Ch. IX,] OF AQUEOUS ROCKS. 97 



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 

 equatorial regions. 



It may be true, as a general rule, that groups of the same species of 

 animals and plant's may extend over wider areas than deposits of homo- 

 geneous composition ; and if so, palseontological characters will be of 

 more importance in geological classification than the test of mineral com- 

 position ; but it is idle to discuss the relative 7alue 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 hj included fragments of older rocks. — It was stated, that inde- 

 pendent proof may sometimes be obtained of the relative date of two 

 formations, by fragments of an older rock being included 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 discover that the 

 more modern rock has been in part derived from the degradation of the 

 older. Thus, for example, we may find chalk with flints in one part of a 

 country ; and, in another, a distinct formation, consisting of alternations 

 of clay, sand, and pebbles. If some of these pebbles consist of similar 

 flint, including fossil shells, sponges, and forarniniferee, of the same species 

 as those in the chalk, we may confidently infer that the chalk is the oldest 

 of the two formations. 



Chronological groups. — The number of groups into which the fossil- 

 iferous strata may be separated are more or less numerous, according to 

 the views of classification which different geologists entertain ; but w T hen 

 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. 16). 



Fig. 102. 



But let the annexed diagram represent seven fossiliferous groups, instead 

 of as many strata. It will then be seen that in the middle all the super- 

 imposed formations are present ; but in consequence of some of them 

 thinning out, No. 2 and No. 5 are absent at one extremity of the sec- 

 tion, and No. 4 at the other. 



In the annexed diagram, fig. 103, a real section of the geological 

 formations in the neighborhood of Bristol and the Mendip Hills, is pre- 

 7 



