RELIEF FEATURES 



17 



of the organic. That part of the mantle rock which is properly 

 called soil ranges from a few inches to a few feet in thickness. 



The distribution and prosperity of the people of the earth often 

 bear a very direct relation to the fertility of the soil. The fertile 

 blue-grass region of Kentucky was the first extensive area to be 

 settled in the Ohio Valley; its inhabitants have always been pro- 

 gressive and well-to-do. The hilly land to the east was slowly 

 occupied by a spare population, condemned by a less fertile soil to 

 relative poverty. 



Where the mantle rock is thicker than the soil, the soil grades 

 down into earthy matter of somewhat different composition, known 

 as subsoil. Between the two there is no distinct separation. Like 

 the soil, the subsoil contains both mineral and organic matter, 

 though the latter is less abundant than in the soil. Only the larger 

 roots penetrate the subsoil in great numbers. The thickness of the 

 subsoil is much greater than that of the soil in some places, but on 

 the other hand, it is sometimes absent altogether. 



Solid rock. Beneath the subsoil is solid rock, which extends 

 down* to the lowest accessible depths, and doubtless far beyond. 

 It is probable, indeed, that the earth is solid to the core. 



Classes of solid rock. The solid rocks of the earth are of many 

 kinds. They differ from one another in color, in strength, in tex- 



Fig. 10. Stratified rock. Trenton Limestone, Fort Snelling, Minn., covered 

 by a thin layer of soil. (From photo by. Calvin.) 



ture, in composition, in origin, etc., but for our purpose the common 

 rocks may be grouped into two great classes; namely, those which 

 are in layers, called stratified rocks (Fig. 10) ; and those which are 

 not in layers, or unstratified rocks (Fig. 2 ; PI. IV, p. 7) . 



