STUDENT’S 
ELEMENTS OF GEOLOGY. 
CHAPTER I. 
ON THE DIEFEKENT CLASSES OF EOCKS. 
Geology defined.—Successive Formation of the Earth’s Crust. “Classifica¬ 
tion of Eocks according to their Origin and Age.—Aqueous Eocks.—Their. 
Stratification and imbedded Fossils.—Volcanic Eocks, with and without 
Cones and Craters.—Plutonic Eocks, and their Eelation to the Volcanic.— 
Metamorphic Eocks, and their probable Origin.—The term Primitive, why 
erroneously applied to the Crystalline Formations.—Leading Division of 
the Work. 
Of what materials is the earth composed, and in what man¬ 
ner are these materials arranged ? These are the first inqui¬ 
ries with which Geology is occupied, a science which derives 
its name from the Greek y?;, ge^ the earth, and Xdyoc, logos^ a 
discourse. Previously to experience we might have imagined 
that investigations of this kind would relate exclusively to 
the mineral kingdom, and to the various rocks, soils, and met¬ 
als, which occur upon the surface of the earth, or at various 
depths beneath it. But, in pursuing such researches, we soon 
find ourselves led on to consider the successive changes 
which have taken place in the former state of the earth’s 
surface and interior, and the causes which have given rise to 
these changes; and, what is still more singular and unex¬ 
pected, we soon become engaged in researches into the his¬ 
tory of the animate creation, or of the various Tribes of ani¬ 
mals and plants which have, at different periods of the past, 
inhabited the globe. 
All are aware that the solid parts of the earth consist of 
distinct substances, such as clay, chalk, sand, limestone, coal, 
slate, granite, and the like; but previously to observation it is 
commonly imagined that all these had remained from the 
first in the state in which we now see them—that they were 
created in their present form, and in their present position. 
The geologist soon comes to a different conclusion, discover- 
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