36 The American Geologist. •^"'^'' ^^'^-• 
esis that it was deposited on the ice, not beneath it." Add to 
this confession of a subglaciahst the inidouljted fact that the 
structure of the typical esker is essentially pell-n::dl, or at least 
chaotic to such a degree that Davis"''' has hesitated to describe 
the very normal eskcrs of the Boston basin as stratified in any 
true or ordinary sense, and that the occasional appearance 
of anticlinal stratification must be chiefly, at least, the result 
of sliding as the gravel gradually adjusts itself to the slowly 
vanishing walls of ice ; and it becomes apparent that no argu- 
ment fatal, or even inimical, to the superglacial origin of esk- 
ers lurks in the coarse, rude, chaotic or anticlinal structure 
of the latter. Even the openwork gravel on which Davis so 
confidently relies, appeals to me as finding its readiest explan- 
ation, as previously noted, in the loosening up and differential 
settling of the coarse and irregular detritus during the melt- 
ing and vertical recession of its ic}^ floor; and T make bold to 
claiml it as specially cogent argument for the superglacial 
origin of the eskers in which it occurs. 
Stone's monograph is the most complete contribution yet 
made to the natural history of eskers ; and no one has dis- 
cussed the theory of eskers more fully and impartially. In 
fact, his work is particularly notable for the judicial and fair- 
minded attitude toward the rival hypotheses which it reveals. 
Although holding, with Chamberlin, Davis and others, that the 
subglacial hypothesis affords the best explanation of a cer- 
tain ideal type, he is disposed, as we have seen, to refer the 
wide eskers, the branching eskers, the reticulated eskers, the 
unstratified eskers and perhaps others, to the agency of super- 
glacial streams. These concessions are certainly sufficient 
to give the superglacial hypothesis a good .standing ; and the 
chief objection urged against a still broader application of 
this hypothesis is the supposed prevalence of crevasses in the 
marginal zone of the ice-sheet, a supposition which is, in 
my opinion, essentially groundless. 
Source of the Material of Which Eskers and Their Terin- 
inal Plains are formed. — The all-important question here is 
as to whether the material was derived chiefly from the en- 
glacial or the subglacial drift ; and if from the former, whether 
it was supplied to superglacial streams through superficial 
t Proc. Bost. Soc. Nat. Hist., 25, p. 
