ESTIMATES OF TIME 811 



slice from his allowance of geological time. Lord Kelvin is willing, I believe, 

 to grant us some twenty millions of years, but Professor Tait would have us 

 content with less than ten millions." ^^ 



Huxley in 1869 had refused to be limited by Kelvin's reasoning, con- 

 sidering that unknown factors may vitiate the results. ^^ Geikie in 1893 

 states of it also : 



"That there must be some flaw in the physical argument I can, for my own 

 part, hardly doubt, though T do not pretend to be able to say where it is to be 

 found. Some assumption, it seems to me, has been made, or some considera- 

 tion has been left out of sight, which will eventually be seen to vitiate the 

 conclusions, and which when duly taken into account will allow time enough 

 for any reasonable interpretation of the geological record." ^^ 



In 1903 Geikie states : 



"Until it can be shown that geologists and paleontologists have misinter- 

 preted the records contained in the earth's crust, they may not unreasonably 

 claim as much time for the history revealed in these records as the vast body 

 of accumulated evidence appears to them to demand. There is a general agree- 

 ment among the geologists that, so far as the phenomena of sedimentation and 

 tectonic structure are concerned, 100 millions of years would probably suffice 

 for the completion of the geological record. But if on paleontological grounds 

 the allowance of time should be found too small, there appears to be no reason, 

 on at least the geological side, why it should not be enlarged, as far as may b(> 

 found needful for the satisfactory interpretation of the evolution of organized 

 existence on the globe." *" 



^Nevertheless, it is evidently true that, from among the various possible 

 interpretations of the data, geologists were in general inlclined to choose 

 those which gave the lower limits to geological time. Such estimates 

 which limited the earth to a duration of 100,000,000 years or less were 

 easy to attain by taking fairly rapid rates of erosion and sedimentation 

 and assuming that these represented the average of geological time. To 

 show how wide are the limits as given by estimates based on geologic 

 processes, the various modes of attack must be enumerated. 



First to be taken up are those concerning the duration of the latest 

 periods as determined especially by the erosion history rather than by 

 the record of sedimentation. 



Upham, who holds to the view that the Pleistocene consisted of one 

 advance and retreat, concluded that the 



37 A. Geikie : Geological change and time. Presidential address, Brit. Asso. Adv. Sci., 

 1892. Ann. Kept. Smithsonian Institution for 1892, pP- 124, 125. 



38 T. H. Huxley: Presidential address, Geol. Soc. London. Quart. Jour. Geol, Soc, 

 vol. XXV, 1869, pp. xxxviii-liii. 



39 A. Geikie : Loc. cit., p. 126. 



*o A. Geikie : Text-book of Geology, 1903, pp. 77-78. 



