674 DYNAMICAL GEOLOGY. 



that the centre was depressed five feet. The temperature in March 

 was below zero, and during the interval it was at all times many- 

 degrees below the freezing-point. 



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

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

 that may be in contact. This principle, first brought forward by 

 Tyndall, is far the most important of the three here mentioned. 

 Any one may test it by breaking a piece of ice and then putting 

 the parts together again : in a few seconds they will be 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 sicle 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, as mentioned by 

 Guyot, — one of the first who noticed this peculiarity of glaciers. 

 The structure is due to the tension or pressure to which the glacier 

 is subjected in making its way between the enclosing walls of a 

 valley, especially where there is a contraction in width, or a pro- 

 jecting point around which a strain is produced. It may also 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. There 



