A Manual of Elementary Geology Part 70

_Relative age of the granites of Arran._--In this island, the largest in the Firth of Clyde, being twenty miles in length from north to south, the four great cla.s.ses of rocks, the fossiliferous, volcanic, plutonic, and metamorphic, are all conspicuously displayed within a very small area, and with their peculiar characters strongly contrasted. In the north of the island the granite rises to the height of nearly 3000 feet above the sea, terminating in mountainous peaks. (See section, fig. 506.) On the flanks of the same mountains are chloritic-schists, blue roofing-slate, and other rocks of the metamorphic order (No. 1.), into which the granite (No. 2.) sends veins. This granite, therefore, is newer than the hypogene schists (No. 1.), which it penetrates.

These schists are highly inclined. Upon them rest beds of conglomerate and sandstone (No. 3.), which are referable to the Old Red formation, to which succeed various shales and limestones (No. 4.) containing the fossils of the Carboniferous period, upon which are other strata of sandstone and conglomerate (upper part of No. 4.), in which no fossils have been met with, which it is conjectured may belong to the New Red sandstone period. All the preceding formations are cut through by the volcanic rocks (No. 5.), which consist of greenstone, basalt, pitchstone, claystone-porphyry, and other varieties. These appear either in the form of dikes, or in dense ma.s.ses from 50 to 700 feet in thickness, overlying the strata (No. 4.). They sometimes pa.s.s into syenite of so crystalline a form, that it may rank as a plutonic formation; and in one region, at Ploverfield, in Glen Cloy, a fine-grained granite (6. _a_) is seen a.s.sociated with the trap formation, and sending veins into the sandstone or into the upper strata of No. 4. This interesting discovery of granite in the southern region of Arran, at a point where it is separated from the northern ma.s.s of granite by a great thickness of secondary strata and overlying trap, was made by Mr. L. A. Necker of Geneva, during his survey of Arran in 1839.

We also learn from the recent investigations of Prof. A. C. Ramsay, that a similar fine-grained granite (No. 6. _b_) appears in the interior of the northern granitic district, forming the nucleus of it, and sending veins into the older coa.r.s.e-grained granite (No. 2.). The trap dikes which penetrate the older granite are cut off, according to Mr. Ramsay, at the junction of the fine grained.

It is not improbable that the granite (No. 6. _b_) may be of the same age as that of Ploverfield (No. 6. _a_), and this again may belong to the same geological epoch as the trap formations (No. 5.). If there be any difference of date, it would seem that the fine-grained granite must be newer than the trappean rocks. But, on the other hand, the coa.r.s.er granite (No. 2.) may be the oldest rock in Arran, with the exception of the hypogene slates (No. 1.), into which it sends veins.

[Ill.u.s.tration: Fig. 506. General Section of Arran from north to south.

1. Metamorphic or Hypogene schists, the oldest formations in Arran.

2. Coa.r.s.e-grained granite sending veins into the schists, No. 1.

3. Old Red Sandstone and Conglomerate containing pebbles exclusively derived from the rocks, No 1., without any intermixture of granitic fragments.

4. Carboniferous strata and red sandstone (New Red?).

5. Trap, overlying and in dikes, pa.s.sing occasionally into Syenites of the Plutonic cla.s.s.

6. _a._ Fine-grained granite, a.s.sociated with the overlying trap, No. 5.

6. _b._ Similar fine-grained granite, sending veins into the older granite, No. 2., and cutting off the trappean dikes, _c_, _d_.[461-A]]

An objection may perhaps at first be started to this conclusion, derived from the curious and striking fact, the importance of which was first emphatically pointed out by Dr. MacCulloch, that no pebbles of granite occur in the conglomerates of the red sandstone in Arran, although these conglomerates are several hundred feet in thickness, and lie at the foot of lofty granite mountains, which tower above them. As a general rule, all such aggregates of pebbles and sand are mainly composed of the wreck of pre-existing rocks occurring in the immediate vicinity. The total absence therefore of granitic pebbles has justly been a theme of wonder to those geologists who have successively visited Arran, and they have carefully searched there, as I have done myself, to find an exception, but in vain.

The rounded ma.s.ses consist exclusively of quartz, chlorite-schist, and other members of the metamorphic series; nor in the newer conglomerates of No. 4. have any granitic fragments been discovered. Are we then ent.i.tled to affirm that the coa.r.s.e-grained granite (No. 2.), like the fine-grained variety (No. 6. _a_), is more modern than all the other rocks of the island? This we cannot a.s.sume at present, but we may confidently infer that when the various beds of sandstone and conglomerate were formed, no granite had reached the surface, or had been exposed to denudation in Arran. It is clear that the crystalline schists were ground into sand and s.h.i.+ngle when the strata No. 3. were deposited, and at that time the waves had never acted upon the granite, which now sends its veins into the schist. May we then conclude, that the schists suffered denudation before they were invaded by granite? This opinion, although not inadmissible, is by no means fully borne out by the evidence. For at the time when the Old Red sandstone originated, the metamorphic strata may have formed islands in the sea, as in fig. 507., over which the breakers rolled, or from which torrents and rivers descended, carrying down gravel and sand. The plutonic rock or granite (B) may even then have been previously injected at a certain depth below, and yet may never have been exposed to denudation.

