XIV. On a collection of rock specimens from socotra

Abstract

In the spring of 1879 the island of Socotra, which lies off the north-east corner of Africa, about 140 miles from Cape Gardafui, was visited by Professor Bayley Balfour. Landing at the north-west extremity, he traversed the northern side of the island up to the eastern end, then returning by a more central course to the sea, he crossed the Haggier Mountains to the southern coast, and returned again to Hadibu, on the north side, by a route lying further to the west. During this journey, in addition to extensive botanical and zoological collections, Professor Balfour obtained about 500 rock specimens illustrative of the geology of a considerable portion of the island. These were sent to the author for examination. A considerable number of them, as was to be expected, were more or less weathered, and so were not in a very favourable condition for precise description; but about eighty of the best preserved specimens have been examined microscopically; from the study of which, and of the remainder the following sketch of the geology of the island may be given. The north-west, inland from Gollonsir Bay, consists of a plateau of a limestone resting unconformably upon a group of highly crystalline gneisses, associated with diorites, which correspond in general character with the Hebridean series of north-west Scotland. The latter group is frequently exposed in the beds of the valleys, the uplands on either side being formed of the limestone. The elevated district traversed between Gollonsir and Kuhmeh Bays is similarly constituted, but it is probable that some true granite also exists among the older series; the limestone extends all along the coast of the latter bay, having its usual foundation, and there is evidence that felsites occur somewhere in this district, most probably inland from the eastern shore. In this part are basalt dykes, which cut the limestone as well as the older rocks.