CR OIL'S "CLIMATE AND TIMEy 821 



believe that it was the first of these periods that produced the glacial 

 epoch. Guided by the rate and amount of post-glacial erosion, how- 

 ever, Dr, Croll conciudes that it was the more recent period which 

 corresponded with the Quaternary glacial epoch ; but he suggests 

 that the earlier period may have coincided with the Miocene glacier. 

 He thinks that the glacial period of the Quaternary lasted from 

 shortly before the last great maximum until about 80,000 years ago 

 (when the eccentricity was 0-0398, corresponding to a diiference of 

 twenty-two days in the length of the seasons), or for about 160,000 

 years — including, of course, the alternating interglacial periods. 



Aside from the strong inherent evidence of the approximate cor- 

 rectness of this determination of the date of the glacial epoch (any 

 uncertainty being due to the imperfection of Leverrier's formula?), 

 there is an abundance of independent testimony leading to substan- 

 tially the same conclusion. Many eminent geologists have calculated 

 the duration of post-glacial time from various data — generally the rate 

 and amount of erosion or deposition in stated localities — with results 

 usually ranging from 100,000 to 300,000 years. The mean of several 

 of the most reliable is a trifle less than 200,000 years. Now, while each 

 of these results, when viewed singly, may properly be regarded, in the 

 words of Sir John Lubbock, "not as a proof, but as a measure of 

 antiquity," they may, when viewed collectively, justly be considered 

 reliable within wide limits, and to prove that no less a period than 

 40,000 or 50,000 years can have elapsed since the retreat of the ice- 

 sheet from temperate latitudes ; and the time has now come when he 

 who endeavors to fix a later date for that event, without showing why 

 these estimates should be rejected, need not be astonished if his efforts 

 only bring him into contempt. So great is the weight of this inde- 

 pendent testimony, indeed, as to warrant the suggestion that the gla- 

 cial epoch of the Quaternary did not extend down to the period of 

 high eccentricity 80,000 years ago, as Dr. Croll intimates, but closed 

 160,000 or 170,000 years ago. This last maximum might, then, be 

 represented by the Reindeer epoch of Europe, and possibly (if it may 

 be permitted in this strongly reactionary age to suggest the bare possi- 

 bility that the pioneers in the field of American archseology, Atwater, 

 Morton, Squier, and their compeers, did not err most egregiously in 

 their estimates of the antiquity of the earlier works of our prehistoric 

 races) by the migration into Mexico and Central America of the 

 mound-builders of the Mississippi Valley — a migration which might 

 thus furnish a parallel with that southerly migration of the Pliocene 

 mammalian fauna at the inception of the glacial period, to which the 

 Indian geologists attribute the richness and variety of the Siwalik and 

 related fossil fauna of the Orient. It might further be urged that, if the 

 duration of the ice age were so great as Dr. Croll suggests, we would 

 be likely to find more unequivocal evidence of the fact in structural 

 variations in those species which survived the cataclysm. 



