GEOLOGICAL ACTION OF WATER 41 



But there is still another way by which rock is changed 

 to soil. Water soaks into cracks and between the layers. 

 When this water freezes it expands, and often breaks off 

 large pieces of rock. Water also soaks into most rocks as 

 it does into a sponge. Some of the rock is dissolved like 

 salt and carried away, and by and by the hard rock crumbles 

 into soil. You can see this decay of rocks wherever rock is 

 exposed to the atmosphere and to rain. By this slow decay 

 and tvearing down of rocks soil is made. A large part of the 

 soil in the Mississippi basin has been made in this way from 

 rock in the Alleghanies and the Kocky Mountains. 



Have you seen buildings or bridges where the rock is 

 wearing away ? 



You see that rock sticks out on both sides of the river, 

 and it extends for many, many miles under the loose soil 

 into the country. In some places it comes to the surface, 

 sometimes we strike it not more than ten to twenty feet 

 below, at other places it lies at a depth of one or several 

 hundred feet ; but we should find it everywhere if we could 

 go down deep enough. 



We have just learned, then, that nearly all of the interior of 

 the earth is composed of solid, hard rock. The loose soil com- 

 pared with the whole mass of the earth is like a thin layer of 

 dust covering a large globe. From observations made on 

 high mountains and in deep mines, people have learned 

 something about the rocks for a depth of about ten 

 miles. 



You have learned now how soil is formed by water, and 

 how ice helps to split up the rock. Some day I must tell 

 you what an important part large ice fields, called glaciers, 

 have played in the formation of soil in our Northern 

 States. 



Do all grasses furnish equally good hay and pasture ? 



