674 DYNAMICAL GEOLOGY. 



off in slabs. It also owes much of its efficiency to the fact that even 

 the hardest rocks are generally much jointed, that is, full of pro- 

 found cracks, which give the waters a chance to gain entrance and 

 leverage. It has had further help in the frequent alternation of softer 

 strata with the hard ; so that a little hammering at the former, if 

 nearest the water's edge, would bring the latter down in fragments 

 within reach. 



The features resulting from degradation are, for the most part, the 

 same that are described on pages 645, 646, as consequences of denu- 

 dation from river action. 



3. FREEZING AND FROZEN WATER. 



Water performs part of its geological work in the act of freezing, 

 and another part when frozen, in the condition of snow and ice. 



1. Water Freezing. 



Rending and Disintegration from Expansion. — Since fresh water 

 expands as the temperature falls below 39^° F., until it freezes, freezing 

 in the seams of rocks opens those seams, tears rocks asunder, and 

 tumbles fragments and masses down precipices ; or, in porous strata, 

 it crumbles off the surface, and causes disintegration. Consequently, 

 bluffs in a cold climate, like the trap hills of Connecticut and the 

 Highlands of the Hudson, have a long talus of broken stone, made 

 mainly by this means, — while, in a tropical climate, the precipices are 

 generally free from fragments. This kind of degradation goes on 

 incessantly in all icy regions, where there are melting and freezing, and 

 may have originated much of the soil and drift material of the globe. 



2. Ice op Rivers and Lakes. 



Ice, forming along streams in which there are stones, envelops the 

 stones in shallow water, even to a depth of two or three feet, or more 

 in the colder climates. Other stones and earth fall on the ice from 

 the banks. When the floods of spring raise the stream, and break up 

 _ the ice, both ice and stones often float down stream with the current, 

 or are drifted up the banks high above their former level, or are 

 spread over the river-flats. 



Ice sometimes forms about stones in the bottom of rivers, when the 

 rest of the water is not frozen, and is then called anchor-ice. In this 

 condition, it may serve as a float to raise the stones, and to transport 

 them, with the aid of the current. 



The same modes of transportation are exemplified in lakes as in 

 rivers, except that there is less current ; and the stones are mostly set 



