4 METHODS OF THEORIZING IN GEOLOGY. [Ch. I. 
they organic or inorganic substances ? That the latter opinion 
should for a long time have prevailed, and that these bodies 
should have been supposed to be fashioned into their present 
form by a plastic virtue, or some other mysterious agency, may 
appear absurd ; but it was, perhaps, as reasonable a conjec- 
ture as could be expected from those who did not appeal, in 
the first instance, to the analogy of the living creation, as 
affording the only source of authentic information. It was 
only by an accurate examination of living testacea, and by a 
comparison of the osteology of the existing vertebrated animals 
with the remains found entombed in ancient strata, that this 
favourite dogma was exploded, and all were, at length, per- 
suaded that these substances were exclusively of organic origin. 
In like manner, when a discussion had arisen as to the nature 
of basalt and other mineral masses, evidently constituting a par- 
ticular class of rocks, the popular opinion inclined to a belief 
that they were of aqueous, not of igneous origin. These rocks, 
it was said, might have been precipitated from an aqueous solu- 
tion, from a chaotic fluid, or an ocean which rose over the con- 
tinents, charged with the requisite mineral ingredients. All 
are now agreed that it would have been impossible for human 
ingenuity to invent a theory more distant from the truth ; yet 
we must cease to wonder, on that account, that it gained so 
many proselytes, when we remember that its claims to proba- 
bility arose partly from its confirming the assumed want of all 
analogy between geological causes and those now in action. 
By what train of investigation were all theorists brought round 
at length to an opposite opinion, and induced to assent to the 
igneous origin of these formations ? By an examination of 
the structure of active volcanos, the mineral composition of 
their lavas and ejections, and by comparing the undoubted pro- 
ducts of fire with the ancient rocks in question. 
We shall conclude with one more example. When the 
organic origin of fossil shells had been conceded, their occur- 
rence in strata forming some of the loftiest mountains in the 
world, was admitted as a proof of a great alteration of the 
