ELEMENTS OF GEOLOGY. 



PART L 



CHAPTER I. 



ON THE FOUR GREAT CLASSES OP ROCKS — THE AQUEOUS, VOLCANIC, 

 PLUTONIC, AND METAMORPHIC. 



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

 of rocks according to their origin and age — Aqueous rocks — Their 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 

 crystalline formations — Division of the work into two parts; the first descrip- 

 tive of rocks without reference to their age, the second treating of their chro- 

 nology. 



Op what materials is the earth composed, and in what man- 

 ner are these materials arranged ? These are the inquiries with 

 which Geology is occupied, a science which derives its name 

 from the Greek, y»7, ge, the earth, and ?toyo5, logos, a discourse. 

 Such investigations appear, at first sight, to 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 these researches, we soon find our- 

 selves led on to consider the successive changes which have 

 taken place in the former state of the earth's surface and inte- 

 rior, 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 dif- 

 ferent periods of the past, inhabited the globe. 



All are aware that the solid parts of the earth consist of dis- 

 tinct substances, such as clay, chalk, sand, limestone, coal, slate, 

 granite, and the like ; but previously to observation it is com- 

 monly 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. Geologists have 



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