So that we have at our command a source of depression on a grand scale, at every period when granitic rocks have originated in the interior of the earth's crust. All mineralogists are agreed that the passage of voluminous masses, from a liquid or pasty to a solid and crystalline state, must be an extremely slow process. It may often happen that, in the same series of superimposed rocks, some are expanding while still solid or while partially melting, while others are at the same time crystallising and contracting; so that the alterations of level at the surface may be the result of complicated and often of conflicting agencies. The more gradually we conceive such changes to take place, the more comprehensible they become in the eyes of the chemist and natural philosopher who speculates on the changes of the earth's interior; and the more fertile are they in the hands of the geologist in accounting for revolutions on the habitable surface. We may presume, that after the movement has gone on for a long time in one determinate direction, whether of elevation or depression, the change to an opposite movement, implying the substitution of a heating for a refrigerating operation, or the reverse, would not take place suddenly; but would be marked by a period of inaction, or of slight movement, or such a state of quiescence, as prevails throughout large areas of dry land in the normal condition of the globe. I see no reason for supposing that any part of the revolutions in physical geography, to which the maps above described have reference, indicate any catastrophes greater than those which the present generation has witnessed. If Man was in existence when the Cromer forest was becoming submerged, he would have felt no more alarm than the Danish settlers on the east coast of Baffin's Bay, when they found the poles, which they had driven into the beach to secure their boats, had subsided below their original level. Already, perhaps, the melting ice has thrown down till and boulders upon those poles, a counterpart of the boulder clay which overlies the forest-bed on the Norfolk cliffs. We have seen that all the plants and shells, marine and freshwater, of the forest bed, and associated fluvio-marine strata of Norfolk, are specifically identical with those of the living European flora and fauna; so that if upon such a stratum a deposit of the present period, whether freshwater or marine, should be thrown down, it might lie conformably over it, and contain the same invertebrate fauna and flora. The strata so superimposed would, in ordinary geological language, be called contemporaneous, not only as belonging to the same epoch, but as appertaining strictly to the same subdivision of one and the same epoch; although they would in fact have been separated by an interval of several hundred thousand years.