PART I. PHYSICAL GEOLOGY 

 CHAPTER I 



WEATHERING 



Nothing endures. The most indestructible rock will, in time, dis- 

 integrate; the mountain peaks will crumble away, and the rough 

 places will be made smooth. The forces which produce these results 

 are called the agents of weathering. They vary in their effectiveness 

 in different places and at different times in the same place, but under 

 all conditions and at all times some agent is at work, reducing the 

 exposed rock to soil. The rate at which rock weathers depends 

 largely upon two factors: (i) the composition and structure of the 

 rock, and (2) the physical conditions to which it is exposed. A 

 sandstone (p. 36) in which the grains are held loosely together will 

 disintegrate rapidly, while another, in which the cementing material 

 is insoluble and abundant, may have a long life. The effect of dif- 

 ferent physical conditions is obvious. In the dry regions of Mexico 

 and Arizona churches and houses built of sun-dried brick (adobe) 

 have lasted for several centuries ; houses made of a similar material 

 would, in New England, crumble to a mound of clay in the course of 

 a few years. 



Mechanical Agencies 



1. Frost. — The property possessed by water of expanding upon 

 freezing is of great importance in the disintegration of rocks in 

 regions where the temperature falls below the freezing point, since 

 upon freezing it expands one tenth and exerts the enormous pressure 

 of 150 tons to the square foot. This force is well illustrated in 

 the bursting of water pipes in which water has frozen. It is stated 

 that in Finland freezing water is sometimes used instead of powder^ 

 and blocks of stone of 400 tons' weight are broken out in this way. 



All rocks, even the most dense, contain pores and fissures in which 

 water may accumulate. Certain sandstones when weighed, then 



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