Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/529

1869.] BLANFORD—ABYSSINIA. 405 Aden series of volcanic rocks.—Along both shores of the southern portion of the Red Sea and of the Gulf of Aden, there is an immense development of volcanic rocks, amongst which are many recent and, it is said, a few active craters. These formations have been described by several travellers*.

Near Zulla, on the west side of Annesley Bay, these beds are but sparingly represented; but all the eastern shore of the inlet consists of them. Near Massowa they are better seen and consist of lava-flows interstratified with beds of ashes and of sedimentary deposits, gravels, and sandstones, some of which contain marine shells. The lava-flows differ entirely from all the volcanic rocks of the highlands, and are assuredly of much later date. Still some of them may be far from recent, the great denudation they have undergone in places indicating considerable antiquity. As a rule they are inclined and present the appearance of having been much disturbed.

Recent formations.—These are not, for the most part, sufficiently important to demand much attention. Large alluvial deposits exist along the coast, north of Massowa and elsewhere, which appear not to be marine, but are more probably due to the washing down of pebbles and sand by rain and small torrents from the hills. They are frequently of considerable thickness.

Black soil precisely similar to the "regur" of India exists in the neighbourhood of the basaltic rocks on the highlands, and is probably derived from their decomposition. Of the plateau (Wadela-Talanta) between the Takkazzye and Jidda rivers, the northern half, where traversed by the British force, was of trachyte and had no black soil on it; the south consisted of basalt covered by black soil. The latter, just like the Indian "regur," seemed peculiarly well adapted to the growth of cereals†.

Denudation.—The enormous gorges cut by the rivers near Magdala attracted general attention, though they are in reality trifling when compared with those of the Takkazzye and other rivers. If it is correct to believe that the trappean formations once covered Abyssinia (and certainly it is difficult to escape the conclusion), then from 4000 to 10,000 feet must have been removed from by far the greater portion of the surface. There is not a trace of marine denudation over the surface examined.

Lake Ashangi.—This small piece of water, the only one of the Abyssinian lakes which it was possible to visit, lies in a small basin of basaltic rocks and has no visible outlet. The perfect sweetness of the waters, and the absence of any evidence of great change of level, prove that a subterranean outlet must exist. It is by no means clear how the rock-basin has been formed. Its form is opposed to the idea of its having been due to glacial action, and no trace of the former existence of glaciers could be detected; but it
 * Ferret et Galinier, 'Rocher d'Hericourt,' &c.

† It should be remembered that some Indian black soil can scarcely be derived from basalt. Such is the case in the extensive deposits of the lower Cauvery valley. Those who have only seen the soils of western India are sometimes not aware of this.