92 TESTS OF THE DIFFERENT AGES [Ch. IX. 



CHAPTER IX. 



ON THE DIFFERENT AGES OF THE AQUEOUS ROCKS. 



On the three principal tests of relative age — Superposition, mineral character, 

 and fossils — Change of mineral character and fossils in the same continuous 

 formation — Proofs that distinct species of animals and plants have lived at suc- 

 cessive periods — Distinct provinces of indigenous species — Great extent of 

 single provinces — Similar laws prevailed at successive geological periods — 

 Relative importance of mineral and palseontological characters — Test of age by 

 included fragments — Frequent absence of strata of intervening periods — Prin- 

 cipal groups of strata in -western Europe. 



In tlie last chapter I spoke generally of the chronological relations of 

 tne four great classes of rocks, and I shall now treat of the aqueous rocks 

 in particular, or of the successive periods at which the different fossilif- 

 erous formations have been deposited. 



There are three principal tests by which we determine the age of a 

 given set of strata ; first, superposition ; secondly, mineral character ; 

 and, thirdly, organic remains. Some aid can occasionally be derived 

 from a fourth kind of proof, namely, the fact of one deposit including in 

 it fragments of a pre-existing rock, by which the relative ages of the two 

 may, even in the absence of all other evidence, be determined. 



Superposition. — The first and principal test of the age of one aqueous 

 deposit, as compared to. another, is relative position. It has been already 

 stated, that where strata are horizontal, the bed which lies uppermost is 

 the newest of the whole, and that which lies at the bottom the most 

 ancient. So, of a series of sedimentary formations, they are like vol- 

 umes of history, in which each writer has recorded the annals of his own 

 times, and then laid down the book, with the last written page upper- 

 most, upon the volume in which the events of the era immediately pre- 

 ceding were commemorated. In this manner a lofty pile of chronicles 

 is at length accumulated ; and they are so arranged as to indicate, by 



