Part 68 (1/2)

[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.