226 The American Geologist. October, 1902. 
In this connection we may note one point more concern- 
ino^ individual crystals. McConnell and Kid have indicated 
* that a single crystal of ice will not change its shape un- 
der either tension or pressure applied at right angles to the 
optic axis, and therefore is non-plastic. Their experiments 
have proved conclusively, however, that glacier ice as a 
zi'hole is plastic, at a certain temperature at least, both for 
tensile and compressive stress. 
From such facts the conclusion is dra^na that the ob- 
served viscosity must be due to an action at the interfaces of 
the crystals, such that their shape is altered in a way to allow 
them to change their relative positions. A hypothesis of this 
character is elaborated by Deeley and Fletcher who arrive 
at the conclusion that glacier grains may change their sizes 
and shapes und'er comparatively small stresses, and further 
that they may readily shear, or slide over each other without 
actual fractures occuring. 
A later contribution by Hobson 7 confirms the crystal- 
line structure ideas of Deeley and Fletcher just considered. 
He saw at Chamounix in September, 1896, without the aid 
a polariscope, similar phenomena to that already noted of 
the crystals in the ice-cave of the Rhone glacier. "The 
ice" he says, "was disintegrating into separate pieces of ir- 
regular form, each an inch or thereabouts in diameter, and 
fitting exactly together, with inter-locking projections auvl 
cavities, so that the structure reminded one of a toy dissect- 
ed map." He also suggests that it might be possible to show 
the same thing by immersing glacier ice in hot water. An 
excellent paper on the movement, melting and interior tem- 
perature, of the Hintereis glacier, of the eastern Alps, was 
published in 1899, by Drs. Blumcke and Hess|. 
Movement in Zones of Fracture and Plasticity. 
While it may be unwise to presume to add materially to the 
vast amount that has been written on the cause of glacier mo- 
tion, it may not be entirely out of place here to make some sug- 
gestions that have come to the writer in connection with a 
study of glaciers. 
*"On the Plasticity of an ice-crystal," McConnell. Proc. Roy. Soc. 
Vol. 49, p. 323. 1S91. 
tGeol. Mag. 1S9G. p. 572. , ^ t t •* i „ 
JUntersuchungen am Hintereisferner Wissensch. Ergans L,. L,eit des 
Du. O. Alpenvereins 1 Bd. 2 Heft. 
