n
action. By what train of investigations were geologists induced at
length to reject these views, and to assent to the igneous origin of the
trappean formations? By an examination of volcanoes now active, and by
comparing their structure and the composition of their lavas with the
ancient trap-rocks.

The establishment, from time to time, of numerous points of
identification, drew at length from geologists a reluctant admission,
that there was more correspondence between the condition of the globe at
remote eras and now, and more uniformity in the laws which have
regulated the changes of its surface, than they at first imagined. If,
in this state of the science, they still despaired of reconciling every
class of geological phenomena to the operations of ordinary causes, even
by straining analogy to the utmost limits of credibility, we might have
expected, at least, that the balance of probability would now have been
presumed to incline towards the close analogy of the ancient and modern
causes. But, after repeated experience of the failure of attempts to
speculate on geological monuments, as belonging to a distinct order of
things, new sects continued to persevere in the principles adopted by
their predecessors. They still began, as each new problem presented
itself, whether relating to the animate or inanimate world, to assume an
original and dissimilar order of nature; and when at length they
approximated, or entirely came round to an opposite opinion, it was
always with the feeling, that they were conceding what they had been
justified _a priori_ in deeming improbable. In a word, the same men who,
as natural philosophers, would have been most incredulous respecting any
extraordinary deviations from the known course of nature, if reported to
have happened _in their own time_, were equally disposed, as geologists,
to expect the proofs of such deviations at every period of the past.

I shall proceed in the following chapters to enumerate some of the
principal difficulties still opposed to the theory of the uniform nature
and energy of the causes which have worked successive changes in the
crust of the earth, and in the condition of its living inhabitants. The
discussion of so important a question on the present occasion may appear
premature, but it is one which naturally arises out of a review of the
former history of the science. It is, of course, impossible to enter
into such speculative topics, without occasionally