A Manual of Elementary Geology Part 68

[2 Ill.u.s.trations: Fig. 490. Fig. 491.

Junction of granite and argillaceous schist in Glen Tilt.

(MacCulloch.)[442-A]]

I have already hinted at the close a.n.a.logy in the forms of certain granitic and trappean veins; and it will be found that strata penetrated by plutonic rocks have suffered changes very similar to those exhibited near the contact of volcanic dikes. Thus, in Glen Tilt, in Scotland, alternating strata of limestone and argillaceous schist come in contact with a ma.s.s of granite. The contact does not take place as might have been looked for, if the granite had been formed there before the strata were deposited, in which case the section would have appeared as in fig. 490.; but the union is as represented in fig. 491., the undulating outline of the granite intersecting different strata, and occasionally intruding itself in tortuous veins into the beds of clay-slate and limestone, from which it differs so remarkably in composition. The limestone is sometimes changed in character by the proximity of the granitic ma.s.s or its veins, and acquires a more compact texture, like that of hornstone or chert, with a splintery fracture, effervescing feebly with acids.

The annexed diagram (fig. 492.) represents another junction, in the same district, where the granite sends forth so many veins as to reticulate the limestone and schist, the veins diminis.h.i.+ng towards their termination to the thickness of a leaf of paper or a thread. In some places fragments of granite appear entangled, as it were, in the limestone, and are not visibly connected with any larger ma.s.s; while sometimes, on the other hand, a lump of the limestone is found in the midst of the granite. The ordinary colour of the limestone of Glen Tilt is lead blue, and its texture large-grained and highly crystalline; but where it approximates to the granite, particularly where it is penetrated by the smaller veins, the crystalline texture disappears, and it a.s.sumes an appearance exactly resembling that of hornstone. The a.s.sociated argillaceous schist often pa.s.ses into hornblende slate, where it approaches very near to the granite.[442-B]

[Ill.u.s.tration: Fig. 492. Junction of granite and limestone in Glen Tilt. (MacCulloch.)

_a._ Granite. _b._ Limestone.

_c._ Blue argillaceous schist.]

The conversion of the limestone in these and many other instances into a siliceous rock, effervescing slowly with acids, would be difficult of explanation, were it not ascertained that such limestones are always impure, containing grains of quartz, mica, or felspar disseminated through them. The elements of these minerals, when the rock has been subjected to great heat, may have been fused, and so spread more uniformly through the whole ma.s.s.

[Ill.u.s.tration: Fig. 493. Granite veins traversing clay slate. Table Mountain, Cape of Good Hope.[443-A]]

In the plutonic, as in the volcanic rocks, there is every gradation from a tortuous vein to the most regular form of a dike, such as intersect the tuffs and lavas of Vesuvius and Etna. Dikes of granite may be seen, among other places, on the southern flank of Mount Battock, one of the Grampians, the opposite walls sometimes preserving an exact parallelism for a considerable distance.

As a general rule, however, granite veins in all quarters of the globe are more sinuous in their course than those of trap. They present similar shapes at the most northern point of Scotland, and the southernmost extremity of Africa, as the annexed drawings will show.

It is not uncommon for one set of granite veins to intersect another; and sometimes there are three sets, as in the environs of Heidelberg, where the granite on the banks of the river Necker is seen to consist of three varieties, differing in colour, grain, and various peculiarities of mineral composition. One of these, which is evidently the second in age, is seen to cut through an older granite; and another, still newer, traverses both the second and the first.

In Shetland there are two kinds of granite. One of them, composed of hornblende, mica, felspar, and quartz, is of a dark colour, and is seen underlying gneiss. The other is a red granite, which penetrates the dark variety everywhere in veins.[444-A]

[Ill.u.s.tration: Fig. 494. Granite veins traversing gneiss, Cape Wrath.

(MacCulloch.)[444-B]]

[Ill.u.s.tration: Fig. 495. Granite veins traversing gneiss at Cape Wrath, in Scotland. (MacCulloch.)]

The accompanying sketches will explain the manner in which granite veins often ramify and cut each other (figs. 494. and 495.). They represent the manner in which the gneiss at Cape Wrath, in Sutherlands.h.i.+re, is intersected by veins. Their light colour, strongly contrasted with that of the hornblende-schist, here a.s.sociated with the gneiss, renders them very conspicuous.

Granite very generally a.s.sumes a finer grain, and undergoes a change in mineral composition, in the veins which it sends into contiguous rocks.

