394 



PROE. J". PRESTWICH ON THE GLACIAL PERIOD, WITH 



directed to the subject. After careful investigation of existing 

 hypotheses he came to the conclusion that the cold of the Glacial 

 period could not be ascribed to any physiographical changes in the 

 distribution of land and water, but that changes in the eccen- 

 tricity of the earth's orbit afforded a probable clue to great secular 

 variations of climate, such as would produce glacial epochs. Follow- 

 ing up Leverrier's calculations, and assuming that the periods 

 of greatest cold were when the eccentricity rose to a high value, 

 and that the warmer periods occurred during the times of lesser 

 eccentricity, Dr. Croll, by an elaborate mathematical computation, 

 extended the inquiry as to the extent and periods of maximum 

 and minimum eccentricity for 3,000,000 years back and 1,000,000 

 to come, and showed that within the last million years there have 

 been two such periods of extreme eccentricity — the one extending 

 from 980,000 to about 720,000 years ago, and the other from about 

 240,000 down to 80,000 years ago * 



As the former period was of greater duration than the latter, 

 and the eccentricity also then attained its highest value, Dr. Croll 

 was at first disposed to refer the Glacial epoch proper to that period, 

 and to consider the latter as corresponding with the extension of 

 the local glaciers towards the close of the Glacial epoch. On this 

 pointhe states that he "consulted several eminent geologists, and they 

 all agreed in referring the Glacial epoch to the former period," the 

 reason assigned being that they considered the latter period to be 

 much too recent and of too short duration to represent that epoch. 



Dr. Croll therefore had good warrant that he was well within 

 the limits of geological probabilities when, on a reconsideration of 

 the subject, he came to the conclusion that the Glacial epoch must 

 be referred, not to the first-named period of eccentricity, but to the 

 later one, commencing 240,000 years ago ; and this is a date now 

 very generally accepted. He considered that " the modern and 

 philosophic doctrine of uniformity had led geologists to over-esti- 

 mate the length of geological periods " (p. 325). Nevertheless he 

 assumes that " the present rate of subaerial denudation does not 

 differ greatly from that which has obtained since the close of the 

 Glacial epoch " (p. 338), and proceeds with the argument in 

 accordance with this view, taking the rate of denudation to be one 

 foot of soil removed from the surface in 6000 years — an estimate 

 founded upon the quantity of sediment now carried down to the 

 sea by such rivers as the Mississippi, the Rhine, the Rhone, the 

 Ganges. &c. This is, in fact (notwithstanding the qualification 

 just admitted), applying the results of experience in recent times 

 to facts observable in climates where the meteorological pheno- 

 mena were totally different, and the conditions therefore not ana- 

 logous. Consequently the data cannot be rightly applicable. If 

 used at all, as a sort of base-line, it must be with modifications, 

 such as taking the modern data as the known quantity, and adding 

 an unknown quantity " oc" which quantity has to be determined 

 before we can get at the true rate of denudation during the period 

 * ' Climate and Time,' chapter xix. (1875). 



