16 



PHYSICAL GEOLOGY 



are more important from the geological standpoint, since 

 they are used by various forms of ocean life for the construc- 

 tion of their shells. The shells of marine organisms have 

 frequently been embedded in the sediments derived from the 

 land, and their remains or the impressions they made (fossils, 

 Fig. 2) constitute an important, though imperfect, record of 

 the life which existed at the time and place the sediments were 

 accumulated. 



The lithosphere. The lithosphere, as the name implies, 

 is composed of rock so far as known ; it is the solid portion of 

 the earth. As the science of geology deals very largely with 

 rocks in one aspect or another, it is essential to study them 

 and their arrangement in some detail. 



THE MATERIALS OF THE LITHOSPHERE 



The mantle rock. Loose, earthy material covers most 



of the land. When capa- 

 ble of supporting plant 

 life, this is called soil. 

 The earthy matter of soil 

 is usually mixed with 

 partly decayed vegetable 

 matter, and then is often 

 dark-colored, even black. 

 Soils are generally com- 

 posed of sandy, clayey, 

 or limy particles, or of 

 combinations of these in 

 any proportion. In ex- 

 cavations for cellars, in 

 railroad cuts, or in other 

 exposures, it may often 

 be seen that the soil gives 

 place below to material which, though loose, is commonly 

 coarser, more compact, and of different color. This is the sub- 



Fio. 3. Decaying granite and resulting 

 rock waste. The granite is cut by a 

 dike (p. 49). Southeastern Wyoming. 



