126 
ELEMENTS OF GEOLOGY. 
And this circumstance it is, which confers on fossils their 
highest value as chronological tests, giving to each of them, 
in the eyes of the geologist, that authority which belongs to 
contemporary medals in history. 
The same can not be said of each peculiar variety of rock; 
for some of these, as red marl and red sandstone, for exam¬ 
ple, may occur at once at the top, bottom, and middle of the 
entire sedimentary series; exhibiting in each position so per¬ 
fect an identity of mineral aspect as to be undistinguishable. 
Such exact repetitions, however, of the same mixtures of 
sediment have not often been produced, at distant periods, 
in precisely the same parts of the globe; and even where 
this has happened, we are seldom in any danger of confound¬ 
ing together the monuments of remote eras, when we have 
studied their imbedded fossils and their relative position. 
Zoological Provinces. —It was remarked that the same spe¬ 
cies of organic remains can not be traced horizontally, or in 
the direction of the planes of stratifications for indefinite dis¬ 
tances. This might have been expected from analogy; for 
when we inquire into the present distribution of living be¬ 
ings, we find that the habitable surface of the sea and land 
may be divided into a considerable number of distinct prov¬ 
inces, each peopled by a peculiar assemblage of animals and 
plants. In the “ Principles of Geology,” I have endeavored 
to point out the extent and ^Drobable origin of these separate 
divisions; and it was shown that climate is only one of many 
causes on which they depend, and that difference of longi¬ 
tude as well as latitude is generally accompanied by a dis¬ 
similarity of indigenous species. 
As different seas, therefore, and lakes are inhabited, at the 
same period, by different aquatic animals and plants, and as 
the lands adjoining these may be peopled by distinct terres¬ 
trial species, it follows that distinct fossils will be imbedded 
in contemporaneous deposits. If it were otherwise—if the 
same species abounded in ever}^ climate, or in every part of 
the globe where, so far as we can discover, a corresponding 
temperature and other conditions favorable to their exist¬ 
ence are found—the identification of mineral masses of the 
same age, by means of their included organic contents, would 
be a matter of still greater certainty. 
Nevertheless, the extent of some single zoological prov¬ 
inces, especially those of marine animals, is very great; and 
our geological researches have proved that the same laws 
prevailed at remote periods ; for the fossils are often identical 
throughout wide spaces, and in detached deposits, consisting 
of rocks varying entirely in their mineral nature. 
