2^2 The American Geologist. October, 1902. 
authorities have doubted the vaUdity of such observations, 
but it does not seem improbable that they were quite correct 
nevertlieless. In the zone of constant plasticity, to be. consider- 
ed, the direction of flow would naturally be in the direction of 
least resistance, so that if a land barrier were interposed in the 
natural path of movement, or the slope of the bed were re- 
versed in direction, the resultant of the two pressure com- 
ponents mio^ht readily force the ice upward and raise the mass 
above it. It is even possible that a large mass of the lower 
portion of the glacier would pass into the zone of fracture, 
carrying; with it the morainic debris from the bed beneath, 
which in time would be extruded from the central portion of 
the ice wall at its melting- terminus. 
It has been sug-g;ested that the comparatively rapid rate 
of motion of the Arctic glaciers may be due in part to the pre- 
sence of infra-glacial material ; it would appear, however, that 
its aid is small, and the rapid rate of flow is! to be more partic- 
ularly attributed to the great amount of ice in the zone of 
constant plasticity. Here the movement would be chiefly 
in this zone rather than in the zone of fracture. It would be 
relatively rapid because of its increased mobility, carrying for- 
ward the whole ice mass, as an inundation, a'^ previously not- 
ed. The power to erode is at the same time relatively in- 
creased, since the kinetic energy of the moving mass is pro- 
portional to the scjuare of the velocity, and thus the production 
of ground moraine is greatly augmented, but does not become 
great enough to cause stagnation of the glacier. 
Ice zvhich is at such depth tJtat the iveiglit of the super- 
inciimhent mass exceeds its yielding strength, or its ultimate 
strength, is in tJw cone of constant phisticity. This depth, as 
shown on previous pages, would be variable, though so far 
as present experiments indicate, would not be less on the aver- 
age than 300 feet. In this zone the weight above is great 
enough to mash the ice and render it like soft wax, and toward 
the bottom of the larger glaciers the very slight increase in 
temperature resulting from the pressure, as shown elsewhere 
in this paper, doubtless aids in producing a condition of pure 
viscofcity. This condition precludes at once the existence of 
cavities of any sort for the ice would flow in the path of least 
resistance, and immediatelv close them. 
