530 Rh. M. Deeley—Structure of Glacier Ice. 
The structure on the glacier ice surface shown by these rubbings 
results from the slow melting and evaporation of the surface ; for if 
by the warm hand or other means the surface be melted until. it is 
quite smooth, in the course of a few hours the structure will again 
develop. Melting takes place most actively between the crystals, 
forming comparatively wide channels, shown in the rubbings by the 
broad white lines. The small white dots are air bubbles. 
In Fig. 3 the crystalline grains vary largely in size and have very 
irregular outlines. The markings on their surfaces much resemble 
‘finger-prints,’ and the lines run at various angles. In Fig. 4 two 
main lines of fracture run parallel with each other across the ice. 
They are not quite straight, the boundaries of the individual grains 
having moved somewhat owing to the growth of some grains at the 
expense of others, since shear took place. One line of shear in Fig. 5, 
though somewhat bent, is almost unaffected by this; but in Figs. 6 
and 7 marked changes have taken place in what were originally 
straight lines of shear. It will be noticed that the coarseness of the 
ridges and furrows on the surfaces of the grains varies very much. 
This is probably due to the angle the optic axis bears to the lined 
surface. 
McConnell! and O. Miigge? have shown that an ice crystal behaves 
as if it consisted of an infinite number of very thin, non-expansible, 
but perfectly flexible layers, somewhat like paper,. between the 
different sheets of which there is a sticky substance, so that the sheets 
can only with difficulty glide over each other. The layers, being non- 
expansible and perpendicular to the optic axis, in the act of beuding 
slide over each other; but the crystal axes still remain at all points 
perpendicular to the bent surface. Muiigge states that the slipping or 
sliding did not commence in the case he tried until the stress reached 
a certain limit, but it does not appear that the experiment was quite 
‘decisive on this point, as the ice used was probably not quite 
homogeneous. Whether the ice is plastic or viscous at right angles to 
the optic axis is therefore uncertain. 
Striations on ice crystals were noticed by Miigge. He states that 
broken surfaces of ice, especially those of bent bars, almost always 
show a fine striation at right angles to the optic axis; but is unable 
to state whether the striations observed on glacier grains run parallel 
to the base and are identical with the striation of translation. I have 
never seen these striations except on strained ice, and although proof 
that the striations on glacier grains run parallel to the base is required, 
I am strongly inclined to think that they do, and that they develop on 
such surfaces by evaporation owing to the irregular stresses in the 
erystals. If this view should prove correct, the irregularities m the 
striations shown on the ice rubbings indicate to us the direction of the 
optic axis and show that what may be regarded as large crystals are 
by no means symmetrical. 
The greater portion of the ice of a glacier has the structure shown 
in Fig. 8. Here the grains are quite irregular, large ones and small 
1 Proc. Roy. Soc., 1890, pp. 48, 259, and 1891, pp. 49, 323. 
* Jahrb. fiir Min., 1895, p. 211. 
