of the Glacial Epoch. 337 



to such an extent as to render the proportion above given between 

 the pre- and post-glacial periods considerably too great. Thirdly, 

 the advent of the severer portion of the glacial epoch would ne- 

 cessarily effect a rapid refrigeration of the ocean, and thus mag- 

 nify the cooling effected during the post-glacial unit of time. 



The experiments of Dalton on the rate of evaporation at dif- 

 ferent temperatures serve to illustrate the truth of the first of 

 these considerations. He found, and his results were afterwards 

 confirmed by Daniel!, that the rate of evaporation increases in a 

 geometrical progression with equal increments of sensible heat. 

 From the same cause, the deposition of most of the pre-glacial 

 sedimentary rocks must have also proceeded at a much more 

 rapid rate than that which has obtained since the glacial epoch ; 

 in fact the rate of deposition depends so essentially upon the 

 amount of precipitation, as to render it by no means improbable 

 that the rate of decrease from a certain point down to the glacial 

 epoch was also approximately a geometrical one : thus the pro- 

 portion between the pre- and post-glacial zoic periods would be 

 considerably reduced. Finally, the descent of the snow-line to 

 the level of the sea along an immense length of coast during the 

 glacial epoch must have had the effect of very rapidly reducing 

 the temperature of the ocean, since the precipitation upon the 

 land, instead of reaching the sea after being warmed at the ex- 

 pense of solar heat, would now be thrown into the ocean, partly 

 as ice-cold water, but chiefly as ice itself; and every ton of the 

 latter in melting would cool more than fourteen tons of sea-water 

 through 10° F. It must also be borne in mind that this ice- 

 cold water would not float on the surface, as in a freshwater 

 lake, but would at once sink into the subjacent warm water, 

 because the point of maximum density of sea-water is, according 

 to Despretz*, 6°*6 F. below the freezing-point of pure water. 

 Thus, taking all these circumstances into consideration, and 

 assuming that the ocean has lost as much as 20° F. of heat since 

 the glacial epoch, I conceive it possible that there would still be 

 a sufficient interval, between a temperature incompatible with 

 marine animal life and that epoch, for the development of the 

 various organisms inhabiting the pre-glacial seas. In the case 

 of land animals and plants no such difficulty arises, since, as 

 already explained, the surface temperature was, during the whole 

 of the period contemplated, very little affected by the internal 

 heat of the earth. 



3. Recent research has led some geologists to the conclusion 



that glacial action existed in the Miocene and even so far back 



as the Permian period ; and although the evidence upon which 



this conclusion rests is by no means generally accepted by geo- 



* Ann. de Chim. et de'Phys. vol. lxx. p. 45. 



Phil Mag. S. 4. Vol. 27. No. 183. May 1864. Z 



