176 THE LURE OF THE LAND 



the nitrogen of the air and form nitric acid, known 

 under the common name of aqua fortis, which has a 

 vigorous solvent action on many kinds of rock. Car- 

 bonic acid arising from the burning organic matter and 

 from the lungs of animals and preexisting as a mineral 

 substance also played an important part in rock decay. 

 " In the process of further cooling, ice was formed, 

 and this also tended to have a disintegrating influence. 

 Water in passing into ice increases in volume, and this 

 tends to break and disintegrate many bodies. Rock 

 saturated with water thus tends to break up when the 

 water becomes ice. During the period of the ice age 

 when large glaciers moved over much of the earth's sur- 

 face, the crushing and grinding effects of the ice had 

 much to do with disintegrating the rock. The vast 

 areas of glacial drift which form the soil of many of our 

 Central Western States are evidences of the gigantic 

 scale on which these ice mills of the gods slowly ground 

 the stones of the earth into soil. When the soil is 

 formed by the decay of rocks without the transporting 

 action of water or ice being active, the soils are said 

 to be formed in situ. When the products of soil dis- 

 integration are carried by water and deposited along 

 the banks of the streams or at their mouths, the soil is 

 called alluvial. When products of rock disintegration 

 are carried by moving ice and deposited therefrom, they 

 are called glacial drift. When they are carried by 

 wind, as is often the case, they are called seolian soils. 

 The above are some of the varieties of soils as deter- 

 mined by their method of formation. Soils are also 

 classified in regard to their chemical characters; as, 

 for instance, when formed from the decay of carbonate 

 of lime, they are called limestone soils. When arising 

 from the disintegration of granite, they are called 



