There can be no doubt, however, that an injection of similar dykes and veins took place after the invasion of the granophyre. These later intrusions are conspicuously displayed along the cliffs that extend from the gabbro junction on the north side of St. Kilda round the eastern coast into the South Bay. They maintain a general parallelism and ascend from the sea-level at varying angles of inclination, running up the pale sea-wall as dark bands. They consist of basalt-rocks, and may often be seen to branch and to die out. Like those in the gabbro, they are not infrequently compound, being made up of two or three or even more distinct dykes. This is well seen on the great precipice below Conacher, where the section given in [Fig. 367] is displayed. Here in a vertical height of about 800 or 900 feet, there must be at least seven dykes, simple and compound. A little further south a triple dyke may be seen to be composed of a thick central zone and two thinner marginal bands, of which the lower strikes off from the others and maintains an independent course through the granophyre ([Fig. 368]).
Fig. 367.—Section of the sea-cliff below Conacher, St. Kilda, showing basic dykes in granophyre.
Fig. 368.—Triple basic dyke, sea-cliff, east side of St. Kilda.
V. THE GRANITE OF ARRAN
The northern half of the island of Arran is mainly occupied by one of the most compact and picturesque groups of granite mountains in Scotland.[413] These heights, rising out of the Firth of Clyde to a height of 2866 feet, present, in their spiry and serrated crests, a contrast to the smoother contours of the older granitic elevations of this country. The granite is surrounded by a ring of schistose rocks, belonging to the metamorphic series of the Southern Highlands, save for a short distance on the eastern margin, where it comes in contact with and indurates the Lower Old Red Sandstone. Macculloch long ago pointed out that no pebbles of the granite are to be found in the surrounding conglomerates and red sandstones of Carboniferous and younger age.[414] Geologists accordingly came to the conclusion that the protrusion of the granite took place after Carboniferous time, and hence that it had no connection with the appearance of the far older granites of the Highlands. In the year 1873 I gave reasons for believing the granite to be not only younger than the Carboniferous formations, but to be referable with most probability to the Tertiary volcanic series.[415] The progress of inquiry has tended to confirm this inference, though no direct proof of its correctness has been obtained. Two lines of investigation may be pursued, and each leads to the conclusion of the probability of the Tertiary age of the granite. One of these proceeds on a comparison of the petrographical characters of the Arran rocks with those of undoubted members of the Tertiary series among the Western Isles. The other inquiry deals with the relation of the rocks to each other in the general geological structure of Arran itself.
[413] The rocks of Arran have often been described. Besides the work of Macculloch above quoted, reference may be made to the paper by Sedgwick and Murchison, Trans. Geol. Soc. 2nd Ser. vol. iii. p. 21; A. C. Ramsay's Geology of the Island of Arran, 1841, the paper of Necker de Saussure quoted on p. 412; J. Bryce's Geology of Clydesdale and Arran, 3rd edit. 1865. The island is at present being surveyed for the Geological Survey by Mr. W. Gunn.
[414] Description of the Western Islands of Scotland, vol. ii. p. 388.
[415] Trans. Edin. Geol. Soc. vol. ii. part iii.