The Desert of Sahara presents some interesting facts of the same nature. It has already been stated that this part of Africa was ocean within a comparatively recent geological period. Tristram and several French officers of scientific attainments, who have made geological examinations of large portions of the desert have shown that the northern margin is lined with ancient sea-beaches and lines of terraces--the “rock-bound coasts” of the old ocean. Numerous salt-lakes exist in the desert which are tenanted by the common cockle. A species of _Haligenes_ which inhabits the Gulf of Guinea is found in a salt lake in latitude 30 deg. north and longitude 7 deg. east, separated, therefore, from its present marine habitat by the whole extent of the great desert, and the vast expanse of Soudan and Guinea. Geologists hence conclude that the existing fauna, including man, occupied Africa long before the Sahara became dry land. Reference has been made in the preceding chapter to the supposed remarkably beneficent effect this great expanse of desert, heated sands, and hot air, has upon the climate, and consequently upon the civilization of Europe. It is probable that from the fact that Sahara was about the last extensive portion of earth to be abandoned by the ocean, that the general opinion became prevalent that the continent of Africa was, geologically, the most recent of the grand divisions of the earth. Though supposed to be the oldest in civilization, it has been supposed to be the youngest in geological constitution. I am informed by scientific men that on account of recent investigations and reasonings, the opinion has for some time been gaining ground that Africa is likely to be shown to be the oldest part of the globe in both respects, and to have been the original birthplace of the race of man.