204 The American Geologist. October, 1904. 
Dakota, where I have made extensive studies of the drift, the 
proportion of the glacial drift or till to the modified drift 
averages, as I estimate, generally about as two or three to one. 
Here, however, .as on cape Cod, Nantucket, Martha's Vine- 
yard, and Long island, the ratio is reversed, so that the modi- 
fied drift is twice or thrice as abundant as the till. 
Referring the modified drift mostly or almost wholly to 
derivation from formerly englacial and finally superglacial 
drift, which had been gathered up by the slowly flowing ice- 
sheet into its lower quarter or third part, but which was at last 
uncovered and exposed on the surface of the icv; when nearly 
all its thickness had been removed in the final melting, and 
thence being washed down by superglacial drainage to the ice 
border, I therefore regard the large volume of these stratified 
drift deposits as good evidence of a very long duration of the 
glaciation. The lower part of this glacial lobe was abundantly 
charged with englacial drift, brought partly from the Cascade 
range on the east and partly from the Olympic mountains and 
the mountains of Vancouver island on the west, but doubtless 
also in an equal or greater degree supplied from the intervening 
area of lowland, fifty miles or more in width, and from the 
bed of the sound, with its many canals, bays, and inlets. 
Pleistocene history probably began with an epeirogenic up- 
lift, elevating this side of the continent at least 3,000 to 5,000 
feet higher than now, as is known on the coast of California by 
the deep submarine valleys described by Davidson,* which 
great elevation caused glaciers to be formed first on the moun- 
tains and to extend thence outwards as piedmont glaciers on 
the lowlands. At length the glaciers of such local origin be- 
came conlluent upon all the coimtry. A general ice-sheet then 
enveloped British Columbia, and indeed extended, as I believe, 
during a very long period, from the Pacific to the Atlantic, 
across the entire width of Canada and the northern United 
States. At last, beneath the long continued ice burden, all this 
ncrthon half of North America sank to its present altitude, 
or mostly a few hundred feet lower, bringing again a 
temperate climate, with plentiful melting of the ice every 
summer along all the border of the continental icefields, which 
then were somewhat rapidly melted away. 
* California Academy of Sciences, Bulletin, yoK ii, pp. 265-268, Jan., 1887; 
do., Proc. Geol., third series, vol. i, pp. 73-103, with nine plates, 1S97. 
