114 GEOLOGY. 



The importance of weathering in the general processes of erosion 

 is shown in many ways. In regions where the mantle rock is the prod- 

 uct of the decay of the sohd rock beneath, and such regions consti- 

 tute a large portion of the earth's surface, the soil and subsoil represent 

 the excess of weathering over transportation. Since most of the earth's 

 surface is covered with soil to a greater or less depth, it is clear that, 

 on the whole, weathering keeps ahead of transportation. Again, 

 it is clear that the loosening of rock by weathering greatly increases 

 the erosion which a given amount of moving water can accomplish. 

 Not only this, but weathering plays a much more important role in 

 the development of valleys than is commonly realized. This is best 

 illustrated by the valleys of young swift streams. The valley which 

 is not at its top ten times as wide as its stream is rare. The stream 

 which has such a canyon has been cutting chiefly at its bottom. Ignor- 

 ing its lateral corrasion, which is shght, the valley which it would cut 

 would have a width equal to its own. This is illustrated by Fig. 96. 



.ocf- 



FiG. 96. — Diagram of a valley the top of which is ten times the width of 



the stream. 



Weathering in its broadest sense is largely responsible for the width of 

 such a valley, in so far as it exceeds the width of the stream. The 

 work of weathering, slope wash, etc., has been to get the material which 

 originally lay between a, h, and c down to the stream. The stream has 

 then carried it away. The above illustration would not apply to old 

 and sluggish streams, for they, by their meandering, widen their valleys 

 independently of weathering. 



Weathering is a part of erosion, but only a part. In so far as it 

 is effected by solution the process involves the transportation of that 

 which is dissolved to some other point. Transportation is also in- 

 volved to some extent in the other processes of weathering, but the 

 central idea of the processes embraced under this term is the loosening 

 and disrupting of rock by which it is prepared for transportation. 



