682 DYNAMICAL GEOLOGY. 



middles of the months of March and May, became so deeply bent that 

 the centre was depressed five feet. The temperature during the in- 

 terval was at all times many degrees below the freezing-point. 



(b.) The facility with which ice breaks, and then mends its fractures 

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

 that are in contact. This fact, first noticed by Faraday, and applied 

 to glaciers by Tyndall, is of prominent importance. Any one may 

 test it, by breaking a piece of ice and then pressing lightly 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, where the motion becomes slow, and the ice feels the 

 pressure from above. 



(c.) The capability of sliding along its bed, but only portions at a 

 time. 



The first of these causes acts universally throughout the mass of the 

 ice, while the second serves to do the immense amount of mending that 

 is required. The third is of less importance. The temperature of 

 the mass of a glacier is at 32° F. throughout the year, its non-con- 

 ducting nature preventing any accession of cold during the winters. 

 " Thus," as Helmholtz observes, " the interior of the masses of n6ve, 

 as well as of the glacier, remains permanently at the melting point." 



(7.) Crevasses. — Along the sides of a glacier, especially when passing 

 prominent angles in the valley, or over places in the valley where there 

 is an increase in the angle of slope, the crevasses are deep and numer- 

 ous. 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. Deep transverse crevasses, and others of irregular 

 courses, are made when a glacier is forcing its way through narrow 

 passes in a valley, or descending rapid slopes. Afterward, on reaching 

 a broader portion of the valley, the ice may return to a solid mass, 

 with a comparatively even surface, having fractures only toward the 

 sides. Forbes mentions one chasm, 500 feet wide, extending quite 

 across the Mer de Glace. 



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 so that the ice appears like a semi-transparent 

 striped marble or agate. This is well seen either side of the middle 

 portion of the Mer de Glace, and in the Brenva and Aar glaciers. 

 The layers are alternations of cellular (or snowy) ice and clear bluish 



