MODIFICATION OF THEORY EXAMINED. 141 



Interglacial Periods Less Strongly Marked in Tem- 

 perate Regions than Glacial. — I quite agree with Mr. 

 Wallace that the interglacial deposits never exhibit 

 any indication of a climate whose warmth corresponded 

 to the severity of the preceding cold. This, however, 

 cannot be urged as an objection, for it is a result which 

 follows as a necessary consequence from theory. It 

 theoretically follows that the cold of the glacial periods 

 will not only exceed in severity the heat of the inter- 

 glacial, but will also be of longer duration. During 

 the glacial periods extreme cold is the characteristic 

 of the winters, which, owing to the presence of snow 

 and ice, only becomes moderated, although, of course, 

 considerably, during the summers. But, on the other 

 hand, during interglacial periods mildness and equa- 

 bility of temperature rather than heat are the 

 characteristics both of summer and winter. 



That the cold of the glacial periods must have con- 

 tinued longer than the warmth of the interglacial will, 

 I think, be apparent from the following considerations. 

 As long as a country remains permanently covered 

 with snow and ice, the climate, as has been repeatedly 

 shown, must continue cold, no matter what the direct 

 heat of the sun may be. Astronomically considered, 

 the interglacial periods are, of course, of the same 

 length as the glacial, — the mean length of which, 

 during the Glacial Epoch, was about 11,600 years ; 

 but the cold of a glacial period would not, as we shall 

 presently see, actually terminate at the end of the 

 period, but would be continued on probably for cen- 

 turies into the succeeding interglacial period. Suppose 

 that during a glacial period the country is covered 

 with a sheet of ice, which, during the continuance of 

 the period, had accumulated to the thickness of 2000 

 or 3000 feet. All this enormous quantity of ice would 



