CRUMBLJNG OR DEGRADATION OF ROCKS. 237 



while from those which are sufficiently inclined the rains wash away 

 the loose materials as soon as they are separated, and carry them down 

 to the vallies. 



The superficial accumulations of which we have spoken, as covering 

 the rocks in many places to a depth of one or two hundred feet, consist 

 of materials thus washed down or otherwise transported — hy water, hv 

 winds, or by other geological agents. Much of these heaps of transported 

 matter is in the state of too fine a powder lo permit us to say from whence 

 it has been derived — but fragments of greater or less size are always to 

 be found, even among the clays and fine sands, which are sufficient to 

 point out to the skilful geologist the direction from which the whole has 

 been brought, and often the very rocks from which the entire accumula- 

 tions have been derived. 



Thus the general conclusion is fairly drawn, that the earthy matter of 

 all soils has been produced by the gradual decay, degradation, or crumb- 

 ling down of previously existing rocks. It is evident therefore — 



1°. That whenever a soil rests immediately upon the rock from which 

 it has been derived, it may be expected to partake more or less of the 

 composition and characters of that rock. 



2°. That where the soil forms only the surface layer of a considerable 

 depth of transported materials, it may have no relation whatever either 

 in rriineralogical characters or in chemical constitution to the immedi- 

 ately subjacent rocks. * 



The soils of Great Britain are divisible into two such classes. In 

 some counties an acquaintance with the prevailing rock of the district 

 enables us to predict the general characters and quality of the soil ; in 

 others — and nearly all our coal fields are in this case — the general 

 character and capabilities of the soil have no relation whatever to the 

 rocks on which the loose materials rest. 



§ 6. On the general structure of the earth's crust. 



Beneath the soil, and the loose or drifted matters on which it rests, we 

 everywhere find the solid rock. This rock in most countries is seen — 

 in mines, quarries, and cliffs — to consist of beds or layers of varied thick- 

 ness placed one over the other. To these layers geologists give the 

 name of strata; and hence rocks which are thus made up of many se- 

 parate layers are called stratified rocks. 



But in some places entire mountain masses are met with, in which no 

 parting into layers or beds is seen, but which appear to consist of one 

 unbroken rock of the same material from their upper surface down- 

 wards, and often as far beneath as we have been able to penetrate into 

 the earth. Such rocks are said to be unstratified. Among these are 

 included the granites, the trap, green-stone, or basaltic rocks, and the 

 lavas. Geologists have ascertained that all these unstratified rocks have, 

 like the volcanic lavas, been in a more or less perfectly melted state — 

 that their present appearance is owing to the action of fire — and hence 

 they are often called igneous* rocks. They often also exhibit a more or 

 less crystalline or glassy structure, or contain, imbedded in them, nu- 

 merous regular crystals of mineral substances ; hence they are some- 

 times called also crystalline rocks. The terms igneous, crystalline, and 



• Sometimea pyrogenous, produced by fire ; but this is an unnecessarily hard word. 



