Origin of Eskers. — Crosby. 27 
tion of its motion wotdd become clog"ged and closed by super- 
glacial drift falling and washing into them, and it will be 
seen that the burden of proof fairly rests upon those who fmd 
in the crevasses of the Pleistocene ice-sheet evidence of the 
non-existence of important superglacial streams. 
During the period of growth and maximum development 
of the ice-sheet the entire volume of the drift must have been 
englacial : and through the processes of shearing and over- 
riding it must have tended constantly to rise to higher and 
higher levels in the ice. As opposed to this elevation of the 
englacial drift we have only the supposed fact that the velocity 
of the ice increases upward from the bottom, tending to bear 
down the upward-sloping shear-planes. Observation shows 
that this is true of alpine glaciers of high surface gradient ; 
and, doubtless, it would be true, in diminishing degree, for the 
lower gradients of an ice-sheet, if the viscosity of the section 
were uniform. It is in the highest degree probable, however, 
that, owing to the outward flow of the terrestrial heat or the 
rise of the isogeotherms, the temperature of the ice-sheet dur- 
ing and after its prime increased downward, being highest at 
the bottom. This accords with Nansen's observations on the 
Greenland ice-cap, previously quoted. Now the mobility of 
the ice, or its tendency to flow through the differential melting 
and freezing of its component granules, is a function of the 
temperature, inasmuch as it must increase with the temperature 
and reach its maximum at the melting point, which, as previ- 
ously noted, may be lowered as much as one degree Cent, by 
the pressure. Here in the lower levels of the ice-sheet, is the 
true zone of flow, passing gradually upward into the colder 
zone of fracture, which normally terminates upward in the 
neve, in which the fractures are promptly healed by the freez- 
ing of infiltrating water, by settling and by fresh snow fall. 
These considerations clearly suggest a tendency, at least, to a 
reversal of the law governing the vertical distribution of veloc- 
ity ; and to the extent or degree of the reversal, the tendency 
of the upper layers of ice to bear down or depress the obliquely 
rising englacial drift will be neutralized. It is even conceiv- 
able that the distribution of velocity may favor or accelerate 
the rise of drift ; and the writer feels that, in view of these 
arguments, the transfer of drift in large volume to a consider- 
