36 PHYSICAL GEOLOGY 



it and carried away. Water obtains its supply of carbon dioxide 

 from the air, from the decay of plants and animals, and from 

 subterranean sources (p. 296). It has been estimated that the 

 surfaces of certain limestones in England have been lowered at 

 rates varying from one inch in 24 years to the same amount in 500 



years. Although solu- 



,..] ,___; ...1 r .j r _i ,__L... tion is most conspicu- 



....... J — , — j__ ...... .!___., : . i...,.._;.. A ous iy exhibited in 



limestone regions, 

 B where the rock is 



' 1 ' I ' ' 1 { 1 »"■' h C often furrowed by the 



Fig. 12.— The formation of the residual soil B from rivulets (Fig. io) 

 the limestone A is shown. The soil was derived from which flow over the 

 the limestone by the removal of the soluble portions and n 1 .1 • • 



the concentration of the insoluble. Large areas of Ken- surtace > ana tne joints 

 tucky and Virginia owe their fertility to this process. and Other cracks are 



widened by its action 

 (Fig. 11), it is also effective on feldspar and even on quartz. 

 Sandstones with calcareous cements are disintegrated by the solu- 

 tion of the cement, causing the rock to fall to pieces and form sand. 

 In regions of impure limestone the insoluble residue, such as clay and 

 flint nodules (p. jj), will be left, covering the unweathered rock 

 (Fig. 12). The depth of this cover often gives a basis for estimat- 

 ing the thickness of limestone which has been dissolved and carried 

 away. Many caves are formed by solution (p. 70). 



2. Oxidation. — Oxygen is effective only on rocks which contain 

 minerals capable of taking up further oxygen and thus forming new 

 compounds. The most important of these are iron compounds, and 

 to them the red and yellow coloring, so conspicuous in rocks, is due. 

 If oxygen alone is added to the iron molecule, a red color (Fe 2 03) 

 results ; if moisture is present, however, the brown or yellow rust 

 (hydroxide), common in moist regions, is formed. One noticeable 

 result of oxidation is an increase in volume; this being the case, the 

 newly formed and bulky minerals crowd the grains of the rock apart 

 and tend to produce disintegration. Complex silicates, such as feld- 

 spar, mica, and hornblende, are attacked by oxygen and carbon diox- 

 ide, and reduced to simpler and more stable compounds. 



3. Hydration. — The union of water with chemical compounds is 

 known as hydration, and is very important in weathering. An im- 

 portant effect is the increase of the volume of the mineral acted upon. 

 1 he operation of hydration and oxidation is well illustrated in the 



