GLACIER CORNICES 785 



which is also fully exposed to the sky, melting is equal on each 

 side of the dirt bands, and they appear as black streaks in the 

 bottom of vertical V-shaped grooves five or six inches deep. 



In the instances described above it is evident that the cornices 

 are due to the more rapid melting of the layer below a dirt band 

 than of the la3^er above it. The evidence sustaining this conclu- 

 sion is, briefly, that when a horizontal dirt band is traced from 

 where it is exposed to the sun, and has a cornice above it, into 

 the cavern where the sun's rays do not exert a direct influence 

 the cornice disappears ; and when followed to where it is vertical 

 and fully exposed to the sky, melting is equal on each side. 



The more rapid melting of the snow below than above a hor- 

 izontal dirt band is evidently due to the absorption of the sun's 

 heat by the dark material, as it is dislodged and washed down- 

 ward so as to stain slightly the surface beneath. The dirt exerts 

 this influence during its passage over the surface of the exposed 

 edges of the layers of clear snow in the face of the precipice. 



In addition to the direct evidence just presented favoring the 

 hypothesis of differential melting to account for the development 

 of cornices, indirect testimony in opposition to the hypothesis of 

 differential motion is furnished by the fact that the distance from 

 the Bergschrund to the head of the neve where it meets the steep 

 upward slope of the mountain ranges from 50 to 200 feet, and, 

 as is probable, there is practically no motion in the wedge of 

 snow thus left clinging to the rocks. Israel C. Russell. 



Note. — That differential melting is the chief factor in the formation of 

 ice cornices, in most cases, may be accepted as established by such evi- 

 dence as is presented in this article, and concurrent evidence elsewhere given. 

 It is much less safe to assume that shearing motion is not a contributitig 

 factor in many cases, especially in the basal and terminal portions of ice 

 tongues. It is quite unsafe to infer that this bears adversely on the doctrine 

 of shearing, which rests on other grounds. It is altogether wholesome, how- 

 ever, to check, by such evidence as that presented in this paper, the too free 

 inference of shearing motion to which the striking features of the cornices are 

 liable to lead. T. C. C. 



