MUSEUM OF COMPARATIVE ZOOLOGY. 223 
rested on the bed, the result might be the formation of those striated 
pavements which have been observed in till deposits. It appears to me 
not improbable that in the end we may be able to account for the for- 
mation of drumlins, those most puzzling of all glacial deposits, by the 
action of pressure upon ice, the compressive action operating in the 
following manner. : 
Let us suppose that a glacier such as covered the drumlin field of 
Southeastern New England had acquired in the process of its movement 
a great store of rock detritus, distributed through several hundred feet 
of the ice which lay next the earth. Let us further suppose that, 
through the thickening of the sheet combined with the development of 
heat near its base, this débris-laden part had been brought to the critical 
point where very slight increments of pressure would bring the impris- 
oned water to the fluid state, and lead to the precipitation of the mineral 
matter, the result would be the rapid formation of a till sheet. Wher- 
ever, through the existence of irregularities on the surface of the earth, 
projections existed of sufficient height to rise into the glacier a little 
above the level at which complete pressure melting occurred, the ice in 
its motion would be subjected to a certain amount of strain as it moved 
over the elevations. As, according to the supposition, the water of the 
glacier was very near the point of fluidity, we may well conceive that a 
very trifling resistance, in amount insufficient to exercise any distinct 
erosive effect on the mass of till, might cause still further melting, and 
thus bring about an increase in the deposit of débris. In this way the 
growing drumlin would rise up into the ice to the point where detritus 
ceases to be supplied, or perhaps to the level where the resistance of the 
glacial material was sufficient to bring about erosion. Even if the mass 
did not at first have the shapely lenticular form proper to these eleva- 
tions, it would, during the subsequent thinning of the ice which probably 
everywhere preluded the disappearance of the envelope, be eroded to the 
arched shape which characterizes the deposits. 
As I propose in this essay only to indicate in a general way the pos- 
sible value of the hypothesis above set forth, I shall not undertake 
further to discuss the explanatory value of this view. Enough has been 
_ set forth to show that, if it proves tenable, it may serve to rationalize 
our views as to the mode of action of continental glaciers, by extending 
our conceptions as to the conditions under which they do their singu- 
larly important work. As the considerations which have been adduced 
are to a certain extent novel and somewhat difficult to grasp it seems to 
me well in closing to submit them to a brief review. Leaving out of 
