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CORRESPONDENCE. 
In the Geologist for May, p. 312, under the caption "How long 
ago was America peopled/' professor Upham expresses the following 
views : 
'"Any estimate of the antiquity of man * * * * must depend on the 
measures or estimates obtained by geologists for the duration of the 
Post-glacial period, and of the much longer glacial period. 
"For the time since the end of the Ice age, apparently nearly alike 
in America and Europe, approximate determinations have been given by 
N. H. Winchell, G. F. Wright and many other glacialists, as summar- 
ized by Hansen, which range from 5000 to 12.000 years. Their average, 
or about 8000 years, may be confidently accepted as near the truth."* 
To the acceptance of the italicized portion of the above quotation, 
the undersigned desires to express an objection. The basis of this ob- 
jection is outlined below. 
1st. The floods of ice which overspread the land areas of the 
northern hemisphere, were apparently synchronously imposed and ex- 
tended at least to Lat. 28 to 40 N. reaching their maximum depths 
about 50 N., from which latitude they flowed both northerly and 
southerly, but more actively in this latter direction, due apparently to 
their having been melted from this side. 
2nd. The retreat has unquestionably been by stages which have 
been given suitable names, generally geographic; these stages of retreat 
may have been interrupted by periods of advance. 
3rd. These periods of retreat, or stages, have been distinctly pro- 
.gressive in the northern hemisphere from more southerly latitudes 
towards the north. No estimate of the period since the retreat of gla- 
cial conditions from Lat. 28 N., in the Mississippi valley has been 
• Italicized by the present writer. 
