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 masses from 50 to 700 feet in thickness, overlying the strata (No. 4.). They sometimes pass 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 associated 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 mass 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 coarse-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 coarser granite (No. 2.) may be the oldest rock in Arran, with the exception of the hypogene slates (No. 1.), into which it sends veins.
Fig. 506. General Section of Arran from north to south.
- 1. Metamorphic or Hypogene schists, the oldest formations in Arran.
- 2. Coarse-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, passing occasionally into Syenites of the Plutonic class.
- 6. a. Fine-grained granite, associated 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 masses 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 entitled to affirm that the coarse-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 assume 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 shingle 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.
Fig. 507.
