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GEOLOGY. 



CHAPTER I. 



THE STRUCTURE AND CLASSIFICATION OP ROCKS. 



N popular language, the word rock instantly suggests the 

 idea of a huge, compact, and hard substance. This is not 

 the geological signification of the term. It is applied to 

 every kind of formation which constitutes the crust of the 

 globe, from the loose sand and gravel of our pathway, 

 and the soft clays that lie beneath the surface soil, to the close-grained, heavy, and 

 majestic masses of granite that form the highest Alps, or of porphyry that crown the 

 summits of the Andes. These materials, so various in texture and diverse in form, 

 are found upon analysis to be compounded of a very few elementary substances. Che- 

 mistry, which, like all science, takes us back from the most complex results to the 

 most simple causes, has been brought sufficiently to bear upon the matter of the globe 

 to show, that but a scanty number of elements enter into its composition. Simplicity 

 in the causation, and amplitude in the result, is one of the laws of the all-potent Creator, 

 which may well excite our admiration. It meets us in every department of nature, and is 

 very strikingly developed by chemical analysis of the ponderous mass upon which we tread, 

 and the magnificent elevations whose tops are screened from our notice by the clouds of 

 heaven ; for nearly the whole of the matter yet known to enter into the composition of our 

 terrestrial spheroid the phenomena of every mountain that rises, and every valley that 

 sweeps every forest that waves and every ocean that roars may be referred to about 

 sixteen simple substances variously combined. The difference between the simple and 

 compound state of these elements is often most remarkable. It is a curious result of che- 

 mical science, that while oxygen and nitrogen compose the principal part of the atmosphere, 

 and oxygen and hydrogen constitute water, a largely diffused fluid, oxygen, which is 

 a light and invisible gas in its simple state, enters largely into combination with the 

 earths and metals, and is supposed to compose fully one half of the solid contents of the 

 globe. The number of simple substances, or those out of which nothing different from 

 themselves can be obtained, amounted in the year 1787 to seventeen, in 1802 to twenty- 

 eight ; but the number at present known amounts to fifty-five. They are : 

 Five gases oxygen, hydrogen, nitrogen, chlorine, and fluorine. 