[Ill.u.s.tration: Fig. 507. Cross section.]

As to the time and manner of the subsequent protrusion of the coa.r.s.e-grained granite (No. 2.), this rock may have been thrust up bodily, in a solid form, during that long series of igneous operations which produced the trappean and plutonic formations (Nos. 5., 6.

_a_, and 6. _b_).

We have shown that these eruptions, whatever their date, were posterior to the deposition of all the fossiliferous strata of Arran. We can also prove that subsequently both the granitic and trappean rocks underwent great aqueous denudation, which they probably suffered during their emergence from the sea. The fact is demonstrated by the abrupt truncation of numerous dikes, such as those at _c_, _d_, _e_, which are cut off on the surface of the granite and trap. The overlying trap also ceases very abruptly on approaching the boundary of the great hypogene region, and terminates in a steep escarpment facing towards it as at _f_, fig. 506. When in its original fluid state it could not have come thus suddenly to an end, but must have filled up the hollow now separating it from the hypogene rocks, had such a hollow then existed. This necessity of supposing that both the trap and the conglomerate once extended farther, and that veins such as _c_, _d_, fig. 506., were once prolonged farther upwards, prepares us to believe that the whole of the northern granite may at one time have been covered by newer formations, under the pressure of which, before its protrusion, it a.s.sumed its highly crystalline texture.

The theory of the protrusion in a solid form of the northern nucleus of granite is confirmed by the manner in which the hypogene slates (No. 1.) and the beds of conglomerate (No. 3.) dip away from it on all sides. In some places indeed the slates are inclined towards the granite, but this exception might have been looked for, because these hypogene strata have undergone disturbances at more than one geological epoch, and may at some points, perhaps, have their original order of position inverted. The high inclination, therefore, and the quaquaversal dip of the beds around the borders of the granitic boss, and the comparative horizontality of the fossiliferous strata in the southern part of the island, are facts which all accord with the hypothesis of a great amount of movement at that point where the granite is supposed to have been thrust up bodily, and where we may conceive it to have been distended laterally by the repeated injection of fresh supplies of melted materials.[463-A]

FOOTNOTES:

[450-A] Silliman's Journ., No. 69. p. 123.

[450-B] See "Principles," _Index_, "Jorullo."

[451-A] "Principles," _Index_, "Volcanic Eruptions."

[453-A] Darwin, pp. 390. 406.; second edition, p. 319.

[454-A] See map of Europe and explanation, in Principles, book i.

[456-A] Elie de Beaumont, sur les Montagnes de l'Oisans, &c. Mem. de la Soc. d'Hist. Nat. de Paris, tom. v.

[456-B] See Murchison, Geol. Trans., 2d series, vol. ii. part ii.

pp. 311-321.

[456-C] Western Islands, vol. i. p. 330. plate 18., figs. 3, 4.

[456-D] Von Buch, Annales de Chimie, &c.

[457-A] Proceedings of Geol. Soc., vol. ii. p. 562.

[457-B] See the Gaea Norvegica and other works of Keilhau, with whom I examined this country.

[459-A] Murchison, Geol. Trans., 2d series, vol. ii. p. 307.

[459-B] Geognostische Wanderungen, Leipzig, 1838.

[461-A] In the above section I have attempted to represent the new discoveries made since 1839, by Mr. Necker and Mr. A. C. Ramsay, in regard to the plutonic formations, 6. _a_, and 6. _b_.

[463-A] For the geology of Arran consult the works of Drs. Hutton and MacCulloch, the Memoirs of Messrs. Von Dechen and Oeynhausen, that of Professor Sedgwick and Sir R. Murchison (Geol. Trans. 2d series), Mr. L.

A. Necker's Memoir, read to the Royal Soc. of Edin. 20th April, 1840, and Mr. Ramsay's Geol. of Arran, 1841. I examined myself a large part of Arran in 1836.

CHAPTER x.x.xV.

METAMORPHIC ROCKS.

General character of metamorphic rocks--Gneiss--Hornblende-schist-- Mica-schist--Clay-slate--Quartzite--Chlorite-schist--Metamorphic limestone--Alphabetical list and explanation of other rocks of this family--Origin of the metamorphic strata--Their stratification is real and distinct from cleavage--Joints and slaty cleavage--Supposed causes of these structures--How far connected with crystalline action.

