MUSEUM OF COMPARATIVE ZOOLOGY. 219 



forth from the earth's interior, would gradually bring a certain thick- 

 ness of the section into a state of more or less perfect fluidity, — into 

 a condition in which the mass would flow, though with much less ease 

 than fluid water, with such facility that in a slow movement it would in 

 no wise aff"ect the condition of its bed. 



The experiments which we are able to make on the surface, either by 

 compressing a mass of ice to the point where a good deal of water 

 appears between its units of structure, or by mingling snow with water, 

 seem to indicate that we may have semi-fluid masses formed containing 

 enough ice to move with a certain speed, and yet, as far as erosion is 

 concerned, behaving like liquids. It appears to me likely that, while 

 in some of the deeper valleys below a continental glacier we might 

 have considerable masses of water in a state of perfect fluidity, the 

 greater part of the material, the cohesion of which was etFected by 

 pressure and heat, would, although the water would be water and the 

 ice ice, have as a mass the essential properties of a fluid. As this ma- 

 terial, ranging in its rigidity between water and ice, moved toward the 

 zones of diminished pressure, it seems to me that it would, through the 

 reduction of pressure, gradually acquire the normal resistance of un- 

 compressed ice. 



The reader has doubtless already perceived the objection which I 

 find suggests itself as an iusuperaiile obstacle to the acceptance of the 

 hypothesis of the central part of the field of ice resting upon water 

 made more or less completely molten by pressure. He will ask how it 

 is possible that this fluid material is not at once driven forward in the 

 direction of the ice front to the point where, on account of the dimin- 

 ished pressure, it would become refrozen. To meet this point, we should 

 attend to certain considerations already presented, though in a some- 

 what preliminary way, concerning the conditions under which this 

 pressure molten semi-fluid is compelled to advance. It should not be 

 supposed that the central portions of the ice field rest upon a deep 

 sheet of pressure molten water, which would be eff"ectively urged 

 towards the margin of the glacier by the weight of superincumbent 

 material. We have to assume the depth of the ice in the neigh- 

 boring portions of the glacier which rested upon the bed rock not to 

 differ considerably from that which rested on the fluid material. A 

 very slight difference in the depth of the section would be sufficient to 

 bring about the change from the rigid to the mobile state. The con- 

 ditions would probably be such as to maintain these two parts of the 

 ice field in a delicate adjustment of their depths. As the central area 



