JVorth American TntergUicial Dsposits. — Upham. 293 
Sankoty head on the east shore of Nantucket* and in Gardi- 
ner's island. f So recent was the Ice age that none of these 
preglacia] molluscan species have become extinct, nor, with 
very rare exceptions, undergone any noteworthy change. 
DivisroN OF THE Glacial period in the Glacial and Cham- 
plain EPOCHS. 
Seeking to subdivide the Ice age with reference to its d}'- 
namic causes and its secular fluctuations in climatic condi- 
tions, we find, first, a long epoch of general snow and ice ac- 
cumulation, interrupted, at least localh' and tenipf)rarily in 
its early part by moderate oscillations of the glacial boundarj\ 
and including, after the ice-sheet attained its maximum Kan- 
san stage in the Mississippi basin, a long interval of extensive 
retreat of that part of the ice-sheet, followed by renewal of its 
growth until it again reached far south toward its former 
limits. This part of the Ice age is well denominated, from its 
envelopment of the land by ice-sheets, the Glacial epoch. Its 
chief cause I think to have been uplifts of the glaciated re- 
gions thousands of feet above their present height. Its grand 
subdivision in the Kansan, Interglacial, and lowfin stages, may 
have been due 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 per- 
ihelion 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 comparatively 
little glacial oscillation.;*; This explanation accords with 
Prof. N. H. Winchell's computations from the rate of reces- 
*Desor and Cabot, in the Quarterlj* .lournal of the Geological Society, 
London, vol. v, 184!), pp. ;540-344, parti}' quoted by Packard in Memoirs, 
Boston Soc. Nat. Hist., vol.i, pp 252-;J. Verrili and Sciidder, Am. .lour. 
Sci., Ill, vol. X, pp. 364-375, Nov., 1875. 
fSanderson Smith, in Annals of the Lyceum of Natural Ilistorv of 
New York. vol. viir, I8G7, pp, 149-151. F. ,1. H. Merrill, in Annals of the 
N. Y. Academy of Sciences, vol. iii, 188G, p. 354, with sections on Plate 
XXVII. 
Also see jjajiers by the present writer. Am. Naturalist, vol. xiii. pp. 
489-502. 552-5(i5, Aug. and Sept., 187!t: .\m. .lour. Sei., Ill, vol. xviii, pj), 
81-92, 197-209, Aug. and Sept., 1879; Proc. I'.oston Soe. Nat. Hist., vol. 
xxiv, pp. 127-141, l)(>c. 1!>, 1888 (also in Am. .lour. Sci., 111. vol. .\xxvh, 
pp. .359-372, May, 1889). 
%.]. D. Dana, Am. .lour. Sci.. Ill, vol. xi.vi, pp. 327-330, Nov., 1893. 
