THE CKUST OF THE EARTH. 



and the superior strata 10 per cent, of the species found among 

 the animal tribes which still continue upon the earth. 



41. In fine, superposed upon these tertiary are several layers of 

 earthy matter, upon which the actual organised world is placed. 

 These are usually resolved into two beds, the lower of which, 

 denominated DILUVIAL, consists of deposits of gravel and clay, 

 with boulder-stones, rounded in different degrees by attrition, 

 giving indication of having been carried from a distance by the 

 extraordinary action of water, from which the general name 

 DRIFT has been given to them. 



The superior bed consists of sand, clay, and gravel, upon which 

 the surface soil, which is the theatre of agriculture, rests. This 

 consists of decayed and decomposed vegetable matter, mixed with 

 more or less of the disintegrated matter of the inferior beds. 

 This uppermost layer is produced chiefly by the ordinary action 

 of water, and is denominated ALLUVIAL. 



42. Such are the five principal groups of rocks, into which 

 geologists have divided the matter which forms the shell of the 

 globe. The transition, secondary, and tertiary groups, have 

 each been subdivided into several layers or strata, each of which 

 is distinguished by the peculiar sorts of mineral matter of which it 

 is composed, and the peculiar species of organic remains which it 

 contains. Geologists, however, are not agreed either as to the 

 limits of the five principal groups, or as to their distribution into 

 subordinate strata. Thus they are not agreed as to where each of 

 the principal groups ends and the next begins. The rocks, 

 which one calls primitive, another denominates transition or 

 metamorphic. Those which one assigns to the upper part of the 

 transition-system, another assigns to the lower part of the 

 secondary system ; and in like manner what one assigns as the 

 highest strata of the secondary, another gives as the lowest strata 

 of the tertiary. These discrepancies, however, arise more from 

 the nature of the things than from any deficiency of our 

 knowledge of them. Between one group and another there is 

 no essential distinction, and their classification into primary, 

 secondary, and tertiary, though convenient, is, like many other 

 classifications, to a certain extent arbitrary. 



43. From what has been stated, respecting the strata constituting 

 the crust of the earth, the following consequences will follow : 



First. The unstratified and igneous rocks existed prior to the 

 stratified or sedimentary rocks. 



Secondly. The stratified or sedimentary rocks were produced 



in the chronological order in which they are found superposed, 



the most ancient being the transition-system, and the others 



being formed in the order of time in which they are superposed, 



46 



