THE ATMOSPHERE AS A GEOLOGICAL AGEXT. 43 



Carbonation. — The production of lime carbonate from rock con- 

 taining calcium compounds, but not in the form of carbonates, is know^i 

 as carhonatio/i , and is one of the important chemical changes effected by 

 the carbon dioxide of the atmosphere in cooperation vnth. water. In 

 the process of carbonation the original minerals of complex com- 

 position are decomposed and simpler ones usually formed. Volumetric 

 changes are involved, which often lead to the disruption of the rock 

 (see Ground water). Furthemiore, carbonates are among the more 

 soluble minerals, and their production therefore brings some of the 

 rock materials into a soluble condition, and their extraction through 

 solution tends still further to disintegrate the rock. The carbonation 

 of crystalline rocks is therefore a disintegrating process, and will be 

 considered further in its many concrete applications. 



Other chemical changes. — A third chemical process which often 

 accompanies oxidation and carbonation is hydration. This is effected 

 by water rather than l^y air, and will be considered in that connection. 

 In general it leads to the disintegration of the minerals and rocks 

 affected. The chemical effects of nitric acid, etc., developed through 

 the agency of atmospheric electricity, and the corresponding effects of 

 the gases and vapors which issue from volcanoes, many of them chem- 

 ically active, are to be mentioned in this connection. 



Conditions favorable for chemical changes. — Conditions are not 

 everywhere equally favorable for the chemical work of the atmosphere. 

 In general, high temperatiu'es facilitate chemical action, and, other 

 things being eciual, rocks are more readily decomposed by atmospheric 

 action in warm than in cold regions. Chemical activity is probably 

 greater where the 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 amomtt 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 drA' reo-ions. 



B. THE ATMOSPHERE AS A COXDITIOXIXG AGEXCY. 



The most obvious mechanical work of the atmosphere is effected 

 by the ^ind, but mechanical results of great importance, conditioned 

 by the atmosphere, are also effected when the air is still. 



