162 R. M. Deeley and G. Fletcher — Structure of Glacier-Ice. 



possible ones ; but I must now draw upon the imagination some- 

 what, and perform operations which cannot be carried out in 

 practice. Take the material removed from the numerous perfora- 

 tions in the plate, and replace it so that the plate becomes whole 

 again. It is evident that though again solid, only that metal which 

 formed part of the original perforated plate is in a state of strain, 

 that filling the holes is taking no share of the load. We will again 

 drill a number of holes, this time in the spaces between the older 

 perforations, and another increase will take place in the deflection of 

 the plate. A strain will also be put upon the metal in the first 

 series of holes bored ; and, in addition, a greatly increased strain 

 upon what remains of the original plate. By repeating the opera- 

 tion the girder could be deformed to any desired extent, and, if 

 necessary, such a violent strain thrown upon any one point that 

 local rupture would ensue." ^ In a glacier, each interfacial molecule 

 which obtains its freedom is regarded as having melted, and, like 

 the material drilled from the plate, ceases to be a support to the 

 mass, and when it becomes attached again {i.e. freezes) it does so 

 under quite different conditions, and may not take any portion of 

 the stress until further movement has occurred. 



Imagine a glacier upon whose mass gravity has previously had 

 no effect to be suddenly put within the sphere of the earth's 

 attraction. It would, of course, immediately undergo a change of 

 form, for ice is an elastic substance. Strains are produced in it, 

 each proportional to the corresponding stress, and the ice takes 

 a small step in the direction of least resistance. But although it is 

 brittle and elastic, and much more cohesive than many rocks which 

 enter into the composition of stable mountain ranges, the fact that 

 the proper motion of its molecules at or near the freezing-point is 

 so energetic that they shake themselves free, renders the mass 

 viscous,* and it slowly flows, even under the influence of very 

 minute stresses. 



In the paper on Glacier-Motion previously referred to, no reference 

 was made to the structure of Glacier-Ice, the illustrations being 

 general ones. At that time it was not known to what extent 

 a crystal of ice was viscous. Any attempt to go into detail under 

 such circumstances could not have been other than unsatisfactory. 

 Neither was the real structure of Glacier-Ice sufficiently known. 

 No doubt it had been described over and over again, but so many 

 of the descriptions are imperfect, and the contradictions are so 

 palpable, that it would have been quite impossible to decide 

 between the various authorities. 



By making polariscopic examinations of thin sections of ice much 

 may be done not only to throw light upon the cause of glacier 

 motion, but also upon the origin of such buried ice-masses as those 

 met with in Arctic regions, just as the microscopic examinations of 

 rock-sections has thrown light upon the history of many rock- 

 masses. 



1 " Theory of Glacier- Motion," by R. M. Deeley, F.G.S., Phil. Mag. Feb. 1888. 



2 For definition see Maxwell's " Theory of Heat," 7th edition, p. 276. 



