28 PRINCIPLES OF GEOLOGY. 



fire; they are, therefore, said to be of igneous formation. Granite, 

 though different in position, agrees with them so closely in its structural 

 characters, 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, the clear traces of watery agency in others, make this ab- 

 solutely 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 may 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 super- 

 abound, as in the coal districts, we must not change a prudent doubt for 

 an insecure conclusion. 



Having thus traced the outlines of a practical system of geology, 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 metalloids. From 

 a careful study of the effects of heat, under different 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 arrange- 

 ments to the action of water. These strata exhibit the results both 



