CHRONOLOGY OF ROCKS. 
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CHAPTER VIII. 
CHRONOLOGICAL CLASSIFICATION OF ROCKS. 
Aqueous, plutonic, volcanic, aud metamorphic Rocks considered chronologic¬ 
ally. — Terms Primary, Secondary, and Tertiary; Palseozoic, 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 con¬ 
tinuous Formation.—Proofs that distinct Species of Animals and Plants 
have lived at successive Periods.—Distinct Provinces of indigenous Spe¬ 
cies.—Great Extent of single Provinces.—Similar Laws prevailed at suc¬ 
cessive Geological Periods.—Relative Importance of mineral and palgeon- 
tological Characters.—Test of Age by included Fragments.—Frequent 
Absence of Strata of intervening Periods.—Tabular Views of fossiliferous 
Strata. 
Chronology of Eocks. —In the first chapter it was stated 
that the four great classes of rocks, the aqueous, the volcan¬ 
ic, the plutonic, and the metamorphic, would each be con¬ 
sidered 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 strat¬ 
ified, that some are calcareous, others argillaceous or silice¬ 
ous, some made up of sand, others of pebbles; that some con¬ 
tain 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 resolutions of the inorganic and organ¬ 
ic creations of former times. For the sedimentary forma¬ 
tions 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 disap¬ 
peared from the earth in succession. 
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