We have now considered three distinct cla.s.ses of rocks: first, the aqueous, or fossiliferous; secondly, the volcanic; and, thirdly, the plutonic, or granitic; and we have now, lastly, to examine those crystalline (or hypogene) strata to which the name of _metamorphic_ has been a.s.signed. The last-mentioned term expresses, as before explained, a theoretical opinion that such strata, after having been deposited from water, acquired, by the influence of heat and other causes, a highly crystalline texture. They who still question this opinion may call the rocks under consideration the stratified hypogene, or schistose hypogene formations.

These rocks, when in their most characteristic or normal state, are wholly devoid of organic remains, and contain no distinct fragments of other rocks, whether rounded or angular. They sometimes break out in the central parts of narrow mountain chains, but in other cases extend over areas of vast dimensions, occupying, for example, nearly the whole of Norway and Sweden, where, as in Brazil, they appear alike in the lower and higher grounds. In Great Britain, those members of the series which approach most nearly to granite in their composition, as gneiss, mica-schist, and hornblende-schist, are confined to the country north of the rivers Forth and Clyde.

Many attempts have been made to trace a general order of succession or superposition in the members of this family; gneiss, for example, having been often supposed to hold invariably a lower geological position than mica-schist. But although such an order may prevail throughout limited districts, it is by no means universal, nor even general, throughout the globe. To this subject, however, I shall again revert, in the last chapter of this volume, when the chronological relations of the metamorphic rocks are pointed out.

The following may be enumerated as the princ.i.p.al members of the metamorphic cla.s.s:--gneiss, mica-schist, hornblende-schist, clay-slate, chlorite-schist, hypogene or metamorphic limestone, and certain kinds of quartz-rock or quartzite.

[Ill.u.s.tration: Fig. 508. Fragment of gneiss, natural size; section at right angles to planes of stratification.]

_Gneiss._--The first of these, gneiss, may be called stratified granite, being formed of the same materials as granite, namely, felspar, quartz, and mica. In the specimen here figured, the white layers consist almost exclusively of granular felspar, with here and there a speck of mica and grain of quartz. The dark layers are composed of grey quartz and black mica, with occasionally a grain of felspar intermixed. The rock splits most easily in the plane of these darker layers, and the surface thus exposed is almost entirely covered with s.h.i.+ning spangles of mica. The accompanying quartz, however, greatly predominates in quant.i.ty, but the most ready cleavage is determined by the abundance of mica in certain parts of the dark layer.

Instead of these thin laminae, gneiss is sometimes simply divided into thick beds, in which the mica has only a slight degree of parallelism to the planes of stratification.

The term "gneiss," however, in geology is commonly used in a wider sense, to designate a formation in which the above-mentioned rock prevails, but with which any one of the other metamorphic rocks, and more especially hornblende-schist, may alternate. These other members of the metamorphic series are, in this case, considered as subordinate to the true gneiss.

The different varieties of rock allied to gneiss, into which felspar enters as an essential ingredient, will be understood by referring to what was said of granite. Thus, for example, hornblende may be superadded to mica, quartz, and felspar, forming a syenitic gneiss; or talc may be subst.i.tuted for mica, const.i.tuting talcose gneiss, a rock composed of felspar, quartz, and talc, in distinct crystals or grains (stratified protogine of the French).

_Hornblende-schist_ is usually black, and composed princ.i.p.ally of hornblende, with a variable quant.i.ty of felspar, and sometimes grains of quartz. When the hornblende and felspar are nearly in equal quant.i.ties, and the rock is not slaty, it corresponds in character with the greenstones of the trap family, and has been called "primitive greenstone." It may be termed hornblende rock. Some of these hornblendic ma.s.ses may really have been volcanic rocks, which have since a.s.sumed a more crystalline or metamorphic texture.

_Mica-schist_, or _Micaceous schist_, is, next to gneiss, one of the most abundant rocks of the metamorphic series. It is slaty, essentially composed of mica and quartz, the mica sometimes appearing to const.i.tute the whole ma.s.s. Beds of pure quartz also occur in this formation. In some districts, garnets in regular twelve-sided crystals form an integrant part of mica-schist. This rock pa.s.ses by insensible gradations into clay-slate.

_Clay-slate_, or _Argillaceous schist_.--This rock resembles an indurated clay or shale, is for the most part extremely fissile, often affording good roofing slate. It may consist of the ingredients of gneiss, or of an extremely fine mixture of mica and quartz, or talc and quartz. Occasionally it derives a s.h.i.+ning and silky l.u.s.tre from the minute particles of mica or talc which it contains. It varies from greenish or bluish-grey to a lead colour. It may be said of this, more than of any other schist, that it is common to the metamorphic and fossiliferous series, for some clay-slates taken from each division would not be distinguishable by mineralogical characters.

_Quartzite_, or _Quartz rock_, is an aggregate of grains of quartz, which are either in minute crystals, or in many cases slightly rounded, occurring in regular strata, a.s.sociated with gneiss or other metamorphic rocks. Compact quartz, like that so frequently found in veins, is also found together with granular quartzite. Both of these alternate with gneiss or mica-schist, or pa.s.s into those rocks by the addition of mica, or of felspar and mica.