LAND AM) WATER. 35 



some saudy regions, lui'g'e rocks lia\e l»een partly worn 

 away by the sand which has for (centuries been blown 

 across their surface;. Whole hills have, in some in- 

 stances, been thus worn away and deposited in other 

 localities. 



The Composition of Soil. — The soil of the eaith may 

 he considered as composed of four different classes of 

 substances : — 



1. Fiitclij Divided Particles of itccA". —Crumbled rock, 

 or sand, constitutes the bulk of many varieties of soil. 

 Even soft, peaty soil will be found, upon careful exam- 

 ination, to contain a greater or less })ercentage of g'ritty 

 substance. 



2. Decaying Vegetable Matter. — Nearly all kinds of 

 soil contain more or less of a soft, pasty, dark-colored 

 substance called humus, or vegetable mold. The grass, 

 leaves, and falling trees, which are continually accumu- 

 lating upon the surface of the soil, and are mixed with 

 it by cultivation, furnish a perjjetual source of this hu- 

 mus. It forms a large part of the peaty soil found in 

 low places. In peat or muck beds it is nearly pure. 



Peat, or swamp muck, is an accumulation of vegetable 

 mntter which has been formed through long periods of 

 the ancient history of the earth. 



In low, marshy places, certain kinds of rank vegeta- 

 tion have grown year by year, or from age to age, and, 

 falling, have been buried one upon another in water and 

 mud. These accumulations have undergone a process of 

 slow decay, or smothered combustion, mider water, which 

 has reduced them to a uniform mass of l)lack or l)rown 

 matter. The dark color is due to the presence of carbon 

 which results from the slow comliustion, as charcoal is 

 obtained from the smothered burning of wood. 



