WEATHERING 37 



weathering of iron pyrite (FeS 2 ) which frequently occurs dis- 

 seminated through rocks. The first, and usually most conspicuous 

 effect is the appearance of a yellow stain on the rock. If the pyrite is 

 abundant, hydration may cause the rock to fall to pieces as a result 

 of the increase of volume and of the formation of sulphuric acid. 

 Building stones which are uniform in color when first quarried some- 

 times become discolored, after an exposure of a year or more, by 

 blotches of brown stain. Upon examination, it is usually found that 

 the stain was formed from the weathering of small crystals of pyrite. 

 From these blotches the stain spreads, sometimes covering an area 

 of 100 or more square inches. 



4. Carbonation. — By the union of carbon dioxide, derived from 

 the air and soil, with the calcium, magnesium, or iron of complex 

 silicates, soluble compounds are formed which upon being carried 

 away in solution cause the rock to crumble. This is an important 

 cause of the disintegration of granite, although oxidation and hydra- 

 tion are also effective in the same process. If organic acids derived 

 from decaying vegetable matter are present in water, they tend to 

 decolorize red and yellow rocks. Such decolorization can often be 

 seen where water trickles over cliffs. For example, the red cliffs of 

 the Vermilion River in northern Ohio are bleached wherever 

 rivulets trickle over them. This is accomplished by the union of the 

 carbon dioxide with the oxides of iron which gave the red and yellow 

 color to the rock. 



5. Organisms. — Although not agents of the weather, the 

 chemical action of plants and animals should be considered in a dis- 

 cussion of rock disintegration. Certain bacteria are found in great 

 numbers on the surface of bare rock. They live not only in low, moist 

 regions, but even on mountain peaks, where they have been found 

 coating the surfaces and crevices of the rocks. They draw their 

 nourishment from the nitrogen and other compounds brought down in 

 snow and rain. Rocks are attacked by the nitric acid which these 

 bacteria form from the ammonia of the air and water. The chemical 

 action of their excretions makes them an important though incon- 

 spicuous agent of disintegration. Other organisms, such as lichens, 

 mosses, and flowering plants, contribute to the decomposition of 

 rocks. The roots of trees not only pry the rocks apart (p. 33), but 

 they also act chemically by producing carbon dioxide and organic 

 acids, which dissolve the lime and transform the silicates into car- 

 bonates and other products. 



