Ch.IV.J 
RELATIVE AGES OF ROCKS. 
39 
cisely tlie 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 
