214 W. Ujpham — Estimates of Geologic Time. 



years, which would give for the Paleozoic era, 36,000,000 

 years ; the Mesozoic, 9,000,000 ; and the Cenozoie, 3,000,000. 

 He disclaims, however, any assumption that those figures are 

 " even an approximate estimate of the real length of the inter- 

 val, but only of relative lengths and especially to make apparent 

 the fact that these intervals were very long? 1 * 



Prof. W. M. Davis, without speaking definitely of the lapse 

 of time by years, endeavors 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. f 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 50 feet distant ; that the formation of the coal beds 

 of Pennsylvania would be 80 or 100 feet back from the 

 present time ; and that the Middle Cambrian trilobitcs of 

 Braintree, Mass., would be 200, 300, or 400 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 Professor 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 vague- 

 ness and acknowledged latitude for probable error, how much 

 time has passed since the Triassic tracks were made, the coal 

 deposited, and the trilobites entombed in the Cambrian slates. 

 Now just this latest and present division of the geologic record, 

 following the Ice age, is the only one for which geologists 

 find sufficient data to permit direct measurements or estimates 

 of its duration. *' The glacial invasion from which New 

 England and other northern countries have lately escaped," 

 remarks Davis, " was prehistoric, and yet it should not be 

 regarded as ancient." 



In various localities we are able to measure the present rate 

 of erosion of gorges below waterfalls, and the length of the 

 postglacial gorge divided by the rate of recession of the falls 

 gives approximately the time since the Ice age. Such measure- 

 ments of the gorge and falls of St. Anthony by Prof. N. H. 

 Winchell show the length of the Postglacial or Recent period 

 to have been about 8,000 years ; and from the surveys of 

 Niagara falls, Mr. G. K. Gilbert believes it to have been 7,000 

 years, more or less. From the rates of wave-cutting along the 



* Manual of Ueology, p. 795. -f Atlantic Monthly, July, 1891, p. 77. 



