238 * The American Naturalist. [Mareh, 
shown in the bluffs of the upper part of the Minnesota river 
valley and by its tributaries, overspread by 25 to 50 feet of the 
later deposits of till.’ 
The two stages of growth of the ice-sheet may have been 
due, aside from their principal dependence on the high eleva- 
tion of the land, to the climatic effects of the last two passages 
in the precession of the equinoxes, with accompanying nuta- 
tion, bringing the winters of the northern hemisphere in aphe- 
lion about 30,000 years ago and again about 10,000 years ago. 
The intermediate time of the earth’s northern winters in peri- 
helion would be the stage of great retreat of the ice margin in 
the upper Mississippi region ; but eastward, from Ohio to the 
Atlantic coast, there appears to have been little glacial oscilla- 
tion’ This explanation accords with Prof. N. H. Winchell’s 
computations from the rate of recession of the falls of St. An- 
thony for the Postglacial or Recent period’ and with his 
estimate of the duration of the interglacial stage from the now 
buried channel which appears to have been then eroded by the 
Mississippi river a few miles west of the present gorge below 
these falls.® 
The chief cause of the Ice age is here thought to have been 
a high epeirogenic uplift; but the very noteworthy sub- 
division of the Glacial epoch in the upper Mississippi basin is 
ascribed to climatic conditions resulting from the same 
astronomic cycle of 21,000 years which Croll supposed to have 
been efficient, during the remote time of maximum eccentricity 
of the earth’s orbit, to produce alternating glacial and inter- 
glacial epochs. Wallace, in his discussion of this subject in 
“Island Life,” thinks that great altitude of the glaciated 
countries coincided with the last stage of maximum eccen- 
tricity, from 240,000 to 80,000 years ago, to cause the Ice age, 
5 Geology of Minn., vol. i, p. 626. 
€J. D. Dana, Am. Jour. Sci., III, vol. xlvi, pp. 327-330, Nov., 1893. 
* Geol. and Nat. Hist. Survey of Minnesota, Fifth An. Rep., for 1876, pp. 175- 
189; Final Report, vol. ii, 1888, pp. 313-341, with fifteen plates (views showing re- 
ant changes of the falls of St. Anthony, gad maps). Quart. Jour. Geol. Soc., 
London, vol. xxxiv, 1878, pp. 886--901. 
. Geo 
s Am logist, vol. x, pp- 69-80, with three plates (sections and a map), 
August, 1892. 