Thus, according to Professor Sedgwick, the main body of the Cornish granite is an aggregate of mica, quartz, and felspar; but the veins are sometimes without mica, being a granular aggregate of quartz and felspar. In other varieties quartz prevails to the almost entire exclusion both of felspar and mica; in others, the mica and quartz both disappear, and the vein is simply composed of white granular felspar.[444-C]

Fig. 496. is a sketch of a group of granite veins in Cornwall, given by Messrs. Von Oeynhausen and Von Dechen.[445-A] The main body of the granite here is of a porphyritic appearance, with large crystals of felspar; but in the veins it is fine-grained, and without these large crystals. The general height of the veins is from 16 to 20 feet, but some are much higher.

[Ill.u.s.tration: Fig. 496. Granite veins pa.s.sing through hornblende slate, Carnsilver Cove, Cornwall.]

In the Valorsine, a valley not far from Mont Blanc in Switzerland, an ordinary granite, consisting of felspar, quartz, and mica, sends forth veins into a talcose gneiss (or stratified protogine), and in some places lateral ramifications are thrown off from the princ.i.p.al veins at right angles (see fig. 497.), the veins, especially the minute ones, being finer grained than the granite in ma.s.s.

[Ill.u.s.tration: Fig. 497. Veins of granite in talcose gneiss.

(L. A. Necker.)]

It is here remarked, that the schist and granite, as they approach, seem to exercise a reciprocal influence on each other, for both undergo a modification of mineral character. The granite, still remaining unstratified, becomes charged with green particles; and the talcose gneiss a.s.sumes a granitiform structure without losing its stratification.[445-B]

Professor Keilhau drew my attention to several localities in the country near Christiania, where the mineral character of gneiss appears to have been affected by a granite of much newer origin, for some distance from the point of contact. The gneiss, without losing its laminated structure, seems to have become charged with a larger quant.i.ty of felspar, and that of a redder colour, than the felspar usually belonging to the gneiss of Norway.

Granite, syenite, and those porphyries which have a granitiform structure, in short all plutonic rocks, are frequently observed to contain metals, at or near their junction with stratified formations. On the other hand, the veins which traverse stratified rocks are, as a general law, more metalliferous near such junctions than in other positions. Hence it has been inferred that these metals may have been spread in a gaseous form through the fused ma.s.s, and that the contact of another rock, in a different state of temperature, or sometimes the existence of rents in other rocks in the vicinity, may have caused the sublimation of the metals.[446-A]

There are many instances, as at Markerud, near Christiania, in Norway, where the strike of the beds has not been deranged throughout a large area by the intrusion of granite, both in large ma.s.ses and in veins. This fact is considered by some geologists to militate against the theory of the forcible injection of granite in a fluid state. But it may be stated in reply, that ramifying dikes of trap, which almost all now admit to have been once fluid, pa.s.s through the same fossiliferous strata, near Christiania, without deranging their strike or dip.[446-B]

[Ill.u.s.tration: Fig. 498. General view of junction of granite and schist of the Valorsine. (L. A. Necker.)]

The real or apparent isolation of large or small ma.s.ses of granite detached from the main body, as at _a b_, fig. 498., and above, fig. 492., and _a_, fig. 497., has been thought by some writers to be irreconcilable with the doctrine usually taught respecting veins; but many of them may, in fact, be sections of root-shaped prolongations of granite; while, in other cases, they may in reality be detached portions of rock having the plutonic structure. For there may have been spots in the midst of the invaded strata, in which there was an a.s.semblage of materials more fusible than the rest, or more fitted to combine readily into some form of granite.

Veins of pure quartz are often found in granite, as in many stratified rocks, but they are not traceable, like veins of granite or trap, to large bodies of rock of similar composition. They appear to have been cracks, into which siliceous matter was infiltered. Such segregation, as it is called, can sometimes be shown to have clearly taken place long subsequently to the original consolidation of the containing rock. Thus, for example, in the gneiss of Tronstad Strand, near Drammen, in Norway, the annexed section is seen on the beach. It appears that the alternating strata of whitish granitiform gneiss, and black hornblende-schist, were first cut through by a greenstone dike, about 2-1/2 feet wide; then the crack _a b_ pa.s.sed through all these rocks, and was filled up with quartz. The opposite walls of the vein are in some parts incrusted with transparent crystals of quartz, the middle of the vein being filled up with common opaque white quartz.

[Ill.u.s.tration: Fig. 499. _a, b._ Quartz vein pa.s.sing through gneiss and greenstone, Tronstad Strand, near Christiania.]

