60 SEVENTH LETTER ON GLACIERS. [1844. 



the frontal resistance, the direction in which the particles will 

 tend to slide over one another, or to produce rents, will approach 

 verticality at the surface, and on the whole will, therefore, tend 

 to produce lines of discontinuity, such as N M. 



(4.) Considering the glacier at different points of its length, 



Fig. 16. 



it is evident, by similar reasoning, that near the region of the 

 neve a the frontal dip will be all but vertical, because there the 

 horizontal resistance is enormous ; whilst at the lower end &> 

 where it tends to vanish, the shells will tend to parallelism with 

 the bed. It is needless to add, that the relative movements of 

 the particles over one another, producing discontinuity, are not 

 to be confounded with their absolute motions in the glacier, 

 exactly as under head (1.) I must, however, observe, that as 

 the tendency of any particle due to the hydrostatic pressure will 

 be to describe ultimately the whole curve N w 4 M within the 

 glacier, this may account for some of the facts, or supposed facts, 

 which indicate a tendency in the ice to expel bodies engaged 

 in it, as well as the convexity of the glacier at all times, and its 

 remarkable rise of surface during winter. 



Lastly r , The ablation of the surface of the glacier during its 

 descent from a to b (fig. 16) will tend continually to give the 

 observed elongated forms of the superficial bands, by cutting the 

 shells of structure obliquely. 



