H. M. Deeley and G. Fletcher — Structure of Glacier-Ice. 157 



the grains have a greater horizontal than vertical extension and 

 that they are rouglily arranged in layers with approximately 

 horizontal shear planes. These shear planes are sometimes very 

 sharp but by no means always so. In some cases it can only be 

 said that the bounding surfaces are more regular in one particular 

 plane than they are in others. 



In many cases where the shear planes are best marked the grains 

 are smallest. Indeed, we have layers an inch or more thick, in 

 which the crystals are large as in Figure 6, and the bounding 

 surfaces utterly irregular. Between them are layers composed 

 of smaller crystals with more or less clearly defined shear planes. 



Fig. 3. (x) Section of ice in ice-cave of Rhone Glacier. The section is vertical 



and parallel with direction of motion. 

 Fig. 4. {I) Horizontal slice. Ice-cave, Obergrindelwald Glacier. 



Here the grains have been broken by excessive strains. Figures 

 4 and 5 show slices cut from the Obergrindelwald Glacier. In 

 both cases the sections were cut nearly horizontally, parallel with 

 the veined structure. Vertically, the grains being flattened, were 

 so small in these particular samples, that the ice melted away before 

 a sketch showing its structure could be made. 



Figure 6 is a sample from the Eismeer of the same glacier. Here 

 the grains were all large, the veined structure being almost absent. 

 Figure 7 is a vertical section from the Mer de Glace. It shows very 

 clearly how greatly even adjacent crystals vary in size. Although 

 we have remarked that the descriptions of the structure given by 

 Forbes are unsatisfactory, it is only fair to point out that his theory, 

 or perhaps his final theory (suggested by the writings of others as 

 well as his own observations), seems to be sound. He says : ^ " The 

 fundamental idea is this, that the veined or ribboned structure of 

 the ice is the result of internal forces, by which one portion of ice 

 is dragged past another in a manner so gradual as not necessarily to 

 . 1 "The Theory of Glaciers," p. 255. 



