ELEMENTS OF GEOLOGY 



CHAPTER I. ; 



ON THE DIFFERENT CLASSES OF ROCKS. 



Geology defined — Successive formation of the earth's crust — Classification of 

 rocks according to their origin and age — Aqueous rocks — Then* stratification 

 and imbedded fossils — Volcanic rocks, with and without cones and craters — 

 Plutonic rocks, and their relation to the volcanic — Metamorphic rocks and their 

 probable origin — The term primitive, why erroneously applied to the crystal- 

 line formations — Leading division of the work. 



Of what materials is the earth composed, and in what maimer are these 

 materials arranged ? These are the first inquiries with which Geology 

 is occupied, a science which derives its name from the Greek y\ ge, the 

 earth, and "koyog, 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 metals, 

 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 unexpected, 

 we soon become engaged in researches into the history of the animate 

 creation, or of the various tribes of animals 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 sub- 

 stances, 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, discovering 

 proofs that the external parts of the earth were not all produced in the 

 beginning of things, in the state in which we now behold them, nor in 

 an instant of time. On the contrary, he can show that they have acquired 

 their actual configuration and condition gradually, under a great variety 



