390 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
Comparison of Hypotheses. 
Although recognizing, as previously stated, that other explana¬ 
tions, such as the filling of abandoned channels by surface slide and 
wash, are entitled to some consideration, and that they are all, 
probably, essential to the complete theory of eskers, it is proposed 
in what follows to take account only of the two main hypotheses; 
viz.^ that eskers have been formed by the active agency of subglacial 
streams, or of superglacial streams. And here again, as already 
noted, it is merely a question of relative importance, since I hold 
with Davis and other advocates of the subglacial hypothesis that 
neither can wisely be discarded in toto. Subglacial tunnels are a 
reality at the present time and were doubtless, to some extent, a 
feature of the Pleistocene ice sheet; and it would certainly be haz¬ 
ardous to deny that deposits formed in them have never escaped 
obliteration on the disappearance of the ice. 
Superglacial Hypothesis. 
This explanation of eskers assumes a stagnant marginal zone of the 
ice sheet at least one hundred miles in maximum width, practically 
free from crevasses, sufficiently wasted by ablation to be more or less 
abundantly covered by englacial drift which has become superglacial, 
with a general southward slope, and, toward the southern border at 
least, thin enough to reflect in its surface contours, in some degree, the 
underlying topography, and even to permit the more prominent 
land forms to rise as nunataks above its surface. At the southern 
margin of the ice, the elevation or grade of the superglacial stream 
finds a limit or control in a barrier of rock or till against which the 
ice may temporarily terminate, or in a body of standing water (a 
glacial lake) held against the ice by such a barrier in northward 
sloping valleys, or, less rigidly, in the detrital cone formed by the 
stream itself as it escapes from the ice. Whatever the character of 
the control, it determines for each superglacial stream a base level, 
towards which it must approximate, but below which it cannot cut 
its channel by merely mechanical erosive action. The stream 
discharging across the ragged southern edge of the ice is but the 
trunk or main stem of a system, deriving both water and detritus 
