It appears then, as far as can be collected from the few specimens above described, that on each side of the central basaltic chain of the Soudah or Black Mountains, the plains of the desert are composed of red sand and sand-stone, containing gypsum and rock salt, and associated with beds of dolomite and common carbonate of lime. All these characters identify most distinctly the sand of the desert of Africa with the new red sand-stone of England. There are no specimens which indicate the existence of any other formations on the south side of the Black Mountains from lat. 29° to 24°, except the marl-stone and green clay (No. 5 and 6); which lead us to suspect strata of tertiary formation in lat. 26°, near Mejdool, on the east of Morzouk. At the north base of the central chain, strata belonging to the same red sand-stone formation seem to extend nearly to Bonjem on the frontiers of Fezzan and Tripoli; where the basis formations appear, and probably repose on them in irregular patches in the desert that divides this place from the mountains on the north of the town of Tripoli. These mountains extend east and west nearly parallel to the coast of the Mediterranean, from long. 15° to 13°; but as no specimens have been brought home from them, it is impossible to do more than conjecture to what formation they belong: from notices inserted in the map, they appear to contain trap and calcareous rocks. The nearest point from which we have a specimen is Benioleed (No. 4): and this is probably referrible to the calcaire grossier of Paris. One solitary specimen from the eastern extremity of these calcareous mountains possesses no character sufficiently distinct to show whether it be calcaire grossier or jura limestone. It seems, however, to belong to one of these two formations. The only specimen remaining to be described is No. 34; a yellow quartzose sand-stone, having a glassy fracture, and in some of its component grains having a sapphire blue colour. It resembles the sandstone of which is composed the statue of the Old Memnon in Egypt; and was found near the north base of the Black Mountains. As it is not a rolled pebble, it indicates, that in addition to basalt there are strata of ancient quartzose sandstone in this chain, which forms nearly the centre of the line along which the specimens above described have been collected. INDEX. * * * * *