THE ATMOSPHERE AS A GEOLOGICAL AGENT. 55 



solid rock. Weathering by mechanical means tends to produce material 

 which, though in a finer state of division, is still like the original rock 

 in chemical composition. AVeathering by chemical means tends to 

 produce a mantle made up chiefly of the less soluble parts of the rock 

 from which it was derived. All processes of weathering prepare material 

 for transportation by vrind and water. 



Many considerations determine the thickness which the mantle of 

 weathered rock (mantle rock) attains. Some of these considerations 

 have to do with the atmosphere, and some T\dth drainage. Since the 

 latter are, on the whole, more important, this matter will be discussed 

 in coimection with the work of water (Chapters III and IV). 



