60 



Mh. HOPKINS, ON THE MOTION OF GLACIERS. 



successive lines in the diagram represent successive positions 

 of any one of these surfaces, or simultaneous positions 

 of successive surfaces originating in the same system of 

 fissures. If we suppose RUR' to have been an open 

 fissure at AA' when SVS' was so at BB', there will be 

 a number of surfaces of discontinuity between RUR' and 

 SVS' corresponding to the number of open fissures be- 

 tween A A' and BB' ; and the same would hold between 

 each consecutive pair of lines which at a previous epoch 

 coincided simultaneously with A A' and BB'. 



If another system of fissures be formed at aa, they 

 will give rise to a corresponding system of surfaces of dis- 

 continuity, of which the dotted line rr' may be taken as 

 the general type. These surfaces will intersect those of the 

 former system at angles more acute as they become more 

 remote from a a*. 



Hence then it follows, as a simple geometrical conse- 

 quence of the existence of transverse fissures and of the 

 more rapid movement of the central portion of the glacier, 

 that the whole mass must be traversed by numerous surfaces 

 of discontinuity; all those originating near the higher e.K- 

 tremity of the glacier becoming very nearly longitudinal as 

 they descend, and others being less so, according as their 

 origin is more remote from that extremity. The whole 

 mass will thus be divided by these intersecting surfaces 

 into innumerable portions. Cohesion, as before intimated, 

 may be partially restored along the surfaces of discontinuity, 

 but the difference of velocity in the central and lateral por- 

 tions will have a constant tendency to give slightly different 

 motions to contiguous portions, and thus to prevent the 

 restoration of cohesion. The wiiole glacier will thus be- 

 come a dislocated mass ; and that it actually is so is indi- 

 cated by the facility with which it breaks up into vertical 

 masses whenever irregularity of motion is superinduced by 

 irregularities in the bottom or sides of the glacial valley. 

 I consider a glacier, therefore, as an aggregate of numerous 

 parts, cohering so imperfectly as to allow a much greater 

 facility of motion among themselves, than if the mass were 

 perfectly continuous. The glacier will thus derive a much 

 greater facility of adapting itself to the configuration of the 

 valley through which it descends, than if its power of adap- 

 tation depended merely on the plasticity and compressibility 

 of glacial ice — properties which it must doubtless possess, 

 though possibly in so small a degree that they may only 

 become sensible under the action of the enormous pressure to which 

 glacier must be subjected whenever its motion is considerably impeded. 



I shall hereafter shew the 



• This exposition respecting the surfaces of discontinuity is 

 similar to tliat given by Professor Forbes witli reference to tiie 



alternate layers of ice of difierent structure, which constitute the 

 ribboned structure. 



