THE COMPOSITION OF THE EARTH 



17 



soil. The soil and subsoil have been called mantle rock, since 

 they form a covering or mantle for the underlying rock, which 

 is usually solid. Since the loose mantle rock is formed by the 

 decay and breaking up of solid rock, it is also called rock 

 waste (Fig. 3). Soil which remains above the solid rock from 

 which it was derived is residual soil, in contrast to transported 

 soil, which has been brought from its place of origin to its 

 present situation by some of the agents which transport 

 materials on the sur- 

 face of the earth. 

 Such soils when de- 

 posited by rivers are 

 alluvial soils, and 

 when accumulated 

 by the wind, eolian 

 soils. Much of the 

 mantle rock of Can- 

 ada and of the north- 

 ern part of the United 

 States was brought 

 to its present posi- 

 tion by the continen- 

 tal glaciers which 

 once covered the re- 

 gion. This ice-trans- 

 ported material is 

 called drift. The 

 mantle rock ranges in 

 thickness from inches 

 to scores and, in exceptional cases, hundreds of feet. 



Classes of rocks. Any considerable amount of mineral 

 matter that has been brought together by natural means 

 constitutes rock. A rock may contain material of one kind, or 

 of several kinds, and may be loose, like sand, or solid, like 

 granite. Popularly, one does not speak of sand or clay as 

 rock, but thinks only of the solid rocks as such. 



B. & B. GEOL. 2 



FIG. 4. Igneous rock. El Capitan, Yosemite 

 Valley. 



