126 MOSTLY MAMMALS 



aqueous formations ; and, secondly, even supposing that 

 they had been so deposited, they would almost certainly 

 have been washed away as the land rose from beneath 

 the sea. Then, again, we do not meet with marine shells 

 in the desert-sands, of which at least some traces ought 

 to have been left had they been marine deposits of com- 

 paratively modern age. 



Whether or no the subjacent strata have ever been 

 beneath the ocean, it is absolutely certain that the sands 

 of all the great deserts of the world have been formed in 

 situ by the disintegration of the solid rocks on which they 

 rest, and have been blown about and rearranged by the 

 action of wind alone. All deserts are situated in districts 

 where the winds blowing from the ocean's surface have 

 to pass over mountains or extensive tracts of land, which 

 drain them more or less completely of their load of 

 moisture. Hence, in the desert itself, when of the typical 

 kind, little or no rain falls, and there is consequently 

 no flow of water to wash away the debris resulting from 

 the action of the atmosphere on the rocks below. 



In other words, as has been well said, desert-sands 

 correspond in all respects, so far as their mode of origin 

 is concerned, to the dust and sand which accumulate on 

 our high roads during a dry summer. On our highways, 

 indeed, the summer's dust and sand are removed by the 

 rains of autumn and winter, only to be renewed the following 

 season ; but in a desert no such removal takes place, and 

 the amount of sand increases year by year, owing to the 

 disintegration of the solid rock here and there exposed. 



Only one degree less untrue than the idea of their 

 submarine origin is the notion that deserts consist of 

 unbroken tracts of sand. It is true that such tracts in 

 certain districts may extend on every side as far as the 



