NATURE AND FORMATION OF SOILS. 19 



into a solid state, expands about one-eleventh of 

 its original bulk. This expansion exerts an ir- 

 resistible force, as is seen in the bursting of 

 iron pipes, the cracking of the rocks into blocks, 

 or shattering them into fragments, thus increas- 

 ing their exposed surfaces many-fold, and ex- 

 posing them to the action of other forces. 



EXERCISE i. Let the student calculate the area of 

 the exposed surface of a cubic foot of rock (a) before, 

 and (b) after it has been broken up into cubic inches. 

 (c) Compare a and b. 



Another effect of the freezing of rock is to 

 cause the fragments to " rise slightly at right 

 angles to the inclined surface, and each thawing 

 produces the reverse movement " * under the 

 influence of gravity. Consequently, they slowly 

 "creep" down the declivity (Fig. 5). 



(7) ICE. The ice of a stream expands with 

 great force, pushing against the bank. It holds 

 in its mighty grasp all loose stones, boulders, 

 and trees along the banks, and when it breaks 

 up transports them to great distances. 



If the student has an opportunity, let him 

 watch the breaking up the ice in a river, or even 

 in a smaller stream, and see with what wonder- 

 ful force the great blocks of ice with their bur- 

 dens are crushing each other to pieces in the 

 fury of a spring torrent. Iron bridges are of ten 

 swept away by the enormous pressure. 



* Scott's Geology, p. 82. 



