984 The American Naturalist. [December 
epochs. The outflow of the upper part of the Pleistocene ice- 
‘sheets probably exceeded the currents of narrow alpine glaciers, 
but was less than the advance of broad and deep polar glaciers 
which end in the sea. For the journey of Pleistocene bould- 
ders 1000 miles in the ice-sheet, somewhat less than 3000 
years would be required if the average of the glacial currents 
was five feet per day. The amount of the glacial erosion and 
of the drift, when compared with the erosion by the Muir 
glacier in Alaska, imply a short rather than a long duration 
of the Ice age. This conclusion is further affirmed by the 
continuance of the same species of the marine molluscan 
faunas from the beginning of the Glacial period to its end and 
to the present day. 
The duration of the Ice age, if there was only one epoch of 
glaciation, with moderate temporary retreats and readvances of 
the ice-borders sufficient to allow stratified beds with the 
remains of animals and plants to be intercalated between 
accumulations of till, may have comprised only a few tens of 
thousands of years. On this point Prestwich has well written as 
follows: “For the reasons before given, I think it possible 
that the Glacial epoch—that is to say, the epoch of extreme 
cold—may not have lasted longer than from 15,000 to 25,000 
years, and I would for the same reasons limit thetime of . ... 
the melting away of the ice-sheet to from 8000 to 10,000 years 
or less." 
Very gentle currents of broad river floods in the Missouri 
and Mississippi Valleys deposited the North American loess, 
attending the maximum extension of the ice-sheet and accom- 
panying its departure up to the time of formation of the 
great marginal moraines. The loess thus testifies that pre- 
vious to the farthest glacial advance the land sank to its pres- 
ent altitude, and probably somewhat lower on the area of the 
early drift, but not to the sea level. The vast weight of the 
continental glacier seems to have been the chief or only cause 
of this subsidence, as was first pointed out by Jamieson for 
the similar depression of the British Isles and Scandinavia at 
"Quart. Jour. Geol. Soc., London, Vol. xliii, 1887, pp. 407, 408. Geology 
Vol. ii, 1888, p.534. — 
