n] FEATURES OF ENVIRONMENT 33 



The disintegration of the rocks goes on quickest when 

 they are subject to great and frequent variation of 

 temperature. The sun heats and expands, the cold 

 of the nights contracts ; cracks are caused, water gets 

 into them, freezes and bursts them, etc. Every river 

 carries some sand. The sand of the seashore is 

 nothing but the ground down mountains of the far 

 inland, carried to the sea by the rivers. A look at 

 the map of Central Asia, Central Australia, or the 

 Sahara shows many river courses which run inland 

 and never reach the sea ; they either end in a land- 

 locked lake, or they lose themselves in the sand, all 

 of which they themselves have helped to carry down. 

 They are burying themselves. The more sand, the 

 more dust, and enormous dustclouds sweep over the 

 country forming shifting dunes, or they deposit 

 other so-called aeolic formations, for instance the 

 loess of China, a kind of loam, verv fertile if irri- 



W 



gated. But the greater the masses of sand, the less 

 becomes the rainfall and the making oT deserts 

 becomes intensified. Sometimes existing outlets are 

 barred by a slow steady upheaval of neighbouring 

 tracts of country, which are then doomed to become 

 deserts. The lakes of deserts are almost invariably 

 alkaline, salty or bitter because the rivers wash the 

 salt, saltpetre and other soluble mineral matter out 

 of the mountains, thus rendering the lakes increas- 

 ingly unfit for life. 



G. 3 



