Geological Societi/ of London. 327 



masses of larger scales. The Eev. P. B. Brodie and Mr. Edward 

 Wilson each appended notes on the Triassic beds from which the 

 fishes were obtained. 



2. " Considerations on the Date, Duration, and Conditions of the 

 Glacial Period with reference to the Antiquity of Man." By Prof. 

 Joseph Prestwich, M.A., F.R.S., F.G.S. 



After showing how the discoveries in the valley of the Sommeand 

 elsewhere, 28 years ago, led geologists who had previously been 

 disposed to restrict the age of man, to exaggerate the period during 

 which the human race had existed, the author proceeded to discuss 

 the views of Dr. Croll on the date of the Glacial epoch. Dr. Croll, 

 who had at first referred this to an eai'lier phase of orbital eccentricity, 

 commencing 980,000 years ago, subsequently regarded it as coinciding 

 with a minor period of eccentricity that commenced 240,000 and 

 terminated 80,000 years since. This last estimate was chiefly sup- 

 ported by the amount of denudation that had subsequently taken place. 



The efficacy of the increased eccentricity of the earth's orbit in 

 producing the cold of the Glacial epoch was shown to be very doubt- 

 ful ; for as similar changes in the eccentricity had occurred 165 times 

 in the last 100 millions of years, there must have been many glacial 

 epochs in geological times, several of them much more severe than that 

 of the Pleistocene period. But of such glacial epochs there was no 

 valid evidence. Another inference from Dr. Croll's theories, that 

 each glacial epoch consisted of a succession of alternating cold and 

 warm or interglacial phases was also questioned, such alternations 

 as had been indicated having probably been due to changes in the 

 distribution of land and water, not to cosmical causes. The time 

 requisite for such interglacial periods as were supported by geological 

 evidence was more probably hundreds than thousands of years. 



Eecent observations in Greenland by Professor Hell and, Mr. V. 

 Steenstrup, and Dr. Rink, had shown that the movement of ice in 

 large quantities was much more rapid, and consequently the denuda- 

 tion produced much greater than was formerly supposed. The 

 average rate of progress in several of the large iceberg-producing 

 glacers in Greenland had been found to be 36 feet daily. Applying 

 these data and the probable accumulation of ice due to the rainfall 

 and condensation to the determination of the time necessary for the 

 formation of the ice-sheet, the author was disposed to limit the 

 duration of the Glacial epoch to from 15,000 to 20,000 years, includ- 

 ing in this estimate the time during which the cold was diminishing, 

 or Postglacial time. 



Details were then given to show that the estimate of one foot on 

 an average being removed from the surface by denudation in 6000 

 years, on which estimate was founded the hypothesis of 80,000 

 years having elapsed since the Glacial epoch, was insufficient, as a 

 somewhat heavier rainfall and the disintegrating effects of frost 

 would produce far more rapid denudation. It was incredible that 

 man should have remained physically unchanged throughout so long 

 a period. At the same time the evidence brought forward by Mr, 

 Tiddeman, Dr. Hicks, and Mr. Skertchly of the occurrence of 



