time the Sahara will disappear under a vegetation of which the mimosas are the forerunner. "Thus," he adds, "while certain timorous spirits fear that the earth may some day be overpeopled, Nature is silently transforming the soil where man will in the future be able to pitch his tent. The Sahara will be covered with green trees, new lakes will be formed, and the rivers whose dry beds now fill the traveler with horror will be running streams of limpid water as abundant as that of the great streams of Europe."
Such a return to what seems pretty certain to have been the state of things ages ago would be most extraordinary without the help of man. The vast tract comprised between the sixteenth and thirtieth parallels of latitude, and extending from the Atlantic to the valley of the Nile, once fertile, became the arid waste of to-day mainly through neglect. A M. Largeau in 1874 visited the valley of the Igharghar, with the intention of branching off to Rhadames to study the commerce of that oasis and test the practicability of diverting to Algeria the caravans that come there by the central route from Soodan. He questioned the chambas on the causes of the drying of the great Saharan streams, and found that all agreed in saying that these dead rivers once ran full through a country more fertile than the Tell (the region north of the Atlas Mountain's crest), but could only explain it by legends more interesting than satisfactory.
M. Largeau gives the following explanation of the change: "It is known that pastoral people have always been great destroyers of forests, for they need large spaces of clear ground to feed the flocks that form their wealth and to promote security against the wild beasts that lurk in forests. Even now the Algerian Arabs are seen firing the woods to enlarge the narrow limits imposed upon them by colonization. So, although the great Saharan streams have not been explored to their sources, yet it is known that they commence on the bare plateaux that are but the skeletons of heights once wooded and fertile. All accounts of the inhabitants of these regions agree on that point. Consequent upon the destruction of the forests the periodical rains were replaced by rare and short though violent storms, the waters from which, instead of soaking in as in past ages, slip by on the rocky masses, carrying away the rich surface mold, and bring about the drying of the springs, and, as a direct consequence, of the rivers."
An admission of this theory leads the way easily and hopefully to the prophecy of Professor Rohlfs, and raises the question whether it would not be better on all accounts to let the salt waters of the Mediterranean circulate in their own proper bed and pursue the more economical work of conquering the desert by assistance from underground. Nearly all the fluvial network of the Algerian Sahara converges toward the Igharghar. Formed by the confluence of several small streams on the slopes of the Ahaggar, it flows northward, and soon sinks through the light sands and pursues its underground course
