ON THE STRUCTURE OF GLACIER ICE 



infiltration cavity, — a precaution which does not seem to have occurred 

 to either Prof. Agassiz or the Messrs. Schlagintweit. 



I will conclude with a few words upon the relation of structure to 

 the arrangement of dirt upon the surface of a glacier. The great 

 " dirt-bands " have never been proved to be connected with any- 

 peculiar structure of the ice on which they lie, and it has been shown 

 that they may be the mere result of the influence of the motion of the 

 glacier upon the form of any patch of dirt scattered accidentally upon 

 its surface ; but besides these " dirt-bands," the dirt on a glacier 

 frequently presents a definite arrangement upon a smaller scale, 

 which is connected with the minute structure of the glacier. We have 

 both observed, for instance, in those parts of the Mer de Glace in 

 which the structure is vertical, that the superficial layer of the wall of 

 a crevasse is weathered into granules of tolerably even size and similar 

 form. Xe\'ertheless, dirt (or a coloured infusion) accumulates in larger 

 proportion in those fissures which are parallel with the cleavage, and 

 thus, from a little distance, the surface of the ice appears as if striated 

 or ruled with lines parallel to the structure. The lines are separated 

 by the width of the granules, and there may be several interposed 

 between two blue veins. 



Why it is that those intergranular fissures which are parallel with 

 the cleavage are the larger, is a question I will not for the present 

 attempt to answer. It may be that the weathering takes place more 

 rapidly in this direction, or it may be that these fissures being in the 

 course of the flow of the water produced by the superficial waste of 

 the ice, become enlarged more rapidly than the others. 



These markings, and the similar ones frequently to be observed on 

 the upper surface of a glacier, might be termed " dirt-lines," to 

 distinguish them from the great " dirt-bands." There is a third mode 

 of arrangement of dirt, which, like the " dirt-lines," is dependent on 

 the weathering of the ice, but the resulting striae are broad streaks, 

 and not mere lines. These may perhaps be termed " dirt-streaks." 



I became acquainted with these quite recently, when, induced by 

 Prof Forbes 's description and representation of the " structure " of 

 the glacier of La Brenva, I paid a visit to that glacier. Prof Forbes 

 states, — 



The alternation of bluish-green and greenish-white bands which 

 compose this structure, gives to this glacier a most beautiful appear- 

 ance as seen from the mule-road. An attempt has been made in 

 plate 5 to give some idea of this most characteristic display, and 

 which is better seen here than in any other glacier whatever with 



