U ngla cial Dr ift .—- Crosby. 
211 
the clear ice, although at temperatures well below freezing the 
reverse relation would probably hold true. We thus arrive at 
the conclusion that at the inception of movement in any part 
of the ice-sheet, the ice is possibly stronger than, but essen¬ 
tially continuous with, the frozen soil, and the latter is clearly 
stronger than the unfrozen soil; and hence it would follow 
that the most probable initial plane of slipping or shearing 
would be in the unfrozen soil, the frozen soil and overlying 
ice moving en masse , and the movement being lubricated by 
the unfrozen soil, which would be at most points of an argil¬ 
laceous and plastic character. Although a residuary soil nat¬ 
urally becomes firmer and more rock-like downward, and is, 
therefore, weakest near the surface, we should not lose sight 
of the fact that, since frost causes a notable expansion of the 
soil, a recession of the lower limit of the frost toward the sur¬ 
face, through the efflux of the terrestrial heat and consequent 
rising of the isogeotherms, would tend to leave at the lower 
surface of the frozen soil a thawed-out layer of loose and 
yielding texture. 
Observations heretofore made on modern glaciers and ice- 
sheets are of little value in this connection, because nowhere 
in the field of observation are realized the conditions that 
must obtain at the base of a sedentary or recently sedentary 
ice-sheet. The true glaciers or ice rivers of alpine districts, 
Greenland, etc., are mere lobes of ice descending under the 
influence of gravity from the edges of neve fields and ice-caps 
into a climatic zone where permanent ice cannot form ; and 
hence they are moving over unfrozen soil, and the ice is wast¬ 
ing by melting on its under as well as its upper surfaces. 
The great desideratum is, evidently, a shaft or boring in the 
interior of Greenland extending through the entire thickness 
of the ice-sheet and a hundred feet into its rocky bed. This 
would expose the true basal relations, thermal and otherwise. 
In his recent paper on the “Influence of debris on the flow 
of glaciers,”* Prof. Russell assumes that ice is analogous to 
pitch in that its plasticity or tendency to flow is diminished 
by enclosed debris. This assumption is undoubtedly safe for 
temperatures well below freezing. But if it is a sound prin¬ 
ciple that when the temperature of the ice is rising and near 
^Journal of Geology, hi, 823-832. 
