GEOLOGICAL WORK OF THE ATMOSPHERE 103 



climate is continuously warm than where there are great changes 

 of temperature. Changes of temperature, on the other hand, tend 

 to disrupt rock, and thus increase the amount of surface exposed 

 to chemical change. Since nearly all the chemical changes worked 

 by the atmosphere on the rocks are increased by the presence of 

 moisture, the chemical activity of the atmosphere is greater in 

 moist than in dry regions. 



Though the chemical changes effected by the air are slow, 

 their importance in the course of the earth's long history must 

 have been very great. The amount of rock which has been thus 

 disintegrated probably far exceeds all that is now above the sea 

 (Chapter VII). 



THE ATMOSPHERE AS A CONDITIONING AGENCY 



The most obvious mechanical work of the atmosphere is effected 

 by the wind, but mechanical results conditioned by the atmos- 

 sphere are also effected when the air is still. 



Temperature effects. The disrupting effects of changes of tem- 

 perature have been described briefly (p. 31) and illustrated (Figs. 

 13 and 14). Several conditions determine the efficiency of this 

 process. The greater and the more sudden the change of temper- 

 ature, the greater the breaking, the suddenness of the change being 

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

 than great annual, changes of temperature 1 favor rock-breaking. 

 A partial exception to this generalization should be noted. If 

 the pores and cracks are full of moisture, 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 considerable expansion (about 

 one-tenth) which water undergoes on freezing is brought into play. 

 This may be called the wedge-work of ice. 



The daily range of temperature is influenced especially by (1) 

 latitude, (2) altitude, and (3) humidity. (1) If other things were 

 equal, the greatest daily ranges of temperature would be in low 



1 It should be noted that it is the change of temperature of the rock sur- 

 face, not the change of temperature of the air above it, which is to be con- 

 sidered. 



