CHAPTER XI 

 THE ATTACK OF THE WEATHER 



The two contrasted processes of weathering. It has already 

 teen pointed out that change and not stability is the order of 

 nature. Within the earth's outer shell and upon it rock altera- 

 tion goes on continually, and from some portions of its surface the 

 changed material is as constantly migrating to neighboring or 

 even far distant regions. Before such transportation can begin 

 the hard rock must first be broken down and reduced to fragments 

 which the transporting agencies are competent to move. 



To accomplish this breaking down, or degeneration, of the rock 

 masses, either a wide range in temperature or chemical reaction is 

 essential. In the atmosphere are found such active chemical 

 agents as oxygen and carbon dioxide, the so-called carbonic acid 

 .gas; and these agents in the presence of water react chemically 

 with the minerals of the rocks and form other minerals such as the 

 hydrates and carbonates, which are lighter in weight and more 

 soluble. This chemical attack upon the outer shell of the litho- 

 sphere is described as decomposition. 



On the other hand the rock may succumb to changes which are 

 purely mechanical and are due either to the stresses set up by dif- 

 ferences between surface and interior temperatures, or to the prying 

 action of the frost in the crevices. Such purely mechanical de- 

 generation of the rocks is in contrast with decomposition and is 

 described as disintegration. The two processes of decomposition 

 and disintegration may, however, go on together ; and the changes 

 of volume that are caused by decomposition may result directly 

 in considerable disintegration, as we are to see. 



The r61e of the percolating water. In order to effect chemical 

 change or reaction, it is essential that the substances which are 

 to react must be brought into such intimate contact with each 

 other as it is seldom possible to attain except by solution. The 

 chemical reactions which go on between the gaseous atmosphere 

 and the solid lithosphere are accomplished through solution of the 



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