[Ill.u.s.tration: Fig. 500. Euritic porphyry alternating with primary fossiliferous strata, near Christiania.]

We have seen that the volcanic formations have been called overlying, because they not only penetrate others, but spread over them. Mr. Necker has proposed to call the granites the underlying igneous rocks, and the distinction here indicated is highly characteristic. It was indeed supposed by some of the earlier observers, that the granite of Christiania, in Norway, was intercalated in mountain ma.s.ses between the primary or paleozoic strata of that country, so as to overlie fossiliferous shale and limestone. But although the granite sends veins into these fossiliferous rocks, and is decidedly posterior in origin, its actual superposition in ma.s.s has been disproved by Professor Keilhau, whose observations on this controverted point I had opportunities in 1837 of verifying. There are, however, on a smaller scale, certain beds of euritic porphyry, some a few feet, others many yards in thickness, which pa.s.s into granite, and deserve perhaps to be cla.s.sed as plutonic rather than trappean rocks, which may truly be described as interposed conformably between fossiliferous strata, as the porphyries (_a c_, fig. 500.), which divide the bituminous shales and argillaceous limestones, _f f_. But some of these same porphyries are partially unconformable, as _b_, and may lead us to suspect that the others also, notwithstanding their appearance of interstratification, have been forcibly injected. Some of the porphyritic rocks above mentioned are highly quartzose, others very felspathic. In proportion as the ma.s.ses are more voluminous, they become more granitic in their texture, less conformable, and even begin to send forth veins into contiguous strata. In a word, we have here a beautiful ill.u.s.tration of the intermediate gradations between volcanic and plutonic rocks, not only in their mineralogical composition and structure, but also in their relations of position to a.s.sociated formations. If the term overlying can in this instance be applied to a plutonic rock, it is only in proportion as that rock begins to acquire a trappean aspect.

It has been already hinted that the heat, which in every active volcano extends downwards to indefinite depths, must produce simultaneously very different effects near the surface, and far below it; and we cannot suppose that rocks resulting from the crystallizing of fused matter under a pressure of several thousand feet, much less miles, of the earth's crust can resemble those formed at or near the surface. Hence the production at great depths of a cla.s.s of rocks a.n.a.logous to the volcanic, and yet differing in many particulars, might almost have been predicted, even had we no plutonic formations to account for. How well these agree, both in their positive and negative characters, with the theory of their deep subterranean origin, the student will be able to judge by considering the descriptions already given.

It has, however, been objected, that if the granitic and volcanic rocks were simply different parts of one great series, we ought to find in mountain chains volcanic dikes pa.s.sing upwards into lava, and downwards into granite. But we may answer, that our vertical sections are usually of small extent; and if we find in certain places a transition from trap to porous lava, and in others a pa.s.sage from granite to trap, it is as much as could be expected of this evidence.

The prodigious extent of denudation which has been already demonstrated to have occurred at former periods, will reconcile the student to the belief that crystalline rocks of high antiquity, although deep in the earth's crust when originally formed, may have become uncovered and exposed at the surface. Their actual elevation above the sea may be referred to the same causes to which we have attributed the upheaval of marine strata, even to the summits of some mountain chains. But to these and other topics, I shall revert when speaking, in the next chapter, of the relative ages of different ma.s.ses of granite.

FOOTNOTES:

[439-A] Bulletin, 2d serie, iv. 1304.; and Archiac, Hist. des Progres de Geol., i. 38.

[440-A] Boase on Primary Geology, p. 16.

[441-A] Bulletin, vol. iv., 2d ser., pp. 1318. and 1320.

[441-B] Syst. of Geol., vol. i. p. 157.

[441-C] Ibid., p. 158.

[442-A] Geol. Trans., 1st series, vol. iii. pl. 21.

[442-B] MacCulloch, Geol. Trans., vol. iii. p. 259.

[443-A] Capt. B. Hall, Trans. Roy. Soc. Edin., vol. vii.

[444-A] MacCulloch, Syst. of Geol., vol. i. p. 58.

[444-B] Western Islands, pl. 31.

[444-C] On Geol. of Cornwall, Camb. Trans. vol. i. p. 124.

[445-A] Phil. Mag. and Annals, No. 27. new series, March, 1829.

[445-B] Necker, sur la Val. de Valorsine, Mem. de la Soc. de Phys. de Geneve, 1828. I visited, in 1832, the spot referred to in fig. 497.