
GLACIAL PHENOMENA AND SUPERFICIAL DEPOSITS. 261 
dredging operations, also, in the vicinity of the south polar glaciers, 
seem to show the possibility of the deposit of non-fossiliferous marine 
boulder-clay. 
603. The second difficulty, is found in the comparatively small amount 
of change, which has been wrought on the extensive area of the plains, 
composed of yielding, scarcely solidified sediments, by so vast a revolution 
in physical geography as that implied by the old water marks. This 
objection, however, applies even more forcibly to any general system of 
glaciation by a northern ice-cap; and it is scarcely possible to imagine 
a mass of ice, like that implied by such a theory, passing southward 
across the soft rocks of the plains, against the general slope of the 
country, and yet not obliterating its pre-formed river valleys and 
features. Such a continental glacier, too, though it might have been 
loaded with Laurentian debris on its eastern margin, and with frag- 
ments from the Rocky Mountains on the west, can scarcely account for 
the west and east transport of these materials for hundreds of miles, 
and their deposit in great quantities in the central region. If such an 
ice mass, be supposed instead, to have moved down at right angles to the 
Laurentian axis, and passed out across the plains in an unbroken sheet; 
setting aside the difficulty of supplying an efficient motor power, and 
other minor circumstances; it will be found necessary to suppose that it 
climbed up and over the abrupt eastern edge of the second steppe, with- 
out destroying it, and proceeded 700 miles westward up an_ incline 
averaging about five feet per mile. 
604. Mr. Belt, in an interesting paper lately published,* deals with 
similar difficulties in explaining the glaciation of Siberia. The northern 
part of Asia, surrounded on all sides save the north, by mountain chains, 
forms an interior continental basin, covered with “vast level sheets of 
sand and loam.” Marine shells are absent from the deposits, except near 
the low ground of the northern coast, and true boulder-clay is apparently 
not found. Mr. Belt, to account for the facts, resorts to a theory first 
suggested by him eight years ago, by which he supposes the existence 
of a polar ice-sheet, capable of blocking up the entire northern front of 
the country, and holding back its waters to form an immense fresh-water 
lake. Prof. N. H. Winchell, in an article in the “Popular Science 
Monthly,” of June, 1873, broadly accounts for the glacial phenomena of 
the North-west, on the supposition of a polar glacier. His illustrations 
—— 


* Jour. Geol. Soc., Nov., 1874. 
