Causes and Conditions of Glacial ion. — Upham. 19 
gentle river currents. The same lapse of time would also 
suffice for the amount of leaching and oxidation of the earli- 
est drift which Mr. Leverett has observed in sections of its 
former surface now covered by thick deposits of later drift in 
Illinois, Indiana, and Ohio (Proc, Boston Society of Natural 
History, vol. xxiv, pp. 455-459, Jan. 1, 1890). It may be 
true also that within this interval the rock gorges in north- 
western Illinois described by Mr. Oscar H. Hershey in the Am. 
Geologist for November, 1893, were eroded ; but it is neces- 
sary to scrutinize very carefully the evidences for such rock 
erosion during the Glacial period, lest preglacial channelling, 
done by small tributaries of the main valle} T s during the 
gradual uplift of the country leading to the Ice age and con- 
temporaneous with the Lafayette erosion, be thus misinter- 
preted. 
The complexit}^ and great length of the Glacial period in 
Europe, with subdivision by interglaeial epochs, which are 
taught by Prof. James Geikie in his admirable books and es- 
says, and by Mr. Andrew M. Hansen in the Feb. -March num- 
ber of the Journal of Geology (also Am. Geologist, vol. xn, 
p. 225, Oct., 1893), may probably be reduced to a much less 
time and to essential continuity of glaeiation, interrupted as 
in America only by moderate fluctuations of the ice borders, 
when we consider the lesson of the Malaspina glacier or ice- 
sheet in Alaska, showing how temperate floras and faunas 
may be enclosed between deposits of till by oscillations of tin- 
ice front requiring no long time. 
One remaining question, which Prof. R. D. Salisbury has 
recently asked in the Journal of Geology (vol. n, p. 222, Feb.- 
March, 1894), concerning the departure of the ice-sheet 
because of epeirogenic subsidence of the glacially burdened 
land, may be readil}' answered. Though the still high surface 
of the greater part of the ice-sheet would not lie affected by 
the temperate climate of the country depressed to its present 
level or slightly lower, the warm summers along the ice border 
would cause it to be rapidly melted. This process extended 
inward until all the ice-sheet disappeared. When the progress 
of the marginal melting in the Mississippi basin had given 
generally steep gradients of the ice-front, its more powerful 
currents formed the retreatal moraines and the many lake 
