674 DYNAMICAL GEOLOGY. 



This property of ice may be one means by which a glacier adapts 

 itself to the uneven slopes of a valley. It is questioned whether 

 it is not dependent on the following. 



(3.) The facility with which ice breaks and mends its fractures 

 by regelalion; that is, by a freezing together again of the surfaces 

 that are in contact. This principle, first applied to glaciers by 

 Tyndall, is the one of prominent importance. Any one may test 

 it by breaking a piece of ice and then putting the parts together 

 again : the surfaces, if moist, will become firmly united. A glacier 

 moves on, breaking and mending itself through its whole course. 

 The multitudes of fractures made on steep slopes may all disappear 

 below when the motion becomes slow and the ice feels the pressure 

 from above. 



Along the sides of a glacier, especially when passing prominent 

 angles in the valley, the crevasses are deep and numerous. The 

 ordinary direction of these crevasses is obliquely up stream, or at 

 an angle of forty to fifty degrees with the margin, being at right 

 angles, nearly, to the lines of greatest tension in the descending 

 glacier. The crevasses at a bend form especially on the convex 

 side of the stream, the ice undergoing a stretching on that side and 

 a compression on the opposite. There are also deep transverse. 

 crevasses, and others of irregular courses, made when a glacier is 

 forcing its way through narrow passes in a valley, and when descend- 

 ing rapid slopes. Afterward, on reaching a border-portion of the 

 valley, the ice may return to a solid mass with a comparatively 

 even surface, having fractures only towards the sides. Forbes 

 mentions one chasm 500 feet wide extending quite across the Mer 

 de Glace. 



7. Structure induced by the movement of a glacier. — The ice of a glacier 

 is often vertically laminated parallel to its sides, and sometimes so 

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

 marble or agate. The layers are alternations of cellular (or snowy) 

 ice and clear bluish solid ice. The melting of the surface some- 

 times leaves the more solid layers 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 pro- 

 jecting 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. 



The resistance to motion in a glacier is not continuously over- 

 come, as in the case of a perfect fluid, but intermittently. This 



