16 PHYSIOGRAPHY 



6. Volcanoes affect the land, sometimes building up lofty 

 mountains. 



7. The surface of the lithosphere seems to be rising in some places 

 and sinking in others. This has also been true in the past, for beds 

 of sediment, such as sand and clay, containing the shells of sea 

 animals are found at levels high above the sea. Great changes in 

 the earth's surface have been brought about in this way. 



Before studying the ways in which these various agencies affect 

 the surface of the land, the materials on which they work must be 

 briefly reviewed. 



THE MATERIALS OF THE LAND 



The land is nearly everywhere covered with vegetation. In 

 some places it is dense enough to form a thick mat over the surface, 

 while in others it is meager, or even wanting. The surface well 

 clothed with vegetation is the surface with which we are most 

 familiar; but there are tracts of sand on which little or nothing 

 grows, and cliffs where the rock is bare, save for scattered patches 

 of moss or lichen. In the polar regions, and on lofty mountains 

 also, the land is often covered by thick beds of snow on which there 

 is almost no vegetation. 



Mantle rock. Beneath the vegetation, there is, in most regions, 

 a layer of loose material, composed of clay, loam, sand, gravel, etc. 

 This layer of earthy matter may be a few inches in thickness, or it 

 may be scores or even hundreds of feet deep. This loose material 

 is mantle rock, but it is also known by other names, among which 

 rock waste is in common use. 



The uppermost portion of the mantle rock is called soil. In 

 color, soil may be black, gray, brown, or even dull red or yellow. 

 It may be either clayey and compact, or sandy and porous. In 

 most cases it is made up largely of small particles of mineral or rock. 

 If a piece of any common sort of rock be put into a mortar and 

 ground to powder, this powder will somewhat resemble soil. In 

 addition to the mineral matter, the soil contains more or less partly 

 decayed vegetable (organic) matter. Both the mineral and the 

 organic matter are necessary parts of a good soil, but their pro- 

 portions vary greatly. The mineral matter is usually far in excess 



