158 THE POPULAR SCIENCE MONTHLY. 



that the argument rests upon many indeterminate premises, since 

 the total extent of the land areas and the depths of the oceans 

 have probably been increasing through the geologic eras, and the 

 effects of tides have probably diminished. The imperfection of 

 the geologic record, so impressively shown by Charles Darwin in 

 respect to the sequence of plants and animals found fossil in the 

 rocks, will also be appealed to as opposing the assumption that 

 the one hundred and seventy-seven thousand two hundred feet, 

 or thirty-three and a half miles, of strata represent the whole, or 

 indeed any more than a small fraction, of the earth's history. To 

 myself, however, this last objection seems unfounded, since in 

 many extensive and clearly conformable sections observed on a 

 grand scale in crossing broad areas, there is seen to have been 

 evidently continuous deposition during several or many successive 

 geologic epochs ; and by combining such sections from different 

 regions a record of sedimentation is made well-nigh complete from 

 the earliest Palseozoic morning of life to its present high noon. 

 But perhaps we may do better to change somewhat the premises of 

 our computation, in view of the extensive regions where the rock 

 strata remain yet to be thoroughly explored, and because of cer- 

 tain large inland tracts having little rain and therefore no drain- 

 age into the sea. If we assume that the total maxima of strata 

 amount to fifty miles, and that the mean rate of the land denuda- 

 tion is only one foot in six thousand years, we then obtain a 

 result three times greater than before, or about eighty-four mil- 

 lion years for the deposition of the stratified rocks. 



Another method of considering this problem is afforded by the 

 determination of one term in a sequence of ratios, whereby the 

 sum of the whole becomes known. Though geologists differ 

 widely in their estimates of the earth's age, up to the seven thou- 

 sand million years claimed by McGee, in an address last year 

 before the American Association, they are approximately in 

 agreement as to the ratios of the several great divisions of geo- 

 logic time. From the thicknesses of the strata and the changes 

 in the animal and plant life, it is comparatively easy to determine 

 the relative lengths of the successive eras, while yet it is very 

 difficult to decide beyond doubt even the approximate length in 

 years of any part of the record. The portions for which we have 

 the best means of determining their lengths 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 for giving us the approximate or probable meas- 



