1884 .] Geological Structure 0/ the Sahara. 543 
a moist climate, which probably continued until the begin- 
ning of the present epoch. 
The characteristic formation of the surface of the Desert, 
the elaboration of many dry valleys, the formation of basin- 
shaped depressions, the origin of steep banks, insulated 
mountains, &c., are due to the erosive aCtion of fresh 
water. 
The sand of the Desert has been produced by the decom- 
position of sandstone, which predominates everywhere in 
the middle and southern Sahara. Its distribution and accu- 
mulation in dunes has been effected by the wind. 
The salt-marshes, and the saline and gypsiferous superfi- 
cial deposits, have been formed by the evaporation of waters 
which had collected in the hollows. There is no proof of 
any essential change in the climate of the Sahara during 
the historical period. 
