Unglacial Drift. — Crosby. 
225 
Chamberlin that, while the upper clear ice of the Greenland 
glaciers increases rapidly in thickness from the lower end up¬ 
ward toward the ice-cap, the basal drift-laden ice increases 
but little if any in thickness, is it a necessary conclusion that 
in'the ice-cap itself, thousands of feet in thickness, the engla- 
cial drift is limited to the lower 100 to 200 feet? To answer 
in the affirmative is to lose sight of the principle that the ve¬ 
locity of the ice increases rapidly upward from the bottom. 
The ice-cap virtually spills over the edge of the plateau 
through deep V-shaped notches; and to my mind the conclu¬ 
sion is unavoidable that a much larger proportion of the 
upper, clear and relatively mobile ice will flow down than of 
the lower, drift-laden, and relatively immobile ice. It is 
probable that increased declivity would in any case accelerate 
the velocity of the upper more than of the lower layers of ice; 
but this contrast would certainly be greatly intensified by the 
section of the valley—broad and open above and narrow 
below. 
It is the general belief of geologists that, if Greenland were 
divested of its ice-cap, it would exhibit continental relief— 
elevated margins and a depressed interior. Hence we may 
assume that the ice-cap attains its maximum thickness in the 
central areas, and that a smothered mountain range separates 
this main body of ice from the overflow fringe of glaciers 
along the coast. The futility of regarding these smaller 
coastal glaciers as representative, in the matter of englacial 
drift, of the great ice-cap from the marginal and superficial 
portions of which they originate, is obvious. 
The Humboldt glacier, and others of the great glaciers of 
the Greenland coast, belong in quite a different category, oc¬ 
cupying as they do, apparently, the lower courses and mouths 
of the great interior valleys. But terminating in the sea, we 
can only judge of their basal conditions by the icebergs to 
which they give rise. It is, perhaps, fair to assume that, 
through basal melting, these giant glaciers, like their smaller 
brothers which fail to reach the sea, are building platforms of 
the ground moraine beneath their extremities, and thus per¬ 
manently shallowing or filling up the bays along the coast. 
But that the icebergs often carry away generous loads of 
drift is well known; and we may specially note, in this con- 
