THEIR PECULIAR CHARACTERS ALSO CONTINUOUS. 2it 



character to many rocks. Thus the per-oxide of iron, which imparts 

 their red colour to many strata — as to the red sandstones — influences 

 not only the mineralogical character of the rock, but also the quality of 

 the soil which is formed by its decay. In like manner the presence of 

 magnesia, sometimes in large quantity, in many lime-stones, produces 

 an important modification in the chemical constitution and mineralogical 

 characters of the rock, as well as in its relations to practical agriculture. 



In consequence of these and other similar causes of diversity, if not 

 every stratum, at least every series of strata, exhibits distinguishing and 

 characteristic peculiarities, by means of which it may be more or less 

 readily recognized. On these peculiarities the special agricultural ca- 

 pabilities of those parts of the globe in which each series of beds occurs 

 are in a great degree dependent. 



4°. This peculiar character is also more or less continuous over very 

 large areas. Thus if a given stratum be found on the surface in any 

 part of England, and again in any part of Russia, the soil formed from 

 that bed will generally exhibit very nearly the same qualities in both 

 countries. A knowledge of the geology, therefore, — that is, of the kind 

 of rock which appears on the surface in every part of a country — ena- 

 bles us to predict generally the kind of soil which ought to rest upon it, 

 if it be not covered by foreign accumulations ; while, on the other hand, 

 a knowledge of the agricultural capabilities of any one district in which 

 certain rocks are known to lie immediately beneath the soil, and of the 

 agricultural practice suited to that district, will indicate the probable ca- 

 pabilities of any other tract in which the same kind of rock is known to 

 appear on the surface, and of the kind of culture which may be most 

 successfully applied to it. 



It is evident, then, that a familiar acquaintance with the general 

 characters and relative positions of all the series of strata that have hith- 

 erto been observed, and of the classifica'.ion of rocks considered geologi- 

 cally, to which this knowledge has led, must be fitted to throw much 

 light upon the principles of a general, enlightened, and philosophical 

 agriculture. 



§ 8. Classification of the stratified rocks, their extent, and the agricultu- 

 ral relations of the soils derived from them. 



It is a received principle, I may say rather, an obvious fact, that in 

 the crust of the earth, as in the walls of a building, those layers which lie 

 lowest or undermost have been first deposited, or are the oldest. In re- 

 ference to this their relative age, the stratified rocks are divided into the 

 primary, the first deposited and most ancient — the secondary, which are 

 next in order — and the tertiary, which overlie both. 



These three series of strata are again subdivided into systems, and 

 these into minor groups, called formations, — the several members of 

 each system and formation having such a common resemblance, either 

 in mineralogical character or in the kind of animal and vegetable re 

 mains found in them, as to show that they were deposited under very 

 nearly the same general physical conditions of the globe. 



The following table exhibits the names, relative positions, thicknesses 

 and mineralogical characters of the stratified rocks, in descending order 

 as they occur in our islands. The annexed remarks indicate also the 

 11 



