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 successive 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--Principal groups of strata in western Europe 96