822 THE POPULAR SCIENCE MONTHLY. 



Interglacial periods are treated of at considerable length in Chap- 

 ters 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 accept- 

 ance 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 sum- 

 mer'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 absorp- 

 tion, 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 accumu- 

 lated during the preceding glacial period. It seems very doubtful, 

 too, even if the melting of the ice took place with the greatest con- 

 ceivable rapidity, whether terrestrial animals or j)lants would spread 

 over the barren wastes of crude glacial d'ehris 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 ex- 

 tensive deposits of lignite — which must have required an immense 

 time for their development. It seems scarcely possible that these ter- 

 restrial 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 ice- 

 cap 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 ex- 

 tended some degrees farthest during the period of greatest eccentricity. 



