[9] Principles, vol. i, p. 265, 1867. CHAPTER VIII. CHRONOLOGICAL CLASSIFICATION OF ROCKS. Aqueous, Plutonic, volcanic, and metamorphic Rocks considered chronologically. — Terms Primary, Secondary, and Tertiary; Palæozoic, Mesozoic, and Cainozoic explained. — On the different Ages of the aqueous Rocks. — 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 palæontological Characters. — Test of Age by included Fragments. — Frequent Absence of Strata of intervening Periods. — Tabular Views of fossiliferous Strata. Chronology of Rocks.— In the first chapter it was stated that the four great classes of rocks, the aqueous, the volcanic, the Plutonic, and the metamorphic, would each be considered not only in reference to their mineral characters, and mode of origin, but also to their relative age. In regard to the aqueous rocks, we have already seen that they are stratified, that some are calcareous, others argillaceous or siliceous, some made up of sand, others of pebbles; that some contain fresh-water, others marine fossils, and so forth; but the student has still to learn which rocks, exhibiting some or all of these characters, have originated at one period of the earth’s history, and which at another. To determine this point in reference to the fossiliferous formations is more easy than in any other class, and it is therefore the most convenient and natural method to begin by establishing a chronology for these strata, and then to refer as far as possible to the same divisions, the several groups of Plutonic, volcanic, and metamorphic rocks. Such a system of classification is not only recommended by its greater clearness and facility of application, but is also best fitted to strike the imagination by bringing into one view the contemporaneous revolutions of the inorganic and organic creations of former times. For the sedimentary formations are most readily distinguished by the different species of fossil animals and plants which they inclose, and of which one assemblage after another has flourished and then disappeared from the earth in succession.