210 W. Upham — Estimates of Geologic Time. 



same month,* reasoning from similar premises of geologic 

 observations, would claim about seven thousand millions of 

 years as the more probable measure of the part of the earth's 

 duration since its earliest fossiliferous rocks were formed, and 

 probably twice as long time since the earth began its planetary 

 existence. 



On the other hand, the most eminent writers who have con- 

 sidered this subject from the standpoint of physical experi- 

 ment and theory and their relationship with astronomy, 

 including Thomson, Tait, Newcomb, Young, and Ball, tell us 

 that geologists can be allowed probably no more than 100 

 millions of years, and perhaps only about 10 millions, since 

 our earth was so cooled as to permit the beginning of life 

 upon it. 



It is comparatively easy to determine the ratios or relative 

 lengths of the successive geologic eras, but is confessedly very 

 difficult to decide beyond doubt even the approximate length 

 in years of any part of the records of the rock strata. The 

 portions for which we have the best means of determining 

 their length are the Glacial and Recent periods, the latter 

 extending from the Cham plain epoch, or closing stage of the 

 Ice age, to the present time, while these two divisions, the 

 Glacial or Pleistocene period and the Recent, make up the 

 Quaternary era. If we can only ascertain somewhat nearly 

 what has been the duration of this era, from the oncoming 

 of the Ice age until now, it will serve as a known quantity 

 to be used as the multiplier in the several ratios for giving 

 us the approximate or probable measures in years for the 

 recedingly earlier and far longer Tertiary, Mesozoic, Pale- 

 ozoic, and Archaean eras, which last takes us back almost or 

 quite to the time when the cooling molten earth became first 

 enveloped with a solid crust. 



Sir William Thomson (now Lord Kelvin) long ago estimated, 

 from his study of the earth's internal heat, its increase from 

 the surface downward, and the rate of its loss by radiation into 

 space, that the time since the consolidation of the surface of 

 the globe has been somewhere between 20 millions and 400 

 millions of years, and that most probably this time and all the 

 geologic record must be limited within 100,000,000 years.f 



Prof. George H. Darwin computes, from the influence of 

 tidal friction in retarding the earth's rotation, that probably 

 only 57,000,000 years have elapsed since the moon's mass was 



* Am. Anthropologist, Oct., 1892, vol. v. pp. 327-344. with a plate showing- 

 relative durations of natural time units, historical eras, and geologic periods. 



f In an article published two months ago in this Journal, since the present paper 

 was written, Mr. Clarence King, from recent physical investigations of diabase 

 when subjected to great heat and pressure, concludes that the age of the earth, 

 deduced by Lord Kelvin's method, is approximately 24,000,000 years. 



