Ashangi group of traps. — The lower subdivision consists entirely of doleritic rocks.
Amygdaloidal basalts prevail, associated with volcanic ash or breccia. The beds are usually disturbed, and frequently inclined at considerable angles.
The only place where the traps of this group were seen resting on the lower rocks was in the Mishek valley, where they are based upon the upper sandstones and conglomerates of the Antalo limestone. The section is obscure ; but there appears to be unconformity. In mineral character there is a marked resemblance between the rocks of this group and those of the great trappean formation of Western India.
Magdala group of traps. — These consist partly of doleritic, partly of trachytic flows and ash breccias, the former predominating in general, but the latter very largely developed, especially in the higher portions of the group, and frequently occurring in beds of great thickness, weathering into immense vertical scarps, to which the peculiar profile of the higher peaks is largely due. To these thick beds the Abyssinian chieftains are indebted generally for the almost impregnable hill forts which form so remarkable a feature of the country. A great proportion of the beds are brecciated; and many, both brecciated and non-brecciated, are highly columnar. Some sedimentary rocks were found interstratified in the gorge of the Jidda river and near Magdala, but no fossils were detected in them.
These beds in Waag and Lasta differ from the subjacent group in being almost perfectly horizontal. To them I am disposed to refer the horizontal beds of the Harat range, west of Adigrat ; and probably the lofty ridges of Simen (or Samen), west of the Takkazzye, are, in part at least, of the same age. If this be the case, this group of beds must once have covered the greater portion, if not the whole of Northern Abyssinia, and have been removed by denudation, which in all probability was entirely subaerial. Their detritus, spread over the valley of the Nile, doubtless accounts in a great measure for the fertility of Egypt.
It is difficult to say whether the singular trachytes of Senafe, which are probably identical in origin with the well-known peculiarly formed ranges around Adowa, belong to the same group. They may be accumulations around the ancient volcanic nuclei.
Nothing positive can be asserted as to the exact geological age of either group of traps. The great outbursts which have covered Western India are Upper Cretaceous* (prenummulitic) ; and the volcanic rocks so largely developed on the south coast of Arabia† are very possibly, in part at least, of the same age. There may be a general connexion between all these enormously developed series of lava-flows, without their being absolutely contemporaneous. Their enormous mass and their persistent horizontality, in many instances over large areas, entitle them to greater attention than they have over received.
- See Memoirs of the Geological Survey of India, vol. vi., Art. 3.
† Carter, Journal Bombay Br. Roy. As. Soc. vol. iv. pp. 28, 91, &c.
