102 GEOLOGY. 



grains up to even large masses. The softer rocks, as the 

 shales, are broken into small fragments as the water gets 

 into their numerous small interstices ; but in the case of 

 the harder rocks, the water enters cracks and crevices 

 that it finds here and there, and the fragments separated 

 by the frost are of considerable size. It is chiefly from 

 this action of frost on rocks that, at the foot of such rocky 

 fronts as are presented by East and West Rock at New 

 Haven, and by the Palisades of the Hudson River, there 

 are accumulations of fragments of rock of various sizes. 

 Such an accumulation is called a talus. In Fig. 39 one 



of these is represented. 

 You see a difference in 

 size of the stones in dif- 

 ferent parts of this talus. 

 This results from the fact 

 so. that every agitation from 



the falling of the fragments, and the action of water and 

 wind, tends, by friction, to make the fragments smaller, 

 and to remove the smallest of them away from the foot 

 of the rock. Some of them are, in process of time, con- 

 verted into sand, and even fine powder, and these the 

 rains may remove, or the tides, if the foot of the rock be 

 so situated as to be exposed to them. 



186. Chemical Action of Water. Water ordinarily 

 contains dissolved in it both solids and gases, which en- 

 able it to act chemically upon the rocks. For example, 

 the limestone or calcareous rocks are acted upon by wa- 

 ter containing carbonic acid gas in it; and as rain, falling 

 through the air, becomes charged w r ith this gas, which it 

 finds there, water is continually carrying oif calcareous 

 matter from such rocks, supplying thus the sea with ma- 

 terial for the shells of the animals that live in it in such 

 abundance with these coverings upon them. Water thus 

 charged also acts with facility upon the oxyds and sul- 

 phurets of iron which are so often present in rocks, and 

 also upon the alkalies that enter into the composition of 



