392 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
factor in letting down and indirectly deepening the deposits in the 
superglacial channels; while the even distribution of the effects due 
to this cause may safely be assumed. But the main question now is, 
will the superglacial channel persist, or retain its walls, until its 
bottom reaches the ground ? In other words, will the melting of its 
floor beneath its increasing load of detritus keep pace with, or at 
least keep ahead of, the general ablation of the interstream surfaces ? 
The familiar instances of medial moraines resting on ridges of ice 
and ^isolated bowlders perched on pillars of ice, through the more 
rapid melting of the drift-free surface, seem to demand a negative 
answer. On the other hand, the innumerable well-like holes in the 
ice, noted by many observers, and varying from an inch to several 
feet in depth, and often containing nothing but the purest of water, 
although commonly a stone or a little sand, clay, or cosmic dust is 
seen resting on the bottom, point to an affirmative answer. These 
holes are usually explained, however, as dependent upon the absorp¬ 
tion of the solar heat by a slight thickness of stony or earthy 
matter; whereas thicker masses protect the ice beneath from the 
solar radiation. The occasional absence of foreign matter indicates 
that this explanation is incomplete; and the lakelets described by 
Russell (’91, p. 120) as a common and characteristic feature of the 
moraine-covered marginal zone of the Malaspina glacier clearly 
demand a different explanation from that usually accepted for the 
relatively small holes; for the basins of the lakelets, which may be 
from fifty to one hundred feet or more in depth and are rarely more 
than one hundred feet in diameter, appearing to differ from the holes 
only in size, contain considerable amounts of drift, and the thickness 
of the deposit in the bottom of each basin constantly increases as 
fresh material slides in from the top and sides. Obviously these nar¬ 
row basins with steep and sometimes vertical sides are formed by 
the downward melting of a cylinder of ice beneath a cover of drift 
so thick that we cannot think of it as sensibly warmed, and still 
less as penetrated, by the solar heat, especially as the turbid water 
usually completely covers the drift. 
These contrasts and seeming contradictions are readily explained 
if we accept the standing water as an essential factor and regard 
the detritus as secondary. Thin stones and particles of drift absorb 
heat and sink into the ice; and thicker stones and dej^osits may do 
the same if they chance to occupy water-tight depressions in the sur- 
