THE ATMOSPHERE AS A GEOLOGICAL AGENT. 45 



Several conditions^ some of which are connected with the atmos- 

 phere and some with the rock, determine the efficiency of this process. 

 Since the breaking of the rock results from the expansion and con- 

 traction clue to its changes of temperature, it follows that, other things 

 being equal, the greater the change, the greater the breaking; but 

 the suddenness of the temperature change is even more important 

 than its amount. It follows that great daily, rather than great 

 annual; changes of temperature^ favor rock-breaking, though vAih. 

 changes of a given frequency their effectiveness is greater the greater 

 their range. A partial exception to this generalization should be 



Fig. 27. — A ^-eatliered summit of granite in the Wichita ^Mountains,. Oklahoma. (Wil- 



Us, U. S. Geol. Surv.) 



noted. If abundant moisture is present in the pores and cracks 

 of the rock a change of temperature from 45° to 35° (Fahr.) might 

 be far less effective in breaking the rock than a change from 35° to 

 25° in the same time, for in the latter case the sudden and very con- 

 siderable expansion (about one-tenth) which water undergoes on freez- 

 ing is brought into play. This may be called the icedge-icork of ice. 

 The daily range of temperature is influenced especially by latitude, 



^ It should be noted that it is the change of temperature of the rock surface, not 

 the change of temperature of the air above it, which is to be considered. Many data 

 concerning temperature changes are to be found in Bartholomew's Atlas of Meteor- 

 ology. 



