WEATHERING 25 



Hardin County, Illinois, a lake varying in size from 100 to 400 

 acres was produced by the stopping of ttie sinkhole outlets. 



The process of solution forms soil material by the removal of 

 soluble substances, as in the case of limestone, leaving the impuri- 

 ties, or as in sandstone, by taking out the cementing material, leav- 

 ing the incoherent sand, and in the case of igneous rocks removing 

 some of the potash, soda, lime, magnesia, or some other compounds, 

 and leaving a residue more or less modified as soil-forming material. 



From the amount of lime carbonate carried by the Thames River 

 it has been estimated that the average amount of this material dis- 

 solved from the limestone area drained by this stream is 1-43 tons 

 per square mile in one year. 9 It is estimated that on the average 

 something like one-third as much matter is carried to the sea 

 in solution as in the form of sediment, and that by this 

 process alone land areas would be lowered something like one 

 foot in 13,000 years. 10 



(g) Plants. The roots secrete acids that attack the rocks and 

 aid solution. The roots of a plant growing on a polished marble 

 surface removed the polish by acid from the roots showing the action 

 of the acids on the rock. While this action in the case of a single 

 root is very slight, yet it plays a rather important part in aiding 

 decomposition because of the infinite number of roots coming in 

 contact with the soil particles, and their long-continued action. 

 This may be shown where the surface^of stones are covered with 

 lichens. Enough rock is broken down to give higher plants, such -is 

 ferns, a chance to grow, and these in turn by the action of their roots 

 and other agencies produce more soil material that encourages still 

 higher plants to grow. These plants hold the material in place and 

 allow sufficient accumulation to form soils. The action of the roots 

 of plants on the minerals in soils is very important while they are 

 alive, and even when they decay they aid materially in the solution 

 and liberation of plant food and* decomposition of rocks. 



(h) Animals. Many animals burrow in the soil, and their 

 action on the minerals tends to aid decomposition and disintegration. 

 This is especially important in the case of earthworms, ants, and 

 similar animals. Many of these carry vegetable matter into the soil, 

 which, by its decomposition, aids in the breaking down of minerals. 

 Earthworms pass large quantities of soil through their bodies, 

 the minerals of which are acted upon by the acids in the alimentary 

 canal ami partly decomposed. Kven the larger rodents, such as 

 gophers, ground squirrels, and mice exert considerable influence in 



