236 HOW THE SUBSOIL IS PRODUCED. 



SO as gradually to establish a more or less manifest difference between 

 the upper and lower layers, in reference even to the earthy ingredients 

 which they respectively contain. 



But, except in the case of very porous rocks or accumulations of earthy 

 matter, these surface waters rarely descend to any great depth, and hence 

 after sinking through a variable thickness of subsoil, we come, in gene- 

 ral, to earthy layers, in which little vegetable matter can be detected, 

 and to whicii the lime, iron, and magnesia of the superficial covering 

 has never been able to descend. 



Thus the character of the soU is, that it contains more brown organic, 

 chiefly vegetable, matter, in a state of decay — of the subsoil, that ihe or- 

 ganic matter is less in quantity and has entered it chiefly in a soluble 

 state, and that earthy matters are presi.'nt in it which have been washed 

 out of the superior soil — and of the subjacent mass, that it has remained 

 nearly unaffected by the changes which vegetation, culture, and atmos- 

 pheric agents have produced upon the portions that lie above it. 



From what is here stated, the effect of trench and subsoil ploughing, 

 in altering more or less materially the proportions of the earthy constitu- 

 ents in the surface soil, will be in some measure apparent. That which 

 the long action of rains and frosts has caused to sink beyond the ordinary 

 reach of the plough is, by such methods, brought again to the surface. 

 When the substances thus brought up are directly beneficial to vegeta- 

 tion or are fitted to improve the texture of the soil, its fertility is increased. 

 Where the contrary is the case, its productive capabilities may for a 

 longer or a shorter period be manifestly diminished. 



§ 5. On the general origin of soils. 



On many parts of the earth's surface the naked rocks appear ovei 

 considerable tracts of country, without any covering of loose mate- 

 rials from which a soil can be formed. This is especially the case in 

 mountainous and granitic districts, and in the neighbourhood of active 

 or extinct volcanoes, where, as in Sicily, streams of naked lava stretch 

 in long black lines amid the surrounding verdure. 



But over the greater portion of our islands and continents the rocks 

 are covered by accumulations, more or less deep, of loose materials — 

 sands, gravels, and clays chiefly — the upper layer of which is more or 

 less susceptible of cultivation, and is found- to reward the exertions of 

 human industry with crops of corn in greater or less abundance. 



This superficial covering of loose materials varies from a few inches to 

 one or two hundred feet in depth, afid is occasionally observed to consist 

 of different layers on beds, placed one, over the other — such as a bed of 

 clay over one of gravel or sand, and a loamy bed under or over both. 

 In such cases the characters and capabilities of the soil must depend 

 upon which of these layers may chance to be uppermost — and its char- 

 acter may often be beneficially altered by a judicious admixture with 

 portions of the subjacent layers. 



It is often observed, where naked rocks present themselves, either in 

 cliffs or on more level parts of the earth, that the action of the rains and 

 frosts causes their suri'aoes gradually to shiver off*, crumble down, or 

 wear away. Hence at the base of cliffs loose matter collects — on com- 

 parativeh level surfaces the crumbling of the rock gradually forms a soil — 



