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 drumlius, 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 debris-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 debris. 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 sliall 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 



