WEATHERING 



35 



Fig. io. — Limestone bowlder channeled by water 

 containing carbon dioxide. 



Chemical Agencies 



The chemically active gases of the atmosphere are oxygen and 

 carbon dioxide. Un- 

 less they are dissolved 

 in water, however, 

 their effect in the 

 weathering process is 

 unimportant, but in 

 the presence of both 

 moisture and heat 

 they accomplish a 

 great part of the work 

 of chemical disinte- 

 gration. It is evident, 

 therefore, that the 

 chemical decomposi- 

 tion of rocks must 

 vary greatly in effec- 

 tiveness in different 

 places and at different times in the same place, and we find that it is 



most active in moist, 

 tropical regions, less 

 rapid in temperate 

 regions, and least im- 

 portant in the frigid 

 zones and in arid 

 regions. 



i. Solution. — Pure 

 water is a poor sol- 

 vent, but when it 

 contains a consider- 

 able quantity of car- 

 bon dioxide its sol- 

 vent power becomes 

 greatly increased, so 

 that limestone, gyp- 

 sum, and other easily 

 soluble rocks are 

 slowly taken up by 



Fig. 



Joints in limestone, widened by solution. 

 (Photo. H. L. Fairchild.) 



