Interglacial periods are treated of at considerable length in Chapters XV. and XVI.; and it is justly held that any theory which does not explain the occurrence of the plant-beds of the drift, as well as its bowlder clays, is unworthy of acceptance. The fossiliferous strata of the marine formations found in Arctic regions, as well as the whole series of fossiliferous deposits found in the drift, are referred to these brief periods of unusually mild climate; but this seems almost too radical. If the general series of marine deposits in the Arctic regions were as nearly unfossiliferous as are the sedimentary strata of tropical India, such an hypothesis would be a little more likely to find acceptance among geologists. The fact developed by Meech, that the polar regions ought to have a warmer summer than the equator, if the solar intensity is a fair criterion, would indicate that these regions should have only a temperate climate if the ice were removed and the summer's heat stored up in the earth; and so slight an additional quantity of heat would accomplish this in a few years that, in view of the known variability of the solar emission and of the terrestrial absorption, it seems quite unnecessary to attach so much importance to the interglacial periods in their relations to Arctic formations. That coal is an interglacial formation, as is suggested in Chapter XXVI., seems still less probable, chiefly because these periods are too short to admit of so great an accumulation of vegetable matter as is stored up in each coal-seam.
It is not improbable, indeed, especially if the marine currents were not seriously affected by the polar snows, that the greater part of each of these periods would be required to melt the ice which had accumulated during the preceding glacial period. It seems very doubtful, too, even if the melting of the ice took place with the greatest conceivable rapidity, whether terrestrial animals or plants would spread over the barren wastes of crude glacial débris so rapidly as to people so wide a zone in the brief period assigned. Some of the intercalated fossiliferous beds of the drift, too, are very rich in numbers as well as species of both animals and plants—the latter sometimes forming extensive deposits of lignite—which must have required an immense time for their development. It seems scarcely possible that these terrestrial deposits can be interglacial, in the sense in which Dr. Croll employs the term, though the aqueous deposits, containing fossil shells of marine and estuarine mollusks, may justly be so considered; for such animals would be likely to keep close to the margin of the icecap as it retreated. To explain the two principal divisions of the drift, which have been recognized over immense areas on both sides of the Atlantic, it seems equally reasonable to refer the uppermost to the last period of high eccentricity, and the lower to that which Sir Charles Lyell supposed to coincide with the glacial epoch; in fact, in support of this collocation, we have the striking coincidence that the ice extended some degrees farthest during the period of greatest eccentricity.
