The Highlands of Ethiopia

by Sir William Cornwallis Harris

When the portion of North-eastern Africa that is to form the province of inquiry received its present configuration, the fountains of the deep may be supposed to have opened at once upon a surface, of which the prior quality and condition has become so shut out from human observation, that analogies, drawn from other countries under similar circumstances, must supply this deficiency.

Porphyry forms the general basis of all the different volcanic formations discernible. From the pinnacle of Jebel Goodah, on the Gulf of Arabia, it may be traced, though indistinctly, in the minor outrunners of the Abyssinian Alps, to the province of Efát, where it passes under red sandstone. The principal Shoan range, and the high westerly plateau towards the valley of the Nile, present solely secondary formations, but the porphyry again emerges on the southerly borders in the ranges of Garra Gorphoo and Bulga; whilst the left; bank of the Háwash valley is distinctly of primitive crystalline formation.

The overlying rocks, which seem to have been poured from the centre of this tract, consist of masses of trachyte and columnar basalt, of pyramids of wacke, and beds of lava and tufwacke, with strata of conglomerates and sandstones. The former of these, the trachyte and basalt, belong to the lofty mountains of Abyssinia; whereas wacke, lava, tuffo, and scoria, cover the surface, and form the hills of the desert below; and many districts present volcanoes which, not half a century ago, were in violent activity.

The hills of Mentshar, Efát, and Giddem, are detached ranges, running nearly parallel to the Shoan alps. Disclosing in some few spots the nature of their interior, it appears that immediately over the porphyry lies a red sand-stone, embedding vast quantities of coal, and presenting a true stratification. It consists of minute but quite perfect hexagon dodecaëders of quartz in a white cement, is very soft, and cleaves sometimes in regular squares. Its depth was not observed to be very great, nor did the overlying formations, a marl and conglomerates, seem to form obstacles to the miner.

The Shoan mountains, of alpine height, exhibit a structure of basalt, wacke, and trachyte; the latter, in all its varieties, surrounds a nucleus of basalt, basaltic wacke, and dolerite. The conglomerates and tuffos at their feet, and partly on their terraces and tops, are of trachytic nature, and sometimes pierced through by small dikes of basalt. Veins of ochre and clay, holes filled with scoria, with intrusions of larger or smaller fragments of various rocks and minerals, and a kind of stratification, are the principal features of this trachytic formation.

When the action began, craters or clefts were formed in the then existing crust of trap-rocks, which in their turn were covered with masses of trachytic lava; a little later, the tuffos and conglomerates were deposited, which prove the importance of augite in their formation by numerous crystals of pyroxen embedded in them. Subsequently new basaltic eruptions either raised these deposits to their present height, or pierced them through in their original sites, both cases occurring on the same locality.

The basalt composing the hills about Ankóber presents no vestige of olivin, nor does it influence the magnetic needle; but a distinction between basalt and greenstone in their finer-grained varieties is difficult; and to determine in words the affinity which they bear to each other in the present instance, the rock might be styled basaltic greenstone. Columns, pentagonal or heptagonal, crown the tops of hills, and seem rather a composition of hornblende than of augite and feldspar. Scoriaceous varieties are found on the outsides of the later protruded masses.

The trachyte is generally a compact mass of grey feldstein, which contains crystals of glassy feldspar, irregularly embedded, and in different quantities. Some varieties are porous, some full of small holes, others black from grains of obsidian; and a few, especially near the dikes, incline to phonolite.

To the westward the ridge terminates in a high plateau, the western Galla provinces of the kingdom. This vast plain is crossed in various directions by hill ranges, the greater part of which do not rise to any considerable height. Here true basalt is disclosed in all the grandeur of its columnar cleavage, but no other species of rock. Deep and narrow ravines carry off the superabundant waters, and pits of tolerable ironstone afford a supply of metal for the manufacture of weapons.

The chief bearing of the mountain chains in Shoa is north and south, with spurs to the west and east. The towering height to which they rise, in a nearly uninterrupted ascent, may be calculated to be from eight to nine thousand feet above the level of the sea, and their single pinnacles far exceed that limit. They are most abrupt, and difficult of access, excepting by the only two passes to the high western plateau. Ravines and chasms of a depth which admits the sun but for few hours, tell of the catastrophe which resulted in their formation. Inaccessible steep cliffs and dismal precipices everywhere line the tiresome footpath of the lonely muleteer.