324 The American Geologist. November, 1898 
owint( to friction, quiclvly die out downwards. Only a relatively thin 
layer of ground-moraine, therefore, can travel onwards underneath the 
ice. Immense quantities of material, however, are interstratified with 
the lower layers of the "inland ice," and these are eventually added to 
the ground-moraine. The amount of this included or intraglacial debris 
depends upon the thickness of the ice, and must thus vary from place 
to place. As the ice diminishes in thickness, its ability to transport 
rock-materials declines, and the rubbish begins to be deposited below. 
Dr. Drygalski thinks that the boulder-clavs of North Germany were in 
all probability deposited in this way. Thus wide sheets of boulder-clay 
and the "end-moraines" of a great ice-sheet have had the same origin — 
they consist of ground-moraine accumulated under the thinner peripheral 
portions of the ice. 
Drygalski's view closely coincides with the opinions 
which the present writer has stated in several papers, that 
the drift was chiefly transported englacially, in the lower 
part of the ice-sheet, and that it was probably thence depos- 
ited in large measure subglacially near the borders of the 
receding ice, while another large part became superglacial 
and was allowed to drop loosely on the surface. From these 
thorough studies of the Greenland ice-sheet, with the equally 
elaborate and valuable observations of it, as bearing on the 
origin of our Pleistocene glacial drift, by Chamberlin and 
Salisbury much farther north, and with Russell's exploration 
of the partly drift-covered and forest-clad Malaspina glacier 
in Alaska, we obtain much aid toward a comprehension of 
the conditions which attended the retreat of the great Amer- 
ican and European ice-sheets. But we have yet much to 
learn, and the numerous expeditions now being made in both 
Arctic and Antarctic regions will doubtless still further ex- 
tend and broaden our knowledge derived from present ice- 
sheets as illustrating the Glacial period. w. u. 
REVIEW OF RECENT GEOLOGICAL 
LITERATURE. 
[New York] Fifteenth Annual Report of the State Geologist [Prof. 
James Hall] for the Year i8q^. Volume I. Pages 738, with many 
plates, maps, and sections ; Albany, 1897. 
Among the many reports that have been published by our state and 
national geological surveys, this stands first, or at least in the front rank, 
