[37] The Great Ice Age, p. 177.
[38] These are named, in descending order, Hessle Boulder Clay, Purple Boulder Clay, Chalky Boulder Clay, and Lower Boulder Clay—below which is the Norwich Crag.
[39] "On the Climate of the Post-Glacial Period." Geological Magazine, 1872, pp. 158, 160.
[40] Geological Magazine, 1876, p. 396.
[41] Early Man in Britain and his Place in the Tertiary Period, p. 113.
[42] Heer's Primæval World of Switzerland Vol. II., pp. 148-168.
[43] Dr. James Geikie in Geological Magazine, 1878, p. 77.
[44] This subject is admirably discussed in Professor Asa Gray's Lecture on "Forest Geography and Archæology" in the American Journal of Science and Arts, Vol. XVI. 1878.
[45] In a letter to Nature of October 30th, 1879, the Rev. O. Fisher calls attention to a result arrived at by Pouillet, that the temperature which the surface of the ground would assume if the sun were extinguished would be -128° F. instead of -239° F. If this corrected amount were used in our calculations, the January temperature of England during the glacial epoch would come out 17° F., and this Mr. Fisher thinks not low enough to cause any extreme difference from the present climate. In this opinion, however, I cannot agree with him. On the contrary, it would, I think, be a relief to the theory were the amounts of decrease of temperature in winter and increase in summer rendered more moderate, since according to the usual calculation (which I have adopted) the differences are unnecessarily great. I cannot therefore think that this modification of the temperatures, should it be ultimately proved to be correct (which is altogether denied by Dr. Croll), would be any serious objection to the adoption of Dr. Croll's theory of the Astronomical and Physical causes of the Glacial Epoch.
The reason of the theoretical increase of summer heat being greater than the decrease of winter cold is because we are now nearest the sun in winter and farthest in summer, whereas we calculate the temperatures of the glacial epoch for the phase of precession when the aphelion was in winter. A large part of the increase of temperature would no doubt be used up in melting ice and evaporating water, so that there would be a much less increase of sensible heat; while only a portion of the theoretical lowering of temperature in winter would be actually produced owing to equalising effect of winds and currents, and the storing up of heat by the earth and ocean.