GLACIERS. 697 



of increased pitch. The ordinary direction is obliquely up stream, at 

 an angle of forty to fifty degrees with the margin. As a glacier's 

 movement is easiest and quickest toward its middle, owing to the fric- 

 tion either side, the Hues of equal resistance to motion run obliquely 

 up stream. Consequently the direction of the pull tending to produce 

 fractures and crevasses (or that of greatest tension) pjg. no6. 



9 



S 



</ \V 



f 



is transverse to this ; that is, obliquely toward the 

 centre down stream; but Hopkins has shown that 

 this pull (p p') is strongest theoretically when it 

 makes an angle of 45° with the sides (g g', g g') of 

 the glacier, and this would give for the crevasses 

 (c c') a direction of 45° with the sides up stream. 

 This angle would be modified by the form of the 

 bottom, and by its pitch. Across the middle por- ^ 



tions of a glacier, the movement often makes transverse crevasses, es- 

 pecially if there is much pitch — an extreme condition of which is 

 shown in the Glacier of the Rhone, as already explained ; and it may 

 make none when it is uniform and the pitch is small. Forbes men- 

 tions one chasm 500 feet wide extending quite across the Mer de 

 Glace. At a bend, crevasses form especially on the convex side of 

 the stream, the ice undergoing a stretching on that side and a com- 

 pression on the opposite. Through narrow passes in the valley, deep 

 and irregular chasms are made ; and then, on reaching a broader por- 

 tion of the valley, the ice may return to a solid mass, with a com- 

 paratively even surface, having fractures only toward the sides. 



7. Veined Structure. — The ice of a glacier, as first observed by 

 Guyot, is often vertically laminated parallel to its sides, and some- 

 times so delicately that the ice appears like a semi-transparent striped 

 marble or agate. This is well i?een either side of the middle portion 

 of the Mer de Glace, and in the Brenva and Aar glaciers. The lay- 

 ers are alternations of cellular (or snowy) ice and clear bluish solid 

 ice. The melting of the surface sometimes leaves the more solid lay- 

 ers projecting. 



The structure is due, as shown by Tyndall, to the pressure to which 

 the glacier is subjected in making its way between the walls of a valley, 

 especially where there is a contraction in width, or a projecting point 

 against which pressure is exerted, and particularly below a place of 

 steep descent. It may be formed when two great glaciers unite, the 

 pressure between the meeting streams being here the cause. In the 

 lower part of the glacier of the Rhone, the laminated structure is pro- 

 duced, according to Tyndall, between the capes m and n (Fig. 1103, 

 p. 690), — the structure-mill, in his language. It appears first in the 

 section s, and is fully developed in the following one, s'. The radi- 



