104 PHYSICAL GEOLOGY 



the water vapor in the air, or of water after it has fallen as 

 rain, with rock material, constitutes hydration. 



Oxidation, carbonation, and hydration are all factors in 

 the decay of rocks. All three involve an increase in the 

 volume of the rock affected, and, unless something is with- 

 drawn simultaneously in solution, the resulting pressure tends 

 to make it crumble. The products of the changes, as in the 

 cases of the iron and calcium carbonates mentioned above, 

 may be soluble, and are likely to be carried away by waters 

 percolating through the rock. Their withdrawal tends to 

 increase the porosity of the rock, thereby weakening it. One 

 of the most important chemical reactions attending the decay 

 of igneous rocks is that by which orthoclase, acted upon by 

 water and carbon dioxide, yields kaolin (p. 22). It may be 

 expressed as follows : 



Orthoclase +Water + Carbon dioxide = Kaolin + Potassium Carbonate + Quartz. 



2 KAlSi 3 O 8 + 2 H 2 O + CO Z = H4Al 2 Si 2 O 9 + K 2 CO 3 + 4 SiO 2 . 



SUMMARY 



The most important phases of the geological work of the 

 atmosphere are the following : (1) Its work as an agent of 

 weathering. Through its effect upon changes in temperature 

 it influences (a) the wedge work of ice, and (6) the splitting 

 of rocks by their expansion and contraction. The oxygen, 

 carbon dioxide, and water vapor of the air unite chemically 

 with various rock substances, and, by so doing, contribute to 

 their decay. These processes prepare materials for removal 

 by various transporting agencies. (2) The transportation and 

 deposition of fine material by the wind. Although most ex- 

 tensive in arid regions, this work has affected, first and last, all 

 land surfaces. Its aggregate effect is to lower the lands and 

 build up the ocean bottoms. (3) The abrasion of rocks. 

 This is most important in deserts, where the atmosphere is 

 often the chief agent of degradation. (4) By controlling the 

 conditions of evaporation and precipitation, the atmosphere 



