96 TESTS OF THE DIFFERENT AGES [Ch. IX 



It may perhaps be suggested that some metamorphic strata, and some 

 granites, may be anterior in date to the oldest of the primary fossilifer- 

 ous rocks. This opinion is doubtless true, and will be discussed in future 

 chapters ; but I may here observe, that when we arrange the four classes 

 of rocks in four parallel columns in one table of chronology, it is by no 

 means assumed that these columns are all of equal length ; one may 

 begin at an earlier period than the rest, and another may come down to 

 a later point of time. In the small part of the globe liitherto examined, 

 it is hardly to be expected that we should have discovered either the 

 oldest or the newest members of each of the four classes of rocks. Thus, 

 if there be primary, secondary, and tertiary rocks of the aqueous or fos- 

 siliferous class, and in like manner primary, secondary, and tertiary hypo- 

 gene formations, we may not be yet acquainted with the most ancient of 

 the primary fossihferous beds, or with the newest of the hypogene. 



CHx\PTER 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 palaeontological characters — Test of age by 

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

 cipal groups of strata in western Europe. 



In the last chapter I spoke generally of the chronological relations 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 fossilif- 

 erous formations have been deposited. 



There are three principal tests by which w^e 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 



