INTRODUCTION. 
CHAPTER I. 
Objects of Geological Science—Descriptive Geology.—Practical Geology.—Composition of rocks. 
Unstratified rocks —Granitic and Basaltic rocks. 
The objects contemplated by the science of Geology may be conveniently distributed under two 
heads—Descriptive and Practical Geology. Descriptive Geology includes the facts derived from 
the examination and study of the various appearances presented by the eartii’s crust, the materials 
of which it is formed, the manner in which these materials are arranged, whether disposed in crys- 
taline masses, or in beds and strata, the order of succession observed in these beds and strata, 
the study of the organic remains which they may enclose, and their comparison with the living 
forms of the present period. It investigates and explains the causes at present in operation, as well 
as those disturbing forces which have acted in past times—which have resulted in the present form 
and appearance of the earth’s crust. 
_ Practical Geology, on the other hand, has for its objects the application to the ordinary affairs of 
life, of the knowledge that may have been thus acquired by a careful study of the phenomena 
exhibited in the earth’s structure, and of the general laws which have governed these phenomena. 
It applies this knowledge to the discovery of substances useful in the arts, such as minerals and 
ores, and thus it directs the practical pursuits of the miner. 
To the engineer it exhibits the character of the country where his operations may be carried on; 
and often enables him to anticipate the difficulties to be encountered, arising from geological struc- 
ture; while to both engineer and architect it may render important aid in the selection of proper 
materials for construction. 
To the agriculturist it points out the origin and character of soils, and their relations to the rocks 
from which they are derived; and in this manner Practical Geology forms, in a great measure, the 
basis of agricultural science. 
DESCRIPTIVE GEOLOGY. 
It is probable that our actual knowledge of the earth’s crust extends to a depth of fifteen miles, 
including the tops of the highest mountains, Sixteen substances, usually considered simple, con- 
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