R. M. Deeley and G. Fletcher — Structure of Glacier-Tee. 161 



reduces than adds to the difficulty of the problem, as does also the 

 viscosity of ice-grains along planes at right angles to the optic 

 axis. We have seen that a very large number of the molecules at 

 the interfaces of the crystals are free ; that is, they sometimes form 

 portions of one crystalline structure, and sometimes of another. 

 Probably within a few minutes all the surface molecules have been 

 free, and have, therefore, been at liberty to assume positions more in 

 accordance with the conditions of stress and strain in the mass. For 

 instance, the conditions of stress will be different for each of the 

 faces separating the three cr3'stals, 2, 4, and 5 of Figure 9. One is 



Fig. 9. 



in compression, the other in tension, and the third in shear, and 

 consequently the structure between 4 and 2 is more open (being in 

 tension) than that between 4 and 5, the adjacent faces of which 

 are in compression. Under such circumstances, it is reasonable to 

 suppose that there would be a migration of molecules from the 

 opposed faces of 4 and 5 to the opposed faces of 4 and 2. An 

 exceedingly slow change of this kind would be sufficient for our 

 purpose. There now remain the faces, such as those between 2 and 

 6, to consider. In these the case is one of simple shear. To make 

 it clear that continuous shear can take place along such a surface 

 ■without fractures, we will quote from the paper on Glacier-Motion 

 previously referred to. This illustration is a general one, applying 

 to all the interfaces : "Take a plate of steel, say twentj'-four inches 

 long, three inches deep, and half-an-inch thick. Firmly fix one end 

 to a suitable support so that the steel plate shall form a girder with 

 its greatest depth in a vertical position. Then distribute a number 

 of weights along the length of the bar. It at once becomes 

 deflected ; that is, shear, elastic shear, is produced." ..." We will 

 now drill a row of holes along the plate, and when this has been 

 done, the girder, having been weakened, will be found to have taken 

 a still greater amount of set. Still further increase the set by 

 drilling several rows of holes. So far, all the operations have been 



DECADE IT. TOL. II. NO. IV. 



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