THE 



AMERICAN JOURNAL OF SCIENCE 



[THIRD SERIES.] 



Art. XLIY. — Unity of the Glacial Epoch ; by G. Frederick 



Wright. 



A distinction between upper and lower " drift deposits " 

 bad been observed some time before Mr. Croll and Professor 

 James Geikie formulated their plausible theory that the ellip- 

 ticity in the earth's orbit was the cause of the great ice age. 

 But the publication of that theory with the exceedingly able 

 advocacy of two so capable and eminent authorities gave the 

 subject such prominence that evidences of successive glacial 

 epochs have since been sought for in every land, while much 

 of that adduced has been allowed to pass without as close 

 scrutiny as would seem desirable. As I have had occasion 

 frequently to review much of this evidence both in field 

 explorations and in the literature of the subject, it may help to 

 a better understanding of the matter to give somewhat fully 

 the results of my observations and study in this direction. I 

 would premise, however, that I shall discuss merely the glacial 

 phenomena of the post-tertiary epoch. With the supposed 

 evidence of glacial periods in earlier times, this paper has no 

 concern except in a very general way. 



At the outset, a question will arise as to what is meant by 

 an " interglacial epoch." The answer to this may show that 

 some who discuss the subject are merely considering the mean- 

 ing of a word. If every temporary recession of the ice front 

 is looked upon as an interglacial period there can be no ques- 

 tion that there have been not merely two but a great many 

 periods in close succession. But the position of most who 

 advocate a succession of glacial epochs is that the continental 



Am. Jour. Sci.— Third Series, Vol. XLIV, No. 263. — November, 1892. 

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