PART 11. CHAPTER XIII. 



159 



Tests of the different Ages of Aqueous Rocks. 



CHAPTER XIII. 



ON THE DIFFERENT AGES OF THE AQUEOUS ROCKS. 



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

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

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

 cessive periods — Test of age by included fragments — Frequent absence of strata 

 of intervening periods — Principal groups of strata in western Europe — Tertiary- 

 strata separable into four groups, the fossil shells of which approach nearer to 

 those now living in proportion as the formation is more modern — Terms Eocene, 

 Miocene, and Pliocene — Identifications of fossil and recent shells by M. De- 

 shayes — Opinions of Dr. Beck. 



In the last chapter I spoke generally of the chronological re- 

 lations of the 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 fossiliferous formations have been deposited. 



Now 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 preexisting 

 rock, which last may thus be shown, even in the absence of all 

 other evidence, to be the older of the two. 



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 the 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 volumes 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 immedi- 

 ately preceding were commemorated. In this manner a lofty 

 pile of chronicles is at length accumulated ; and they are so 

 arranged as to indicate, by their position alone, the order in 

 which the events recorded in them have occurred. 



In regard to the crust of the earth, however, there are some 

 regions where, as the student has already been informed, the 

 beds have been disturbed, and sometimes reversed. (See pp. 73, 

 74.) But the experienced geologist will not be deceived by 



