72 BACTERIA AND SOIL FERTILITY 



PHYSICAL AGENCIES 



Temperature Changes. — Probably the greatest single factor 

 in rock disintegration is change in temperature, for by it rocks 

 weighing tons are torn from their mooring and then broken into 

 fragments. It is a well-known fact that most substances expand 

 when heated and contract when cooled. It is less well known 

 that the extent of this expansion varies with different substances. 

 Marble expands more on heating than does granite, and sand- 

 stone still more than either. Now, most rocks are composed of 

 a number of different minerals. These when heated expand and 

 contract unequally, thus putting parts under a strain which at 

 times cause cracks of varying size. Even substances of uniform 

 composition if quickly heated and cooled often break. This 

 principle is made use of by the farmer who builds a fire upon a 

 large bowlder and later pours cold water upon it. Extremes of 

 many degrees in temperature may occur in some places within a 

 few hours. 



During the day the rock becomes so highly heated as to be un- 

 comfortable to the touch, while at night the temperature sinks 

 nearly to the freezing point. In regions of such great extremes 

 of daily temperature the splitting of pieces from the parent ledge 

 is sometimes attended with gun-like reports sufficiently loud to be 

 heard at a considerable distance. At the foot of cliffs in such 

 regions are sharp angular flakes varying from small particles to 

 masses of several tons. Many of these are so clear cut that the 

 uninitiated may conclude that they have been torn from the 

 mountain by the hand of man. 



The small crevices produced by the intermittent heating and 

 cooling soon become filled with water, and during the cold nights 

 of autumn this changes into ice. Water on freezing expands 9 

 per cent and may exert a force of 150 tons to the square foot. 

 This is sufficient to tear off huge rocks, which if on the mountain 

 side roll into the valley below where they are slowly ground into 

 smaller and smaller particles. When water enters a very porous 

 rock and then freezes the rock may be quickly disintegrated. 



