•226 
The American Geologist. 
April, 1896 
nection, (1) that perhaps many of what pass for small bergs 
are really large bergs deeply laden ; (2) that bergs well bal¬ 
lasted with drift cannot possibly be overturned so as to ex¬ 
pose the drift to observation: and (3) that the drift-bearing 
part of a berg, under the combined influence of the higher 
specific heat of rocky debris and gravitation, must melt away 
very rapidly w T hen the temperature of the water is above 
freezing. Again, these giant glaciers are simply ice rivers 
draining at lower levels the great mer de glace or interior sea 
of ice; and, just as in the case of the water of a lake and its 
outlet stream, the velocity of the ice must be greatly acceler¬ 
ated in passing from the mer de glace to the glacier. From 
this premise the conclusion follows irresistibly, as previously 
noted, that the glacier will consist in much larger proportion 
than the mer de glace of the upper drift-free ice. Hence we 
can hardly suppose that even a clearly exposed section of the 
Humboldt glacier would reveal to us a true and undistorted 
vertical section of the Greenland ice-cap. for the basal layers 
certainly w T ould not be adequately represented. Thus obser¬ 
vation is baffled at every point, unless, indeed, a boring should 
some time be made in the interior of Greenland. I see no 
reason, however, to doubt that the mer de glace is well sup¬ 
plied wdth englacial drift, or that wherever the ice is actively 
abrading its bed it holds in its mass the entire volume of de¬ 
tritus, moving, full armed and without any intervening shield 
or ground moraine, over the unprotected bed-rock. 
That the englacial drift rises to a great proportional hight 
in the Greenland or any other ice-cap appears to me, however, 
by no means a necessary conclusion. Probably very little 
rises to a greater hight than 500 feet, or possibly 1,000 feet, 
even where the thickness of the ice is one to two miles. In 
fact, none of the suggested processes of absorption seem com¬ 
petent to diffuse the detritus through any considerable thick¬ 
ness of the ice, or to carry it far above the summits of the 
obstructions which give rise to the shearing and flexing 
movement. I conceive it, rather, to be somewhat crowded in 
the slow-moving basal strata of ice, whence it is early set free 
by basal melting to form the ground moraine, and to thin 
out. rapidly upward. Among the causes tending especially 
To check or limit the upward movement of drift in the ice.is 
