EXPERIMENTS IN ICE MOTION. 931 
definite planes, at least under the very slow movement to which 
it was subjected. The phenomena may be explained fully as 
well by recourse to the drag of the bottom. The lower layers of 
the ice were retarded while the upper flowed over the obstruc- 
tion and onto its rear face. It was there protected from further 
push as well as retarded, and the lower layers having meanwhile 
backed up against the front of the escarpment, overflowed and 
passed out into the ice in a horizontal line resulting from a 
stretching of the viscous body as seen in all the experiments. 
It is to be noted that while the general depression of a viscous 
mass is caused by the pull of gravitation, the motion at any one 
point is due to the motion of the mass behind it and is largely a 
shove, rather than a pull. 
The result arrived at by Professor Chamberlin from his 
observations was that the ice was rigid in great part and moved 
along planes of shear between well individualized layers. He 
says, p. 212: ‘‘My observations seems adverse to anything 
which can be properly termed viscous fluency.” And again, 
p. 213: ‘Everywhere the aspect of the ice was that of rigidity 
rather than viscous fluency. The rigidity, to be sure, did not 
prevent contortions and foldings of the laminations, such as take 
place in the crystalline rocks, but faulting and vein structures 
also occur, and there seems no more occasion to assume viscosity 
in the one case than in the other. Even if a certain measure of 
viscosity be admitted, it does not follow that viscosity was an 
essential agent in motion.” 
From the peculiarly perfect manner in which the results of 
the experiments in wax and the observed phenomena in the ice 
parallel each other, it seems that the obvervations of Professor 
Chamberlin may be interpreted ina slightly different manner and 
that his ‘‘certain measure of viscosity’ may have afforded a 
very considerable part in the mechanics of the ice bottom. At 
least some method of movement must be admitted which will 
produce currents such as would be produced in a viscous body 
under like conditions. And this motion, it seems to me, cannot 
be produced by shearing alone. 
