224 
Tne American Geologist. 
April, 1896' 
rated drift, other than that derived from lateral and medial 
moraines through the agency of crevasses, is, no doubt attrib¬ 
utable to the facts that their courses were long since swept 
relatively bare of detritus, and that in their lower courses 
they are undergoing basal melting and hence depositing 
rather than absorbing drift. 
According to the observations of Chamberlin and Salisbury* 
the numerous glaciers descending from the margin of the 
Greenland ice-cap present, in this respect, two types: (1) the 
drift-laden glaciers, which have commonly vertical sides and 
ends, and predominate north of lat. 76° ; (2) the apparently 
drift-free glaciers, which are commonly without prominent 
vertical sides or ends, and predominate south of lat. 76°. 
Both these able observers state emphatically that in the drift¬ 
laden glaciers the drift is strictly a basal feature, rarely rising 
to greater hights in the ice than 100 to 150 feet, even where 
the glacier may be a thousand feet or more in thickness. 
Furthermore, these observations are regarded as fully con¬ 
firmed by those made upon the countless icebergs of the 
neighboring seas. We may, perhaps, reasonably suppose that 
the greater abruptness of the northern glaciers is due in some 
measure to the more rapid melting of the drift-laden basal 
layers of the ice, in consequence, as already explained, of the 
low specific heat of the imbedded debris; and that, possibly, 
the northern glaciers are more generally drift-laden because 
the severe climate tends to prevent basal melting. Certain it 
is that the englacial drift is, in general, most in evidence 
where the basal conditions most closely approximate those of 
an ice-sheet in its prime. 
It appears to me, however, extremely improbable that all 
the englacial drift of the Greenland glaciers has been absorbed 
by the ice during its comparatively short and steep descent 
from the margin of the ice-cap. I would suggest instead that 
a considerable part of it represents the lower drift-laden por¬ 
tion of the ice-cap itself. Whether these lobes of the ice-cap 
are well charged with drift or not is of no special significance 
in a study of the Pleistocene ice-sheet of North America, un¬ 
less we can regard them as reliable indications in this respect 
of the constitution of the parent ice-cap. Assuming with 
* Journal of Geology, jii, 875-902. 
