156 ELEMENTARY GEOLOGY 
each rain is reserved as a permanent supply for the 
rivers; and so for their perennial supply, streams de- 
pend in great degree upon springs. 
Where forests have been removed, the water flows off 
more readily, and less is left to be supplied from the 
springs after the rains are over. Hence in forested 
regions, rivers are moderately permanent in size and 
volume, while in those sections which have been de- 
forested, they swell to violent torrents after every 
heavy rain, and then, if small, may quickly become 
mere dry channels. 
Chemical Action. — Although it was once believed 
that valleys were formed by other means, and then 
occupied by rivers, we now know that the streams 
have shaped their own valleys. Most of the river 
valleys in the world have been carved by the action 
of the waters that occupy them, aided by weathering. 
They do this work by the combination of two different 
processes, — chemical and mechanical. 
River water is always at work in a chemical way; ~ 
and just as underground water dissolves minerals in 
its passage through the rocks, so the surface water, as 
it flows over its rocky bed, is always taking some 
mineral matter. into solution. The amount thus dis- 
solved depends partly upon the materials that the 
water carries, and partly upon the rock over which it 
flows. When very impure with organic materials, the 
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