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 others, 
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 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 Teasoning, 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 primeval lake of fresh water existed over every spot 
Where such fossils occur; and even where they superabound, as in 
ne coal districts, we must not cha ange a prudent doubt for an inse- 
eure conclusion. : sa ze . bated 
_ 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 
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 
*ppearance to the effect of partial or general fusion. Above this 
ic series we find, certainly, the effects of deep and overruling 
Many of the primary, and all of the secondary rocks, owe 
Sent appearances and arrangements to the action of water. 
Strata exhibit the resnits both of agitated and of tranquil 
Waters—mechanical ageregates—sedimentary deposits—and chem- 
ical Precipitates, in frequent repetition. ‘This circumstance, com- 
with the facts relating to organic remains, teaches us, that dur- 
‘ 0. I. 4 
Vou. XXL—N 
Water, 
their pre 
cs 
