W. Upham— Work of Prof. H. C. Lewis. 159 
Pennine Chain! are similar to the driftless area of South-western 
Wisconsin. 
Comparison of the drift in North America and Great Britain 
enabled Prof. Lewis to refer the British modified drift, both that 
often intercalated between deposits of till and that spread upon the 
surface in knolly and hilly kames and more evenly in plains and 
along valleys, to deposition from streams supplied by the glacial 
melting, the material being washed out of the ice-sheet. These 
beds are to be carefully distinguished from others, similar to them 
in condition and material, which are of interglacial and post- 
glacial age. In this connection it is also important to discriminate 
between subglacial till or ground moraine and englacial till that 
was contained in the ice-sheet. The differences marking these 
deposits in New England and generally through the northern 
United States? are the remarkable compactness and hardness of 
the subglacial till, due to compression under the ice-sheet, con- 
trasted with the looseness of the englacial till; the abundance of 
glaciated stones and boulders in the former, and their comparative 
infrequency in the latter ; and the usually greater proportion of large 
boulders in the englacial till. Weathering has changed the small 
ingredient of iron from the protoxide combinations which it still 
retains in the lower or subglacial till to hydrous sesquioxide in the 
upper or englacial till, giving to the latter a yellowish or reddish 
colour, in contrast with the dark grey or blue of the former. 
Beds of modified drift, that is, of gravel, sand and clay, brought 
by streams from the melting ice-sheet, may occur (1) enclosed within 
subglacial till, (2) intercalated between the subglacial and englacial 
till, or (8) overlying all other drift formations. In the first and 
second cases they were deposited beneath the ice-sheet, or sometimes 
in the second case were laid down in front of the ice-border and 
afterward became covered with englacial till by an advance of the 
ice. Prof. C. H. Hitchcock describes abundantly fossiliferous marine 
modified drift which he believes to have been thus overlaid by en- 
glacial till at Portland, Maine. ‘To the third case, where the 
modified drift is superficial, belong osars and kames, formed in ice- 
walled channels, and the more extensive plains and valley drift 
spread along the course of the floods that descended from the ice- 
border to the sea. In districts to which the ice-sheet transported 
fragments of marine shells, these are liable to be found not only in 
both divisions of the till but also in any portion of the modified drift. 
The geologists of Sweden record a similar order of drift formations 
in that country, there being generally recognizable subglacial and 
englacial till, with associated beds of stratified gravel, sand and clay.‘ 
1 A. Geikie’s Text-Book of Geology, p. 903; Quart. Journ. Geol. Soc. vol. 
XXxil. 1876, pp. 184-190. 
® Grou. Mac. Dec. II. Vol. VI. 1879, p. 283; Third Annual Report of the U.S 
Geological Survey, p. 297. 
3 Geology of New Hampshire, vol. iii. pp. 279-282; Grou. Mac. Dee. II. Vol. 
VI. 1879, pp. 248-250. 
* American Journal of Science, III. vol. xiii. 1877, pp. 76-79; Great Ice Age, 
second ed. p. 400. 
