62 THE JOURNAL OF GEOLOGY. 



time, in position, and in the sequence of geological events, as to 

 be regarded as separate epochs of the same glacial period.^ On 

 the other hand they might be so widely separated from each 

 other in time, in position, and in the sequence of geological 

 events, as to make their reference to separate glacial periods 

 more appropriate. In any case their separation would be suffi- 

 ciently marked to necessitate their reference to separate ice 

 epochs. So far we believe there would be no disagreement. 



If, instead of entirely disappearing, the first ice-sheet suffered 

 great reduction of volume and area, and if this reduction were 

 followed by a second great expansion of the ice, might the time 

 of such expansion be regarded as a second glacial epoch of the 

 common glacial period? To this question, too, as thus stated, 

 we apprehend there would be but one answer, and that affir- 

 mative. 



It seems certain that the edge of the continental ice-sheet 

 was subject to more or less extensive oscillations, as are the ends 

 of glaciers and the edges of ice-sheets to-day. How much of 

 an oscillation is necessary, and under what attendant conditions 

 must it take place, in order that the recession of the ice-edge 

 shall mark an interglacial and its readvance a distinct glacial 

 epoch? When the question takes this specific form, and when 

 inquiry is made concerning the quantitative value of the differ- 

 ent elements entering into the problem, we reach the battled 

 ground. It is the battled ground, partly because it is the ground 

 of misunderstanding. It is the ground of misunderstanding, 

 partly because glacialists are not agreed as to the meaning of 

 certain terms in common use by them. 



Four elements seem to enter into the idea of an ice epoch 

 as distinct from other ice epochs. These are (i) the distance to 

 which the ice retreated between successive advances; (2) the 

 duration of the retreat, or the time which elapsed between suc- 

 cessive ice extensions; (3) the temperature of the region freed 

 from ice during the time between maxima of advance; and (4) 



' The terms period and epoch are here used in the sense in which they have been 

 used most commonly in the literature of glacial geology in the United States. 



