THE OEIGIN OF SOILS 25 



consisted, and subsequently out of the remains of previous 

 rocks of aqueous as well as of igneous origin. It is toler- 

 ably certain that the original crust of the earth has entirely 

 disappeared, and that the igneous rocks of the present day 

 are of more recent origin. The processes are still in opera- 

 tion. Both igneous and aqueous rocks are constantly being 

 formed ; they suffer erosion and denudation, and new 

 aqueous rocks are again formed from the remains of both. 



Erosion of the earth's crust is effected by glaciation and 

 by weathering. 



Glaciation. Glaciers, in their progress, detach frag- 

 ments of rock and carry them down, crushing and grinding 

 them by the weight and movement of the ice. The stones 

 and boulders brought down with the finely pulverised 

 material are usually rounded and scratched. In some cases 

 the detritus has been carried considerable distances and 

 has accumulated in places to a depth of many feet. It is 

 called glacial drift or boulder clay. 



Weathering. The weathering of rocks is caused by 

 changes of temperature, frost, water, carbonic acid, and 

 oxygen acting more or less continuously for long periods 

 of time. 



Changes of temperature cause expansion and contraction 

 of rocks as of other substances, the particles become gradu- 

 ally loosened, and, finally, the forces which hold them 

 together are overcome. It is not any particular tempera- 

 ture neither heat nor cold but the frequent change from 

 one to the other that ultimately causes the rock to crumble. 

 The greatest effects are produced in hot dry climates, in 

 which the widest difference between the temperatures of 

 day and night occur. The formation of sandy deserts in 

 tropical regions is probably due mainly to this cause. In 

 those climates the other weathering agents are compara- 

 tively inoperative. 



Water assumes its maximum density at the temperature 



