ures in years for the recedingly earlier and far longer Tertiary, Mesozoic or Secondary, Palæozoic or Primary, and Archæan 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 Archæan, Palæozoic, and subsequent 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 Alexander Winchell from the thicknesses of the rock strata are closely harmonious, the durations of Palæozoic, Mesozoic, and Cenozoic time being to each other as 12:3:1. The Tertiary and Quaternary 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 sensibly 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 Tertiary 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 approximate 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 acknowledged latitude for probable error, how much time has passed since the Triassic tracks were made, the coal deposited, and the