GLACIAL PERIOD, "WITH REFERENCE TO THE ANTIQUITY OE MAN. 393 



29. Considerations on the Date, Duration, and Conditions of the 

 Glacial Period, ivith reference to the Antiquity of Man. By 

 Joseph Prestwich, M.A., F.R.S., Professor of Geology in the 

 University of Oxford. (Head May 25, 1887.) 



"When, twenty-eight years ago, the barriers which restricted the 

 age of Man to a limited chronology were overthrown by the disco- 

 veries in the Yalley of the Somme and Brixham Cave, the pent-up 

 current of geological opinion tended to the other extreme of 

 assigning to Man (Post-glacial) an excessive antiquity ; and now the 

 belief in that great antiquity seems to me to be exerting, in a 

 manner somewhat similar to that which at first caused the rejection 

 of the Kent's Cave and Somme Valley evidence, an imperceptible bias 

 on the questions raised as to the Glacial and Preglacial age of Man — 

 apart from the question of Pliocene or Miocene Man, into which, 

 on the present occasion, I do not purpose to enter. 



The extreme opinions which dealt with millions of years are now 

 probably held by few ; but still, with many and, probably, the majo- 

 rity of geologists the Glacial and Postglacial periods, which involve 

 the question of the antiquity of Man, are measured by very great 

 terms of time. At the outset of the discussion, and when the 

 antiquity of Man was limited to the Postglacial period, I saw no 

 reason for assigning to that period the length of time then claimed by 

 many geologists. The data, however, on which to form an opinion 

 with respect to the duration of this and of the Glacial period were 

 too imperfect to form any definite conclusion upon. Since then 

 further observations in the Alps, and more especially the observa- 

 tions of the Danish geologists in Greenland, have brought forward 

 facts of great importance in their bearing on ice-action and 

 growth, and have furnished us with data which may warrant 

 the estimates I now beg to lay before the Society, not as a complete 

 discussion of the subject, but as a preliminary inquiry. 



Measured by our own limited experience, the excavation of the 

 Postglacial valleys, the life of the successive generations of the 

 Pleistocene Mammalia, and the dying-out or extinction of a large 

 number of species might seem to demand a long period of time. Con- 

 sequently at first it was suggested that the Glacial period com- 

 menced possibly about a million years since, and that the Post- 

 glacial period had lasted about 200,000 years. It was felt, how- 

 ever, on the other hand, that the very large proj3ortion of existing 

 species of Yertebrata and Invertebrata which came in with the 

 Pleistocene period and had since undergone no change, and this 

 combined with the stationary condition of Man himself during so 

 long an interval, presented serious objections to adopting such 

 lengthened periods of time. But on neither side were the conclusions 

 based on any definite data. 



The question was in this state when Dr. Croll's attention was 



