Yol. 5 7.] CLIilATE OF THE PLEISrOCEXE EPOCH. 473 



au abstract of the present paj^er ; and the second had not then 

 appeared. 



These writers adopt the view, not only that the more important 

 changes of climate during past ages, such as that, for example, 

 which is supposed to have occurred between the Carboniferous and 

 the Permian, were due to variations in the amount of carbon - 

 dioxide in the atmosphere, but also that to some extent the minor 

 climatal oscillations of the Glacial Period may be traced to the 

 same cause. Such an alteration in the atmospheric constitution must 

 have been, however (as Prof. Chamberlin points oat), of general and 

 not of local operation, and the hypothesis that the latter group of 

 events was so caused is inconsistent with that suggested by me that 

 the maximum glaciation of one region may have been contempor- 

 aneous with the existence of genial conditions in another, situated in 

 a similar latitude. 



It does not follow, however, that because the carbon-dioxide 

 theory may account for the climatal change at the end of the 

 Palaeozoic Era just named, or for the long and gradual refrigeration 

 which went on, apparently without intermission, during the 

 Miocene,^ and until the end of the Pliocene, as well as for the 

 general rise in temperature which has taken place since the end of 

 the Glacial Period, that it must necessarily have been the cause of 

 all, or even part of, the marked, repeated, and possibly sudden 

 changes which, commencing at the close of the Pliocene, continued 

 during the Glacial and apparently, though with less intensity, 

 during the post-Glacial Period.'- 



I cannot help doubting whether the suggested alterations in the 

 constitution of the atmosphere could have been sufficiently rapid in 

 operation to have originated the latter. Dr. Ekholm, indeed, 

 remarks {op. c'lt. p. 26), dealing with the gradual reduction of 

 temperature in Miocene times, that 



' the temperate Polar climate of that age, with its slowly-proceeding deterior- 

 ation, may have occurred during a rate of carbonic acid not much greater 

 perhaps than the present one, the cooling influence of a slow decrease of 

 carbonic acid exhibiting its full strength only much later.' 



Prof. Chamberlin points out that the reduction of the thermal 

 absorption of the atmosphere during the Ice Age consequent on a 

 deficiency of carbon-dioxide, would have intensified the difference in 

 temperature between the Equatorial and Polar regions, and between 

 that of the land and of the sea,^ and Dr. Ekholm expresses a some- 

 what similar opinion.^ This, however, would have tended to pro- 

 duce, under the special circumstances of the Glacial Period, increased 



^ The fossil moUusca of the various Miocene horizons of the Mediterranean 

 region, for example, show, in Prof. Sacco's opinion, no indication of alternations 

 of climate. 



^ Prof. Chamberlin indeed believes that some of the less important climatic 

 changes of the Glacial Period may have bean due to variations in the atmo- 

 spheric circulation, Journ. Geol. Chicago, vol. vii (1899) p. 772. 



^ Ibid. p. 555. ^ Quart. Journ. Eoy. Met. Soc. vol. xxvii (1901) p. 24. 



