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 V 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 E. Emden, Ueber das GletscherJcom, Neue Denkschnften der allgemeinen Sehto. 

 GeselL, xxxiii. i. p. 21. 



2 Ewing and Kosenhain, Phil. Trans., 1899, A. 353. 



3 Emden, Inc. cit., p. 22. 



4 1700 metres above sea level. 



3—2 



