of the Surface Ice of Glaciers. 
35 
The lines are known as Forel’s ‘Streifen’ or streaks. Their 
explanation is, as Emden 1 remarks, the darkest point in our 
knowledge of ice granules, but I think some light may be thrown 
upon the process of their formation if a comparison be made 
between the photographs in his paper and those of Ewing and 
Rosenhain 2 in their Bakerian Lecture on ‘ the Crystalline 
Structure of Metals.’ The resemblance is very striking. Com- 
pare, for instance, the markings in Emden’s Figure 6 and Ewing’s 
Figure 14. Professor Ewing has shown that “ the structure of 
metals is crystalline, and remains crystalline when the form of the 
metal is altered,” and that plastic deformation in metals is due “to 
slips on cleavage or gliding planes within each individual crystalline 
grain and partly (in some metals) to the production of twin 
crystals.” There is no true shearing as in viscous deformation. 
I think it is probable that Forel’s streaks are evidence of a similar 
property in ice crystals, and that they are indications on the 
crystal surface of slips which have happened in the ice crystal 
under stress. The relation of the general direction of these slip- 
bands to the crystal axis has not been made out in the case 
of metals, but it appears it may vary from crystal to crystal. 
Emden 3 was unable to find any relation between the axis and 
the direction of these lines. In my own observations I have seen 
them only on the hexagonal bases of the crystals, but it must be 
remembered that the exposed surface of the cave ice consists prin- 
cipally of the irregular hexagonal bases. 
The ice cavern contains air saturated with moisture at a 
temperature very near 0° C. If such slips were formed these 
conditions would be suitable for their preservation. 
In Figure 2 a cast is shown of the crystalline structure of 
a block of compact glacier ice after half an hour’s exposure to the 
sun. This fragment of ice was cut from a portion of a blue band 
on the upper Grindelwald glacier opposite the place called ‘ Im 
Schlupf 4 .’ The block at first was quite compact, transparent with 
a continuous surface. After the exposure the cast shows that 
fusion has commenced at the joints of the crystalline granules 
and so the surface is traversed by furrows more or less straight. 
The general appearance of the cast is like a bird’s-eye view of a 
hilly country divided by hedges into fields of varying sizes. The 
spaces enclosed by the furrows are the exposed faces of the 
granules, and it will be seen how very much they vary in size. 
The horizontal width of the cast is 12 centimetres. Between the 
1 K. Emden, Veber das Gletscherkorn, Neue Denkschriften der allgemeinen Schiv. 
Gesell., xxxm. i. p. 21. 
2 Ewing and Rosenhain, Phil. Trans., 1899, A. 358. 
3 Emden, loc. cit., p. 22. 
4 1700 metres above sea level. 
3—2 
