404 



PEOP. J. PEESTWICH ON THE GLACIAL PEEIOD, WITH 



both in North-eastern America and North-western Europe. It is 

 therefore more than possible that the mean annual surplus of ice 

 was, independently of the extra quantity due to the increasing cold, 

 larger than in Greenland at present. 



The growth of the ice-sheet is not, however, dependent only on 

 the rainfall. The experiments of MM. Dufour and Eorel have shown 

 that when the temperature of the air on the Rhone glacier varied 

 from 41° to 52° P. there was a condensation of moisture equal to 

 150 cubic metres of water per square kilometre, and this in- 

 creased proportionally the volume of the glacier-ice and water. 

 Under these circumstances it is not difficult to conceive that a foot 

 or more, taking the total precipitation, might be added annually to 

 the thickness of the ice. Even in recent times a difference of level 

 in the surface of some of the Swiss glaciers to the extent of from 

 80 to 100 feet has been known to have been effected in the course 

 of 20 years. 



Taking as the known quantity the results supplied by the Green- 

 land observations, the equation will be — a surplus-ice overflow equal 

 to one mile advance in eight or twelve years, minus the retardation 

 due, 1st, to friction and irregularities of surface ; 2ndly, to seasonal 

 changes of temperature (the so-called interglacial periods) ; plus, 1st, 

 the increase of discharge due to progressive secular refrigeration, and 

 2ndly, the increased precipitation and condensation. The one known 

 quantity gives from 4000 to 6000 years. Of the unknown quantities 

 we can at present but form a distant idea. We can only feel assured 

 that they must, in all probability, be subordinate to the known 

 quantity. After full consideration of the subject, my own opinion, 

 based on the facts T have here breught forward, is, that it will be 

 found that the time required for the formation and spread of the 

 great ice-sheets in Europe and America need not be extended be- 

 yond from 15,000 to 25,000 years, if so much. 



I am taking this to represent the interval between the time when 

 the ice-sheet commenced its progressive march and that when the 

 climatal change was such, as to cause its full retreat. The fact is, as 

 we use the terms, they have not the meaning that might be attached 

 to them. Preglacial does not signify a separate period before the 

 Glacial,nor Postglacial another subsequent to it. The former term 

 merely applies to the earlier stages of the Glacial epoch, and the latter 

 to the later stages. The lines are arbitrary ones. We might equally 

 well adopt two periods, the one from the inset of the cold period 

 to its zenith, and the other from its zenith to its termination. I, 

 however, here adopt the usual divisions — the so-called Glacial epoch 

 representing a certain length of time when the cold was at its 

 maximum, and the others the periods of first increase and last de- 

 crease. It is to the latter, to which, on Dr. Croll's hypothesis, a 

 term of 80,000 years has been assigned, that I now refer. 



The adoption of this length of time has been very much the result 

 of the belief that no shorter time would account for the excavation 

 of the valleys supposed to have been formed during this period. I 

 myself may have been partly instrumental in giving currency to the 



