196 ON THE ORIGIN^ MATERIALS, COMPOSITION, 



question at large. Original differences in the mate- 

 rials are the foundations of the leading distinctions; 

 and the remainder must be sought in chemical actions. 

 Thus also, changes or alternations are the results of 

 changes in the materials, combined sometimes with 

 variations of the chemical forces or affinities. Hence 

 it also is, that rocks preserve the same characters 

 wherever they occur ; a. circumstance otherwise cal- 

 culated to excite our surprise. In every other de- 

 partment of nature, her productions vary according 

 to the climate and situation, but granite is the same 

 in Egypt and in Greenland. It is with the laws of 

 organization alone that climate interferes. 



As the secondary, or later, strata have been chiefly 

 formed from the waste of these antient and definite 

 rocks, it is less surprising that they should preserve a 

 general constancy of character throughout the globe, 

 however individuals may vary in different places. Even 

 these variations are still remarkable ; as well from 

 their steadiness, as from the extent through which 

 that uniformity can sometimes be traced. The dif- 

 ference between compact limestone and chalk, is no 

 less remarkable than the similarity which, in distant 

 places, occurs between strata which we can scarcely 

 conceive to have formed parts of one deposit. It is 

 worthy of remark however, that, in the secondary 

 strata, the most conspicuous variations occur in the 

 limestones ; and these, it is obvious, have been sub- 

 ject, in many instances, to chemical laws, as well as 

 to the influence of organized bodies, from which the 

 others have been comparatively exempted. That the 

 secondary strata should contain sandstone and schists, 

 is easily accounted for, by recollecting that these 

 must be the result of the destruction of the older 

 rocks ; the more durable mineral remaining distinct, 



