute details. The unity of character so long ago recognized among stratiried rocks of different ages was now being extended to the igneous rocks: and these also would in time be correlated according to their respective ages.
The President inquired whether Mr. Bonney had met with indications of gas-passages in the serpentinous rocks, such as might simulate Eozoon.
Rev. T. G. Bonney stated that he had examined the serpentines of Elba. With respect to Mr. McPherson's paper referred to, he said that some of that gentleman's figures might be taken as representing the Lizard serpentines. In reply to the President he stated that he had seen gas-passages, but nothing resembling Eozoon, although in the first stages of the decomposition of olivine the nummuline layer sometimes seemed to be simulated. For his own pair he believed in the organic nature of Eozoon.
PITCHSTONES & PERLITES FROM SHROPSHIRE.
- ↑ Mikroscop. Beschaff. der Mineralien und Gesteine, p. 365.
- ↑ Mikroscop. Physiographie, p. 124.
- ↑ Elemente der Petrographie, p. 221.
- ↑ "On some Structures in Obsidian," &c, read before the Royal Microscopical Society, March 1876.
- ↑ Quart. Journ. Geol. Soc. yol. xxxii. p. 140.
- ↑ I recently described the occurrence of an analogous case of crystallization without disturbance of previous texture, in one of the altered slates near Penzance. See Quart. Journ. Geol. Soc. vol. xxxii. p. 410.
- ↑ For an admirable description of laminated volcanic rocks, see Darwin's 'Geological Observations on Volcanic Islands,' p. 74, second edition.
- ↑ Beiträge zur Petrographie. Berlin, 1869.
- ↑ Die Gesteins-Analysen. J. Roth. Berlin, 1861.