306 Gi:OLOGIC TIME. 



The results obtained by Wallace are (j[uesti()iied byT. Mellard Reade, 

 wbo states tliat Wallace lias not allowed for the erosiou and re-dejwsi- 

 tion of the same sediments a number of times.* 



Winchell. — Dr. Alexander Wincliell reviews the opinions of physicists 

 and geologists on the age of the world or of ceitain periods, and enu-- 

 merates the grounds for the various estimates, as follows: 



" (1) The time recjuired tor the sun to contract from a nebulous con- 

 dition or froui the orbit of the earth to its present limits. 



" (2) The time which the sun will re(]uireto cool from its present con- 

 dition to a darkened or planetary state. 



" (3) The time recjuired for the earth to cool from incipient incrusta- 

 tion to its present state, based on the thermal conductivity of rock 

 masses and the rate of inci'eased heat toward the earth's center. 



"(4) Kelative times required for the deposition of all the rocky sedi- 

 ments. 



" (o) Calculation based on the obliteration of the rotational effects of 

 the upheaval of a continental mass. 



" (0) The time since the middle of the last glacial period, based on 

 the theory that epochs of glaciation on the northern hemisphere have 

 been caused by extreme eccentricity of the earth's orbit. 



" (7) Estimates based on rates of erosions and dei)osition. 



"(8) The rate of Bluff-recession and Terrace formation. 



" (0) Decrease of temperature of ground covered by ice during the 

 glacial period, as compared with temperature of ground not chilled by 

 the ice sheet." t 



Dr. Winchell was inclined to accord at least equal contidence to the 

 later results of geologic action, such as erosion of river gorges and 

 lakeside and seaside bluff's, as he would give to the mathematical 

 methods of the x)hysicist. On this basis he deduced the result that the 

 whole incrusted age of the Avorld would be 3,000.000 years. In con- 

 clusion he says: 



"If our attemi)ts to ascertain the age of the world, or the duration 

 of any single period of its evolution, yield only uncertain results, they 

 suffice at least to demonstrate that geological history has limits far 

 within the wild cimcf ptions of a certain class of geologists. They 

 show, if we may credit the indications here regarded most trustworthy, 

 a restriction of the modern epoch within limits not exceeding one-tenth 

 or one-twentieth the duration sometimes assigned to it."| 



GeUcie. — Sir Archibald Geikie has recently summed up the case of the 

 geologist and physicist in a very clear statement, as follows: 



"In scientific as in other mundane questions there may often be two 

 sides, and the truth may ultimately be found not to lie wholly with 

 either. I frankly confess that the demands of the early geologists for 

 an unlimited series of ages were extravagant, and even, for their own 

 pur])oses, unnecessary, and the physicist did good service in reducing 

 them. It may also be freely admitted that the latest conclusions Ironi 

 physical considerations of the extent of geological time require that the 

 inter])retation given to the re^-ord of the rocks should be vigorously 

 revised, with the view of ascertaining how far that interpretation may 



* Geol. Mag., 1883, vol. X, pp. 309, 310. 



t World Life, or Comparatife Geology, Chicago, 1883, pp. 355-376. 



X Loc. cit., p. 378. 



