12 STEPPES AND DESERTS. 



western coast of Peru, is believed to be covered with per- 

 petual snow in the gold districts of Machinga and Mocanga. 

 But all these mountains, with the abundant waters to which 

 they give rise, are far remote from the immense Desert which 

 stretches from the southern declivity of the Atlas to the 

 Niger. 



Possibly, however, all the causes of heat and dryness 

 which have been enumerated may have been insufficient to 

 transform such considerable parts of the African plains into 

 a dreadful desert, without the concurrence of some revolution 

 of nature, such, for instance, as an irruption of the ocean, 

 whereby these flat regions may have been despoiled of their 

 coating of vegetable soil, as well as of the plants which it 

 nourished. Profound obscurity veils the period of such an 

 event, and the force which determined the irruption. Perhaps 

 it may have been caused by the great ff rotatory current" 

 ( 24 ) which sends the warmer water of the Mexican gulf over 

 the banks of Newfoundland and to the shores of the old 

 continent, and causes West India cocoa-nuts' and other 

 tropical fruits to reach the coasts of Ireland and Norway. 

 There is still at least at the present time, an arm of this 

 current directed from the Azores to the south-east, which 

 sometimes produces disasters by carrying ships upon the 

 west coast of Africa, which it strikes at a part lined by 

 sand-hills. Other sea coasts (I particularly recall that of 

 Peru between Amotape and Coquimbo) shew that in these 

 hot regions of the earth, where rain never falls and where 

 neither Lecideas nor other Lichens ( 25 ) germinate, centuries 



