376 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
relative importance, or as to which best accounts for the more 
typical and important examples and has, therefore, the best claim to 
be regarded as the theory of eskers. 
Eyidexce of Existing Glaciees ais^d Ice Sheets. 
It must be conceded at the outset that nowhere have observations 
having any obvious bearing upon this problem been made upon 
existing ice masses which realize, even apj^roximately, the essential 
conditions of the Pleistocene ice sheet as it existed upon the plain 
country remote from mountainous tracts, where, chiefly, eskers are 
now found. In Alaska, we have, besides the Malaspina glacier, only 
alpine glaciers in lofty mountain valleys of high gradient; and the 
Malaspina glacier, the type of piedmont glaciers, is simply a lake of 
ice existing at a level where permanent ice could not form, due to 
the confluence on the lowlands of the powerful alpine glaciers of 
the St. Elias range, and deriving its movement, in part at least, from 
the thrust of these tributary ice streams. On Greenland, which 
appears to have mountainous borders with an inner lowland, we find 
a true ice cap, with an area estimated by Peary at 600,000 square 
miles, and a maximum thickness of probably several thousand feet 
and possibly a mile or more; and it is well known that in the 
recent past this ice cap, which has evidently passed its culmination 
or maximum stage, has covered the whole of Greenland and the 
islands which fringe its coast, extending, possibly, far into the 
adjoining seas. But observation, naturally, has been chiefly confined 
to the margins of the ice, and to the overflow portions of the great 
mer de glace descending as lobes and valley glaciers to and toward 
the coast, and, as in the case of the Malaspina glacier, to levels at 
which permanent ice cannot form under existing climatic conditions. 
In both Alaska and Greenland, the drainage of the ice is chiefly 
subglacial; and at many points powerful streams of water, carrying 
heavy burdens of detritus, are seen to issue from beneath the 
margins or extremities of the ice lobes ; while the superficial streams, 
due to ablation of the upper surface of the ice, rarely if ever reach 
its margin, being swallowed by crevasses to form moulins and 
becoming tributary to, and the main sources of, subglacial rivers. 
In the case of the Malaspina glacier, the principal rivers discharging 
