176 SUBMERGENCE OF THE SAHARA. [Ch. XII 



The changes alluded to in the " Principles of Geology " as capable 

 of affecting the climates of the globe at successive geological periods, 

 consisted chiefly of the conversion of sea into land and land into sea, 

 the increased or diminished height of mountain chains and conti- 

 nents, and the preponderance of land and water in high and low lati- 

 tudes, together with the new direction given to the principal currents 

 of the ocean, such as the Gulf-stream. But although I did not omit 

 to mention the vast heat which is carried by the winds from the great 

 desert of Africa to those parts of Europe which lie immediately north 

 of it, I was not able to avail myself of a geographical fact since ascer- 

 tained by geologists respecting the Sahara, namely, that this desert 

 must have formed part of the sea when the cold of the glacial epoch 

 was at its height. Kitter had suggested in 181.7, that the African 

 desert had been under water at a very modern period, and M. Escher 

 von der Linth gave it as his opinion in 1852, that if this submergence 

 were true, it would explain why the Alpine glaciers had attained in 

 the Post-pliocene period those colossal dimensions which Venetz and 

 Charpentier, reasoning on geological data, first assigned to them. 

 Since this hint was thrown out by the distinguished Swiss geologists, 

 Messrs. Laurent and Tristram, and in 1863 M. Escher himself, to- 

 gether with MM. Desor and Martins, have found marine shells, espe- 

 cially the common cockle, Cardium edule, scattered far and wide, 

 from west to east, over the desert, while the shells of these and other 

 living species have also been found in boring Artesian wells, at the 

 depth of many feet below the surface. 



The space now occupied by the Sahara, instead of forming a tract 

 of parched and burning sand, from which the south wind or sirocco 

 now derives its scorching heat and dryness, constituted formerly a 

 wide marine area, stretching several hundred miles north and south 

 and east and west. From this area the south wind must formerly 

 have absorbed moisture, and must have been still farther cooled and 

 saturated with aqueous vapor as it passed over the Mediterranean. 

 When at length it reached the Alps, and, striking- them, was driven 

 into the higher and more rarefied regions of the atmosphere, it 

 would part with its watery burden in the form of snow, so that the 

 same aerial current which under the name of the Fohn or Sirocco 

 now plays a leading part with its hot and dry breath, sometimes even 

 in the depth of winter, in melting the snow and checking the growth 

 of glaciers, must, at the period alluded to, have been the principal 

 feeder of Alpine snow and ice. 



METEORITES IN DRIFT. 



As my account of the glacial period has led me to speak at some 

 length of post-pliocene drift, I may take this opportunity of referring 

 to the discovery of a meteoric stone at a great depth in the alluvium 

 of Northern Asia. 



