SOIL FORMATION 77 



Carbon dioxid probably plays even a more important role 

 in the weathering of rock than does oxygen. This unites with 

 water which readily carries into solution marble or reacts with 

 the calcium, magnesium or iron of complex silicates with the for- 

 mation of soluble compounds. These on being carried away 

 cause the rock to crumble. This is an important cause of the 

 disintegration of granite. When rocks take up carbon dioxid, 

 as occurs in the true carbonation, they increase in bulk and 

 thereby rupture some of the rocks. 



The sulfur brought to the soil from the atmosphere often 

 amounts to 40 pounds per acre annually* This is oxidized by 

 bacteria to sulfuric acid which readily attacks even the more 

 resistant rocks. 



In addition to this, small quantities of ammonia, nitric acid 

 and other acids are continually being carried from the atmos- 

 phere to the earth. They likewise play their part in rendering 

 soluble the constituents of the rocks. 



Water. — Water is a universal solvent, as is exemplified by the 

 fact that pure water is a chemical curiosity found only in the 

 laboratory where it is obtained with great difficulty. Hence, 

 just as fast as the other weathering agents render the rock con- 

 stituents soluble they are dissolved by the water, and where the 

 rainfall is sufficient they are leached from the soil. It is esti- 

 mated that the rivers of North America carry to the ocean each 

 year 474,000,000 tons of soluble constituents — a quantity suf- 

 ficient to cover one hundred acres to a depth of nearly three thou- 

 sand feet. 



Although pure water slowly dissolves the more insoluble rocks, 

 yet when it becomes charged with carbon dioxid from the at- 

 mosphere or from the decay of organic matter its solvent powers 

 are increased many times. Such water dissolves limestone, gyp- 

 sum, and similar rocks comparatively fast. In limestone regions 

 rocks are often furrowed by rivulets which charged with carbon 

 dioxid have flowed over their surfaces. 



It is estimated that the surface of certain limestone regions 

 has been lowered at rates varying from one inch in twenty- four 



