120 [May 22, 



tiveness of these two causes, therefore, in producing the veined 

 structure in any particular locality is not at present determined. Its 

 determination would require more accurate and detailed observations 

 than have yet been made on this subject. 



The differential theory of the veined structure is then considered ; 

 but here the author dissents entirely from all Professor Forbes's 

 mechanical reasoning, by which he professes to determine the 

 positions of the surfaces of maximum differential motion, which, 

 according to this theory, are coincident with the structural veins. 

 Mr. Hopkins contends that the actual differential motion of two con- 

 tiguous particles must necessarily take place in the common direction 

 of their motions. He cannot understand the effectiveness of such 

 motion in any other sense, in producing the phenomena in question. 

 He has investigated for this case the forms of the veined surfaces, 

 but finds them altogether different from the observed forms ; and 

 with respect to Prof. Forbes's investigation he cannot possibly admit 

 it, as he at present understands it. 



The author then examines the intensity of the dislocating forces 

 acting on the glacier. He demonstrates Prof. Forbes's error in 

 supposing that it is much augmented by an enormous hydrostatic 

 pressure within the mass, tending to push it onward in the direction 

 in which it may be most free to move. It is proved that, under the 

 existing conditions of a glacier, the hydrostatic pressure from the 

 water contained in the pores of the mass can but little exceed the 

 atmospheric pressure on its surface. Bat Mr. H. shows that there 

 must in many localities be a very large increase in the intensity of 

 the internal tensions and pressures arising from the free sliding 

 motion of the whole glacier. Where the motion of a particular part 

 of the mass is retarded by local circumstances, there will probably be 

 an enormous pressure upon it h tergo, from the mass behind ; or 

 there may, in other cases, be a great additional tension, arising from 

 the freer motion of the mass in front. Hence the dislocating forces 

 must often be greatly increased, the dislocation is ensured, and the 

 operation of regelation brought into action ; and the continued motion 

 of the glaciers is preserved when it might otherwise be arrested. 



