The desert of the Adaïel, spreading from the sea to the foot of the Shoan Alps, is not altogether a plain, as it has been most likely in remote ages, numerous wadies, with banks more or less high, now intersecting the greater part of it. These banks rise in some instances to hills of firm rock, generally wacke. They however consist of but lightly cemented conglomerates, or loose boulders. Towards the middle, as the ground rises, extinct volcanoes make their appearance, sometimes scattered and solitary, with indistinct cones and craters, completely covered with volcanic cinders, and sending off sheets of lava in all directions; or in whole clusters, with cones and craters complete, connected with each other, and environed by belts of their products. The extensive plain of Eyrolúf is a solid level of a dark, black, undecayed lava.
The tract of land between Killulloo and Dathára especially has been visited and overwhelmed by the action from below, which, having reversed the original disposition, has covered the surface with the effects of its violence. There is little to be seen of the under parts, although here and there some of the later formation, the residuum of the calcareous waters, has spread like a thin coat over the low grounds; but violent commotions have again and again altered and destroyed the first appearance, and it is now difficult to determine the centre pool from whence the fiery stream issued. In the absence of a main volcano and a main volcanic range, it may be concluded that, similar to some violent eruptions in South America, large mountains have been thrown up in the midst of former extensive plains, the fluid and half-fluid matter having burst forth wherever they were nearest to the surface.
Small extinct volcanoes were found on the plains of Sultélli and Eyrolúf. The road passes close to the isolated cone of one of these, called Jebel Hélmund. The walls are straight and black, covered with several smaller cones of ashes; the hill itself is about four hundred feet in height; the crater is on the eastern side, a little below the top; and the sides, which are steep and sloping, are clothed with shrubs towards the base.
On the road to Moo stands a similar volcano; but the influence of these craters does not seem to have extended far beyond the immediate neighbourhood, although there is a connexion between the whole cluster on the plain of Mittur, which may be seen in the small lava streams and débris of volcanic product on the adjoining plains of Sultélli and Eyrolúf. It is not, however, apparent that they alone have formed the present state of the surface, as the south-eastern side of the plains is terminated by a much older formation of wacke.
Between Meinha-tólli and Madéra-dubba, obsidian, pumice, clinkstone, and fresh-water limestone containing shells of Melania, were strewed about Excellent soil is found in all these situations, the low grounds being overflowed at some seasons, and, as in all volcanic countries, producing much vegetation. The extensive plains of Moolu and Burdúdda are thickly covered with grass, and intersected by small brooks and pools, terminating towards the Háwash in very broken, hilly ground, and the large plain on the eastern bank of the river bears every sign of being annually deluged.
The country of the Adaïel is throughout very sparingly watered. During spring and autumn the hills collect sufficient rain-water for numerous rivulets, which after a course of scarcely one mile are absorbed by the sands, and dry up altogether by the end of the rains, whilst the deep hollows and clefts in the firmer rock preserve small quantities for the dry months of the year. The Háwash itself, although receiving all the rivers of Efát, and of the eastern declivity of the Shoan mountains, does not reach the sea. The banks, thickly overgrown, are about thirty feet in height, and very abrupt. Its fall is scarcely perceptible, yet the rush of the water is very considerable.
On the western bank volcanic hills and sheets of water again appear, the latter being situated lower than the bed of the stream. One of these, impregnated with alkali, is evidently an old crater filled up, and supplied by a hot mineral spring. The water is much esteemed for washing clothes; it possesses an hydrothionic smell and a bitter taste, resembling that of the salt of magnesia; but the borders are verdant, and a species of Cyperus grows luxuriantly in the water.
This portion of the country, though still sparingly supplied with the means of subsistence, is more favourable for specimens of zoology than the burning tract between Tajúra and Killulloo.
Of Beetles the family Coprophaga had many representatives: Scarabaeus, Copris, Ateuchus, Onitis, Aphodius, Trox; Melolontha; four species of Cetonia (on the Aloe); one Silpha, Hister, Abax, Graphipterus, Anthia, Staphylinus, Elater, Cantharis, Erodius, Moluria, Pimelia, Mylabria, Chrysomela.
Of Orthoptera, large flights of Gryllus migratorius were observed near Azbóti. Acrydium and Gryllotalpa very common throughout. Also many Neuroptera, and termite cone studding the face of the country.