304 GEOLOGIC TIME. 



period.* At a later date, wlien attempting to assign a minor limit to the 

 duration of geologic time, be was driven to the conclusion that geologic 

 climates are due to the combined cooling of the earth and sun. On com- 

 paring the rates of cooling of such a body as the earth with the maxi- 

 mum measured thickness of the several strata, there is a remarkable 

 proportion between them, which leads toward the conclusion that the 

 maximum thickness of the strata is iirojiortional to the times of their 

 Ibrmation. From the combined conclusions deduced from the rate of 

 cooling of the eartli and the time required for dei)osition of the sedi- 

 mentary rocks, he gives for the whole duration of geologic time a mini- 

 mum of 200,000,000 years.t 



Croll. — Dr. James Groll began his studies of denudation as a factor 

 in estimates of geologic time in 1805, and reference is made to it in 

 I8O74 111 the following year a more elaborate paper was published, 

 and subsequently numerous references have been made to it and other 

 factors that are to bo considered in the estimate of geologic time. § Dr. 

 Croll agrees with Sir W. Thompson that Prof. Tait probably under-esti- 

 mated the time when he afltirms that 10,000,000 years is about the 

 utmost that can be allowed, from the physical point of view, for all the 

 changes that have taken place in the earth's surface since vegetable 

 life of the lowest known form was capable of existing there. He re- 

 marks : 



"And this is certainly all that ever can be expected from giavitation; 

 mathematical computation has demonstrated that it can give no more. 

 The other theory, founded on motion in space — a cause as real as gravi- 

 tation — labors under no such limitation. According to it, so far at 

 least as regards the store of energy which may have been possessed by 

 the sun, plant and animal life may date back, not to 10,000,000 years, 

 but to a period indetinitely more remote. In fact, there is as yet no 

 known limit to the amount of heat which this cause may have i>ro 

 duced; for this depended upon the velocities of the two bodies at the 

 moment prior to collision, and what these velocities were we have no 

 means of knowing. They might have been 500 miles a second, for 

 anything which can be shown to the contrary. Of course, I by no 

 means aftirm that it is as much as 100,000,000 years since life began 

 upon our earth, but I certainly do affirm that, in so far as a possible 

 souice of the sun's energy is concerne<l, life may have begun at a ])erio(l 

 as remote." II 



Dr. Croll considers the geological evidence relating to the age of the 

 sun's heat on the principle that, " in order to determine the present 

 rate of sub-aerial denudation, we have only to ascertain the quantity of 

 sediment annually carried down by the river systems.''* After extended 

 consideration of the evidence he concludes that a period of 24,000,000 



* Manual of Geology, 3d od., 1871, p. 101. 



\ Nature, July 4, 1878, vol. xviii, pp. 267,268. 



tPhil. Mag., Feb. 1867, vol. xxxiii, p. 130. 



^Fhil. Mag., London, May, 1868, voL xxxv, pp. 363-.384; Nov., 1868, vol. xxxvi, 

 P]). 362-886. <leol. Mag., London, 1871, pp. 97-102. Climate and Time, London, 1875, 

 pp. 329-367. Stellar Evolution and its Relations to Geological Time, 1889, pp 39-68. 



\\ Evolution and it.<! lielalions to Geological Time. 1889, p. 36. 



