376 Sketch of Henri/ Carvill Lewis— Upham. 
This removes one of the most perplexing questions which gla- 
cialists have encountered, for nowhere else in the British Isles 
is there proof of any such submergence during or since the 
glacial period, the maximum known being 510 feet near Airdrie 
in Lanarkshire, Scotland, f At the same time the submergence 
on the southern coast of England was only from 10 to 60 feet,J 
while no traces of raised beaches or of pleistocene marine for- 
mations above the present sea level are found in the Orkney and 
Shetland islands. 1| The work and writings of professor Lewis 
emphasize the principle that glacially transported marine shells 
and fragments of shells, which occur in both the till or boulder- 
clay and the modified drift in various parts of Great Britain, are 
not to be confounded with shells imbedded where they were 
living or in raised beaches, for only these prove the former 
presence of the sea. 
The drift deposits of Engla'id south of the terminal moraines 
traced by Lewis were regarded by him as due to floating ice upon 
a great f resh-Avater lake,held on the north by the barrier of the 
ice-sheet which covered Scotland, northern England, and the 
area of the North sea, and on the south-east by a land barrier 
where the strait of Dover has since been eroded. Under this 
view he attributed the formation of the Chalky boulder-clay in 
East Anglia and of the Purple and Hessle boulder-clays in 
Lincolnshire and much of Yorkshire to lacustrine deposition, 
and believed that there was only one advance and recession of 
the ice-sheet. But shortly after the British Association meeting 
in 1887 his observations on Frankley hill in Worcestershire and 
thence westward§ led him to accept the conclusion, so thoroughly 
worked out by other glacialists both in America and Great 
Britain, that there were two principal epochs of glaciation, 
divided by an interglacial epoch when the ice-sheet was mostly 
melted awdj. There can be little doubt that the continuation 
of Lewis' study of the drift in England, if he had lived, would 
opinion was held fifty years ago by Mr. James Smith (Researches in Newer 
Pliocene aul Post-Tertiary Ge<:)logy, pp. 11 and 16), though he attributed 
the drift to debacles instead of glaciation. 
■j- Quart. Journ. Geol. Boc, vol. vi, 1850, pp. 386 8; xxi, 1865, pp. 319-221 
t Quart. Journ. Geol. Soc, xxxiv, 1878, pp. 454-7 , xxxix, 1883, p. 54; and 
Geol. Mag., II, ii, 1875, p. 229; II, vi, 1879, pp. 166-172. 
I Quart. Journ. Geol. Soc, xxxv, 1879, p. 810; xxxvi, 1880, p. 663. 
§ Geol. Mag., Ill, iv, pp. 515-517, Nov., 1887; v, p. 430, Sept., 1888. 
