NATURE AND FORMATION OF SOILS. 17 



These underground streams perform various 

 kinds of work, such as weakening rocks, dissolv- 

 ing minerals, carving channels, rising in springs 

 or in artesian wells, bringing mineral matter to 

 the surface, and forming caves and making 

 peculiar deposits in them. 



(3) LANDSLIDES are caused by the undermin- 

 ing of masses of rock and soil by water, which 

 produces a slippery surface of bed-rock, and 

 makes it easy for gravity to move an enormous 

 quantity of soil or rock down the declivity. 



(4) LAKES differ from oceans in being (usu- 

 ally) above the sea-level ; in size ; and in the 

 freshness of their waters, provided they have an 

 outlet. Their chief movements are waves pro- 

 duced by winds. These waves often erode the 

 shore. They carry with them and distribute 

 over the bottom of the lake the sediment 

 brought by the rivers, thus making stratified 

 rock. 



(5) THE OCEAN, with its waves, tides, and cur- 

 rents, which constantly beat upon the shore, 

 plays an important role in this great drama. As 

 we have seen, the material transported by the 

 rivers may form deltas and bars, or be widely 

 distributed, according to the strength of the 

 tides and the power of the currents along the 

 shore. 



More than one-half of the rocks have been 

 laid down in the sea and then raised above it. 



