402 ASTRONOMICAL CAUSES. 



hemisphere is raised by the equatorial currents, and the 12 

 " only represent the number of degrees that the mean normal 

 temperature of our island stands above what is called the 

 normal temperature of the latitude." 



There is yet another cause to which the present mild tem- 

 perature of Europe is partly due, and which must not be alto- 

 gether neglected. At the period under consideration, indeed, 

 the geography of Western Europe must have been very nearly 

 what it is now. There is, however, good reason for consider- 

 ing that the Desert of Sahara then formed part of the Atlantic 

 Ocean. Mr. Tristram has called attention to cliffs, ancient 

 sea-beaches, and lines of terraces along the northern margin 

 of the desert, and the common cockle is still found living in 

 some of the salt lakes. Mr. Tristram also discovered a species 

 of ffaligenes, which inhabits the Gulf of Guinea, in a salt lake 

 in lat. 32 N. and long. 7 E., separated, therefore, from its 

 present marine habitat by the whole extent of the great 

 Desert. Moreover, as we have already seen, the present geo- 

 graphical distribution of animals can only be explained on 

 the hypothesis that the existing fauna, including man, occupied 

 Africa long before the Sahara became dry land. 



Mr. Croll has shown in the memoir already cited, that cur- 

 rents of warm water produce a far greater effect upon climate 

 than aerial currents of equal volume and temperature ; yet it 

 is evident that such a change would have a great effect on the 

 climate of Europe. At present we receive from the south hot 

 dry winds, which warm us both directly and also indirectly 

 by melting the snow and ice on our mountain-tops. If the 

 Sahara was a sea, the " Eohn," instead of being a burning, dry 

 wind, which strips the snow off the Alps, both by melting and 

 evaporation, would be a moist, damp wind, and when it reached 

 the mountains would produce dense clouds and thick fogs, 

 which would prevent the sun's rays from warming the earth 

 or melting the glaciers. So that to the barren desert of the 



