THE WORK OF RUNNING WATER. Ill 



which the letters were cut has weathered away. Some of it has 

 crumbled off as the result of the expansion and contraction induced 

 by changes of temperature, and some of it has been dissolved by the 

 rain which has fallen upon it. In this case the weathering is effected 

 partly by the atmosphere and partly by w-ater. These are the chief, 

 but not the only agents concerned in the general processes of weather- 

 ing. Those phases of weathering which are the result of the activities 

 of the atmosphere, whether physical or chemical, have been discussed 

 in connection with the atmosphere (pp. 42 and 54). 



The rain which falls upon the surface of exposed rock, and that 

 Tvhich sinks through the soil to the solid rock below, dissolves, even 

 if slowly, some of the rock constituents. Each constituent of a rock 

 composed of several minerals may be looked upon as a binding material 

 for the others. When one is dissolved the rock crumbles, much as 

 mortar does when the lime which cements the sand is dissolved. 



The solution of mineral matter by ground water, as well as the 

 other chemical changes it effects, is greatly augmented by the impuri- 

 ties, especially carbonic and other organic gases, dissolved by the 

 water from the atmosphere and the soil. The commonest chemical 

 changes effected by the joint action of water and air, oxidation and 

 carbonation, have been referred to in Chapter 11. Hydration is more 

 exclusively the work of water, and is one of the commonest processes 

 of rock change, and often of rock disintegration. Numerous other 

 less simple chemical changes resulting from the activities of ground 

 water are constantly in progress, and in so far as they lead to the dis- 

 integration of rock are processes of w^eathering. Many chemical 

 changes involve notable changes in volume of the mineral matter 

 concerned. Merrill has calculated that in the conversion of the gra- 

 nitic rock of the vicinity of Washington, D. C, into soil, its volume has 

 been increased 88 percent., largeh^ as the result of hydration.^ Even 

 when the chemical changes do not themselves directly involve the 

 disintegration of the rock, the accompanying increase of volume is 

 sometimes sufficient to cause its physical disruption. This also may 

 be regarded as a phase of weathering. 



The weathering accomplished by water, or under its influence, 

 proceeds at rates which vary with the composition of the rock, the 

 amount and composition of the water, the temperature, and certain 



^ Jour, of Geol., Vol. IV, p. 718. An excellent summan^ of the principles of Rock 

 Weathering. 



