248 COMPTE-RENDU. QUATRIEME PARTIE 
sition, or somewhere between 40 000 and 80 000 years. The 
Lafayette period thus comprised two paris or epochs, the first 
characterized by the déposition of the formation, the second by 
ils extensive érosion and the culmination of the continental up- 
lift. ^ 
TIxb Glcicicil pcriod, Gomparison of the work of the g’iaciers 
and ice-sheets of the présent time with those of Pleistocene time 
seems to me best accordant with a référencé of ail oiir g’iacial 
drift to a single continuons period of glaciation, which, tliougli 
occupying probably 20 000 years or more, was yet brief as 
compared with the duration of most other recognized géologie 
periods or epochs. The outflow of the upper part of the Pleis- 
tocene ice-sheets probably cxceeded the curreiils 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 boulders 1000 miles in the ice-sheet, somewhat less 
than 3000 years would be required if the average of (he glacial 
currents was five feet per day. The amount of the glacial éro- 
sion and of the drift, when compared with the érosion by the 
Muir glacier in Alaska, imply a short rallier than a long dura- 
tion of the Ice âge. Tins conclusion is furllier affirmed' by the 
continiiance of the saine species of the marine molluscan fauiias 
from tlie beginning of the Glacial period to its end and to lhe 
présent day. 
lhe duration of the Ice age, if therc was only one epoch of 
glaciation, with moderatc temporary retreats and re-advances 
of Üie ice-border sufficient to allow stratified beds with the re- 
mains of animais and plants to be intcrcalated between accumu- 
lations of till, may hâve comprised only a few lens of thousands 
of years. On tins point Pi’of. Prestwich bas well written as fol- 
lows : « For the reasons before given, I think it possible lhat 
lhe Glacial epoch lhat is to say, the epoch of extreme cold, 
— may noL hâve lasted longer than from 15 000 to 25 000 years, 
and I would for the same reasons limit the time of.... the melt- 
ing away of the ice-sheet to from 8000 to 10 000 years or 
less h » 
Very gentle currents of broad river floods in the Missouri 
and Mississippi valleys deposited the North American loess, at- 
tending the maximum extension of lhe ice-sheet and accompany- 
^ Quart. Jour. Geol. Soc. London, vol. XLIII, 1887, n. 407, 408. GeoIoiïV, 
vol. II, 1888, p. 534. 
