CHRONOLOGY OF FOSSILIFEROUS STRATA. 
129 
it may be dispersed by a current over an area several hun¬ 
dred leagues in length, so as to pass from the tropics into 
the temperate zone. If the bottom of the sea be afterwards 
upraised, the organic remains imbedded in such yellow or 
red strata may indicate the different animals or plants which 
once inhabited at the same time the temperate and equato¬ 
rial regions. 
It may be true, as a general rule, that groups of the same 
species of animals and plants may extend over wider areas 
tlian deposits of homogeneous composition; and if so, palaeon¬ 
tological characters will be of more importance in geological 
classification than the test of mineral composition ; but it is 
idle to discuss the relative value of these tests, as the aid of 
both is indispensable, and it fortunately happens, that where 
the one criterion fails,we can often avail ourselves of the other. 
Test by included Fragments of older Rocks. —It was stated, 
that proof may sometimes be obtained of the relative date 
of two formations by fragments of an older rock being in¬ 
cluded in a newer one. This evidence may sometimes be 
of great use, where a geologist is at a loss to determine the 
relative age of two formations from want of clear sections 
exhibiting their true order of position, or because the strata 
of each group are vertical. In such cases we sometimes dis¬ 
cover that the more modern rock has been in part derived 
from the degradation of the older. Thus, for example, we 
may find chalk in one part of a country, and in another strata 
of clay, sand, and pebbles. If some of these pebbles consist 
of that peculiar flint, of which layers more or less continu¬ 
ous are characteristic of the chalk, and which include fossil 
shells, sponges, and foraminifera of cretaceous species, we 
may confidently infer that the chalk was the oldest of the 
two formations. 
Chronological Groups. —The number of groups into which 
the fossiliferous strata may be separated are more or less 
numerous, according to the views of classification which dif¬ 
ferent geologists entertain; but when we have adopted a 
certain system of arrangement, we immediately find that a 
few only of the entire series of groups occur one upon the 
other in any single section or district. 
The thinning out of individual strata was before described 
(p. 42). But let the annexed diagram represent seven fos¬ 
siliferous groups, instead of as many strata. It will then be 
seen that in the middle all the superimposed formations are 
present; but in consequence of some of them thinning out, 
No. 2 and No. 5 are absent at one extremity of the section, 
and No. 4 at the other. 
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