DISTINCT GLACIAL EPOCHS. 63 



the intervention between successive advances, of changes inter- 

 rupting the continuity of geological processes. 



(i.) It would be arbitrary to name any definite distance to 

 which the ice must recede in order to constitute its re-advance 

 a distinct ice epoch. It would be not so much a question of 

 miles as a question of proportions. Considering this point alone, 

 we presume it would be agreed that an ice-sheet should have 

 suffered the loss of a very considerable proportion of its mass, 

 and that it should have dwindled to proportions very much less 

 than those subsequently attained, before its re-advance could 

 properly be called a separate glacial epoch. To be specific, if 

 the North American ice-sheet, after its maximum extension, 

 retreated so far as to free the whole of the United States from 

 ice, we should be inclined to regard a re-advance as marking a 

 distinct ice epoch of the same glacial period, if in such re-advance 

 the ice reached an extension comparable with that of the earlier 

 ice-sheet. Especially should we be inclined to refer the second 

 ice advance to a second glacial epoch, if it, as well as the pre- 

 ceding retreat, were accompanied by favoring phases of some or all 

 the other three elements entering into the notion of a glacial epoch. 

 In this statement we do not overlook the fact that a northerly 

 region — as Labrador or Greenland — might be continuously cov- 

 ered with ice throughout the time of the two glaciations of the 

 more southerly regions. But this is not regarded as a sufficient 

 reason for discarding the notion of duality. Greenland has very 

 likely been experiencing continuous glaciation since a time 

 antedating that of our first glacial deposits. The renewal to-day 

 of glaciation comparable in extent to that of the glacial period 

 would certainly be regarded as a distinct glacial epoch, if not a 

 distinct glacial period, even though Greenland's glaciation may 

 not have been interrupted. Scandinavia and Switzerland have 

 probably not been freed from ice since the glacial period. Their 

 snow and ice fields are probably the direct descendants of the 

 ice fields of the glacial period. An expansion of the existing 

 bodies of ice in these countries to their former dimensions, would 

 constitute a new glacial epoch, if not a new glacial period. 



