Ch. IV.] RELATIVE AGES OF ROCKS. 39 



cisely the same mineral character at very different periods ; as, 

 for example, two formations of red sandstone, with a great 

 series of other strata intervening between them. Such repeti- 

 tions might have been anticipated, since these red sandstones 

 are produced by the decomposition of granite, gneiss, and mica- 

 schist ; and districts composed exclusively of these, must again 

 and again be exposed to decomposition, and to the erosive 

 action of running water. 



But notwithstanding the variations before alluded to in 

 the composition of one continuous set of strata, many rocks 

 retain the same homogeneous structure and composition, 

 throughout considerable areas, and frequently, after a change 

 of mineral character, preserve their new peculiarities throughout 

 another tract of great extent. Thus, for example, we may 

 trace a limestone for a hundred miles, and then observe that it 

 becomes more arenaceous, until it finally passes into sand or 

 sandstone. We may then follow the last-mentioned formation 

 throughout another district as extensive as that occupied by the 

 limestone first examined. 



Proofs of contemporaneous origin derived from organic 

 remains. 



We devoted several chapters, in the last volume, to show 

 that the habitable surface of the sea and land may be divided 

 into a considerable number of distinct provinces, each peopled 

 by a peculiar assemblage of animals and plants, and we endea- 

 voured to point out the origin of these separate divisions It 

 was shown that climate is only one of many causes on which 

 they depend, and that difference of longitude, as well as lati- 

 tude, is generally accompanied by a dissimilarity of indigenous 

 species of organic beings. 



As different seas, therefore, and lakes are inhabited at the 

 same period, by different species of aquatic animals and plants, 

 and as the lands adjoining these may be peopled by distinct 

 terrestrial species, it follows that distinct organic remains are 

 imbedded in contemporaneous deposits. If it were otherwise 



