W. Up ham — Estimates of Geologic Time. 213 



As a confirmation of the validity of his estimate of 28,000,- 

 000 years, Wallace cites the estimates differently obtained 

 through the geologic time ratios of Lyell and Dana, in combi- 

 nation with Dr. Croll's astronomic theory of the causes of the 

 Ice age, which attributes the accumulation of ice-sheets to 

 stages of high eccentricity of the earth's orbit. The Quater- 

 nary Glacial period is assigned by this theory an extent of 

 about 160,000 years, from 240,000 to 80,000 years ago. The 

 next preceding epoch of great eccentricity was about 850,000 

 years ago, and to that time are referred large ice-borne blocks 

 in Miocene strata of northern Italy. The union of this assump- 

 tion with the time ratios for the Tertiary and earlier eras is 

 explained as follows by Wallace in " Island Life," chapter x. 



Sir Charles Lyell, taking the amount of chauge in the species of roollusca as a 

 guide, estimated the time elapsed since the commencement of the Miocene as 

 one-third that of the whole Tertiary epoch, and the latter at one-fourth that of 

 geological time since the Cambrian period. Professor Dana, on the other hand, 

 estimates the Tertiary as only one fifteenth of the Mesozoic and Paleozoic com- 

 bined. On the estimate above given [that the time since a Miocene glacial epoch 

 has been 850,000 years], founded on the dates of phases of high eccentricity, we 

 shall arrive at about four million years for the Tertiary epoch, and sixteen mil- 

 lion years for the time elapsed since the Cambrian, according to Lyell, or sixty 

 millions according to Dana. The estimate arrived at from the rate of denudation 

 and deposition (twenty-eight million years) is nearly midway between these, and 

 it is, at all events, satisfactory that the various measures result in figures of the 

 same order of magnitude, which is all one can expect on so difficult and exceed- 

 ingly speculative a subject. . . . The time thus arrived at is immensely less than 

 the usual estimates of geologists, and is so far within the limits of the duration 

 of the earth as calculated by Sir William Thomson as to allow for the develop- 

 ment of the lower organisms an amount of time anterior to the Cambrian period 

 several times greater than has elapsed between that period and the present day. 



Professor Haughton has estimated time ratios from two 

 series of data. His results deduced from the maximum thick- 

 ness of the strata for the three grand divisions of Archaean, 

 Paleozoic, and subsequent time, expressed in percentages, are 

 34-3: 42*5 :23 - 2 ; aud from his computations as to the secular 

 cooling of the earth, 33*0 :410 : 26*0. From his consideration 

 of the present rates of denudation and the maximum thick- 

 ness of the strata, he obtains " for the whole duration of 

 geological time a minimum of two hundred millions of years." 

 In my opinion, this is a large rather than a small total esti- 

 mate ; but the length of Archaean or pre-Cambrian time seems 

 to me proportionately much greater than is here allowed. 



The ratios reached by Profs. J. D. Dana and Alexander 

 Winchell, from the thicknesses of the rock strata, are closely 

 harmonious, the durations of Paleozoic, Mesozoic, and Ceno- 

 zoic time being to each other as 12 : 3 : 1. The Tertiary and 

 Quaternary ages, the latter extending to the present day, are 

 here united as the Cenozoic era. Professor Dana has further 

 ventured a supposition that these three vast eras, from the 

 Cambrian dawn until now, may comprise some 48,000,000 



