CONDITIONS FAVORING THE FORMATION OF DESERTS. 167 



Avinds. The same is the case with the desert portion of Arabia ; and these 

 accumulations of sand seem to have come from the disintegration of sand- 

 stones, with which the areas in question were oi'iginally underhiin. 



Another cause of the formation of really desert areas is the drying up 

 of lacustrine areas covered by very saline water. In such cases the dry 

 bottom of the old lake is at first too strongly impregnated with mineral mat- 

 ter to allow any vegetation to take root and flourish. A long exposure to 

 atmospheric influences would, in such cases, be required to enable the soil 

 to get rid of its excess of saltness ; and as such cases of desiccation must 

 necessarily be confined to regions of small rain-fall, it will be easily under- 

 stood that such desert areas must, as an ordinary thing, remain unoccupied 

 by vegetation for an indefinite period. 



Since moisture is essential to the vigorous growth of trees, so that very 

 dry regions are not, as a general rule, covered by forests, it will not be 

 difficult to understand why treeless areas are usually found in the interior 

 of tlie great continental masses, as is so well illustrated by the position of 

 the plains of North America and that of the pampas and llanos of the south- 

 ern division of the New World. As will be more fidly explained farther on, 

 the edges of the continents are the regions where the larger portion of the 

 rain-fall, on the land, takes place. To this rule there are but few exceptions ; 

 the most striking one is the existence of a rainless belt along a considerable 

 part of the west coast of South America, a condition of things chiefly depend- 

 ent on the position of the chain of the Andes, in that region, with reference 

 to the trade-winds. 



Besides excessive dryness and cold, there is another cause which is effec- 

 tive in preventing a natural growth of forests over certain areas, which are 

 not unfrequently of very considei-able extent. This condition sometimes 

 occurs independently of other agencies, so that certain regions remain tree- 

 less when precipitation is abundant, and the temperature conditions perfectly 

 fiivorable to the growth of arboreal vegetation. In other places the cause in 

 question is more or less effective in combination with some other condition 

 tendincr to brino; about the same result. It is the mechanical texture of the 

 soil, and especially its fineness, to which allusion is here made. In the 

 region of the so-called prairies of the ^ILssissippi Valley, especially, there are 

 very large areas where the rain-fall is ample, but where, over a large portion 

 of the surface, trees iire wanting, their place being supplied by a vigorous 

 growth of grasses. Here, moreover, it is evident enough that temperature 



