HOW OLD IS THE EARTH? 159 



ures in years for the recedingly earlier and far longer Tertiary, 

 Mesozoic or Secondary, Palaeozoic or Primary, and Archaean or 

 Beginning eras, which last takes us back almost or quite to the 

 time when the cooling molten earth became first enveloped with 

 a solid crust. 



Haughton has estimated time ratios from two series of data. 

 His results deduced from the maximum thickness of the strata 

 for the three grand divisions of Archaean, Palaeozoic, and subse- 

 quent time, expressed in percentages, are 34'3 : 42'5 : 23 '2 ; and 

 from his computations as to the secular cooling of the earth, 33'0 : 

 41'0 : 26'0. The ratios reached by Profs. J. D. Dana and Alexan- 

 der Winchell from the thicknesses of the rock strata are closely 

 harmonious, the durations of Palaeozoic, Mesozoic, and Cenozoic 

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

 nary ages, the latter extending to the present day, which are here 

 united as the Cenozoic era, Dana would rank approximately in 

 the ratio of 3 : 1, giving to the Quaternary a sixty-fourth part of 

 all time since the beginning of the Cambrian period, to which our 

 earliest well-preserved fossil faunas belong. For reasons to be 

 stated later, I think that this estimate of the relative length of 

 Quaternary time is greatly exaggerated ; but this would not sen- 

 sibly affect the general ratios. 



Prof. W. M. Davis, of Harvard University, without speaking 

 definitely of the lapse of time by years, has endeavored to give 

 some conception of what these and like estimates of geologic 

 ratios really mean, through a translation of them into terms of a 

 linear scale. Starting with the representation of the postglacial 

 or recent period, since the North American ice-sheet was melted 

 away, as two inches, he estimates that the beginning of the Ter- 

 tiary erosion of the Hudson River gorge through the Highlands 

 would be expressed by a distance of ten feet ; that the Triassic 

 reptilian tracks in the sandstone of the Connecticut Valley would 

 be probably fifty feet distant ; that the formation of the coal 

 beds of Pennsylvania would be eighty or one hundred feet back 

 from the present time ; and that the Middle Cambrian trilobites 

 of Braintree, Mass., would be two hundred, three hundred, or four 

 hundred feet from us. 



Having such somewhat definite and agreeing ratios, derived 

 from various data by different investigators, can we secure the 

 factor by which they should be multiplied to yield the approxi- 

 mate duration of geologic epochs, periods, and eras, in years? If 

 on the scale used by Prof. Davis we could substitute a certain 

 time for the period since the departure of the ice-sheet, we should 

 thereby at once determine, albeit with some vagueness and ac- 

 knowledged latitude for probable error, how much time has passed 

 since the Triassic tracks were made, the coal deposited, and the 



