In the preceding exposition the object has been to give, in a condensed form, the leading facts and conclusions of geology. These facts are so firmly established by observation, and the conclusions from them so clearly demonstrated, that every rational mind must assent and be prepared to admit the principle on which Geology, like all physical sciences, must rest—that geological phenomena must be referred to physical causes.

When the student has obtained sufficient familiarity with these established propositions, he will be prepared to follow in the discussion of the less elementary problems of the science, many of which are subjects of controversy. Before, however, going into these speculations, Mr. Hopkins devotes several pages to the evidences of the vast antiquity of the globe, dwelling both on the evidences afforded by the organic remains of successive creations embedded in the rocks, and on those drawn from the inanimate kingdom. Of the first we find that an immense number of different tribes of animals and plants which in former ages have given their passing phases to terrestrial life on our planet have disappeared with a transition so slow that, according to the present order of nature, there is not the slightest evidence of the introduction of a new species of animal since the creation of man; whilst of the latter, it appears certain that the deposition of sedimentary strata, and that process of denudation necessarily contemporaneous with it, must have proceeded at very much the same rate in former as in recent times; and, admitting this conclusion, we obtain something like a rough conception of the enormous lapse of time necessary for the deposition of the whole mass of sedimentary formations, by simply comparing such mass with the proportion which has been transported and deposited, within the last two or three thousand years.