SEDIMENTARY ROCKS 



265 



change the chemical nature of the rock, but they change its volume, \ 

 allow some of its material to be carried off in solution, and in many | 

 cases cause it to fall to pieces. The result is decayed rock or 

 one variety of rock waste. It is to be observed that the rock waste 

 which ari>es from decay is unlike the original rock in composition. 





l*'ig. -242. !'. x foliation on the slope of a granite mountain near Royal Arch Lake, 

 Yosemite Quadrangle. (Turner, U. S. (ii-o!. Surv.) 



Some things have been added, and others taken away. In this 

 respect, the waste arising from decay is unlike that arising from rock 

 breaking. 



The products of decay may remain where formed, or may be 

 taken away. If they remain where formed for long periods of time, 

 they may come to make a thick mantle of residual earth. Decayed 

 rock is scores of feet in depth in many places, and hundreds of feet 

 in some. Chemical decomposition is greatest in warm regions, and 

 products of decay are least readily removed where there are forests. 

 The products -of decay are therefore likely to be deepest in warm, 

 forested regions. They are very deep, for example, in some parts 

 of Hrazil. 



SI.DIMKM \lli)\ \M> SI.DIMr:\TARY ROCKS 



Removal of decayed rock. The breaking-up of igneous rock 

 prepares the way for other processes. The loosened material may 

 be blown away by the wind, washed away by running water, or 



