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 affect 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 effected 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 insuperable 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 effectively 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 
