746 THE JOURNAL OF GEOLOGY. 



affirmation is concerned, whether either one or both of these intervals 

 is of sufficient importance to constitute the succeeding ice advance a 

 separate glacial epoch. It is clear, however, from the discussion, that 

 the author believes in at least a bi-fold division of the glacial forma- 

 tions of sufficient importance to allow each to be assigned to a sepa- 

 rate epoch. It is also clear that he is hospitable to a threefold divi- 

 sion each with the rank of an epoch, and the way is left open for 

 recognition of a fourth. 



In this connection Professor Chamberlin makes some suggestions 

 of general interest concerning the subdivisions of the drift. He says : 



" If the ice age consisted of distinct glaciations separated by climatic conditions 

 as genial as those of today, they might as properly be called periods as epochs of gla- 

 ciation. If the intervals of ice retreat, whether they amounted to complete disappear- 

 ances or not, were comparable to the post-glacial period in duration, in the amount of 

 erosion, weathering, soil production, vegetal accumulation, orographic movement, or 

 other work done, or in the geniality of their climate or the character of their life, they 

 are surely entitled to be recognized as marking epochs. If the intervals fall notably 

 short of this, it is doubtless best to regard them as marking episodes, rather than 

 epochs. The need for recognizing them would still remain, however, if we are to 

 decipher and delineate the intimate history of the Ice Age." 



We suspect that many glacialists would not be willing to follow the 

 above suggestion in full. We suspect that many of them would hold 

 that an interval of deglaciation might fall " notably " short of the post- 

 glacial interval, and still there-advance of the ice constitute a separate 

 glacial epoch, especially if the retreat and the subsequent re-advance 

 of the ice were very considerable. If, for example, the ice retreated so 

 far from its extreme position as to free the territory of the United States, 

 and if, during this retreat, the region freed became temperate, a subse- 

 quent advance of the ice to the limit of the East Iowan formation might 

 perhaps not improperly be regarded as a distinct glacial epoch, even if 

 the deglaciation interval were notably shorter than the post-glacial 

 epoch. Especially would this be true, if the ice remained long in 

 retreat, and if other events, such as changes of continental attitude, 

 intervened. Even on the basis which Professor Chamberlin has pro- 

 posed, there is in the minds of many geologists no doubt but that at 

 least two, and very likely three distinct glacial epochs have affected the 

 North American continent. 



Professor Chamberlin very properly insists that just at present it is 

 a matter of subordinate importance whether the several divisions of the 

 ice period be called epochs or episodes ; that the thing which is impor- 



