Principles of Geology. 25 



their peculiar appearances, and have produced the remarkable phe- 

 nomena which accompany them, from the agency of fire ; they are, 

 therefore, said to be of igneous formation. Granite, though differ- 

 ent in position, agrees with them so closely in its structural charac- 

 ters, and in the phenomena which accompany its contact with other 

 rocks, that it is now admitted to have been in a state of igneous fusion. 



Numerous facts of a different kind, generalized with equal caution^ 

 leave not a shadow of doubt that all the secondary strata, and many 

 of the primary, were deposited from water. The shells which fill 

 so many of the rocks, and the clear traces of watery agency in otherSj 

 make this absolutely certain. From the different characters of these 

 shells, we can clearly determine, in many instances, whether they 

 belonged to marine, fluviatile, or terrestrial species; and we maj 

 thus, with great probability, conjecture the nature of the aqueous 

 fluid which deposited so many rocks. In the application of this last 

 method of reasoning, however, too much caution cannot be used ; 

 for, surely, fresh water shells may have been as easily swept down to 

 the sea, and buried in its deposits, as the wood which lies in so many 

 secondary rocks, and it would, therefore, be hazardous to conclude 

 that a great primaeval lake of fresh water existed over every spot 

 where such fossils occur ; and even where they superabound, as in 

 the coal districts, we must not change a prudent doubt for an inse- 

 cure conclusion. 



Having thus traced the outlines of a practical system of geologyj 

 I shall conclude with a very brief sketch of the series of changes 

 which appear to have visited the earth. From chemical researches 

 it seems highly probable that the whole crust of the earth is to be 

 viewed as originally produced by oxidation of fluid metals and met- 

 alloids. From a careful study of the effects of heat, under differ- 

 ent circumstances, and of the habitudes of earthy compounds under 

 its influence, it seems probable that the granitic rocks, which are 

 the lowest of the primary series, owe their present condition and 

 appearance to the effect of partial or general fusion. Above this 

 granitic series we find, certainly, the effects of deep and overruling 

 water. Many of the primary, and all of the secondary rocks, owe 

 their present appearances and arrangements to the action of water. 

 These strata exhibit the results both of agitated and of tranquil 

 waters — mechanical aggregates — sedimentary deposits — and chem- 

 ical precipitates, in frequent repetition. This circumstance, com- 

 bined with the facts relating to organic remains, teaches us, that dur- 



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