34 GEOLOGY 



volume of the mineral matter. The result is that the rock affected 

 crumbles. Thus the iron rust formed on a knife blade crumbles 

 off. So the iron rust formed when oxygen and water unite with 

 the iron in the rock, causes the rock in which the change takes 

 place to crumble, partly because of the expansion involved. 



Again, some of the simple compounds, especially the carbonates, 

 formed when the rock decays, are somewhat soluble and may be 

 dissolved and taken away. This tends to make the rock less com- 

 pact by taking away one of its ingredients. 



Oxidation, carbonation, and hydration therefore not only 

 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 arises from decay is unlike the original rock in composition. 

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

 respect, the waste sediment arising from decay is unlike that 

 arising from rock breaking. 



The products of decay may remain where they are formed, or 

 they 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. The decayed rock is scores of feet in depth in many places, 

 and hundreds of feet in some places. Chemical decomposition 

 is greatest in warm regions, and the 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 Brazil. 



SEDIMENTATION AND SEDIMENTARY ROCKS 



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

 prepares the way for other processes, for the loose material which 

 results from the disruption may be blown away by the wind, washed 

 away by running water, or moved by any agency which shifts 

 loose materials about on the surface of the earth. If the products 

 of rock disintegration are coarse, they may become gravel after 

 being rounded by streams or waves. If the material is finer, say 



