Mr, Hopkins on the Mechanism of Glacial Motion, 239 



the lonioer extremity of the glacier, its inclination to the axis 

 constantly diminishing as it recedes from the sides : and since 



Fig. 1. 



the fissures must be perpendicular to the direction of maxi- 

 mum tension, they must follow the same law as the curve 

 line M N, and approximate more nearly to parallelism, in- 

 stead of perpendicularity, to the axis, the more nearly they 

 approach it. 



3. I now pass to the mechanical reasoning on which Prof. 

 Forbes founds his theory of the laminar structure of glacial 

 ice, as exhibited in the alternate bands of blue and white ice. 

 According to his views, if I understand them correctly, the 

 more rapid motion of the centre of a glacier tends to form 

 longitudinal planes of discontinuity, along which the cohesion 

 of the mass is entirely, or in a great degree, destroyed, so as 

 to Jacilitate the infiltration of water descendinjj from the sui*- 

 face, which water is afterwards frozen and constitutes the 

 bands of blue ice. The general direction of these planes of 

 discontinuity, however, is not exactly parallel to the sides, but 

 inclined to them, the more accurate law being that they meet 

 the crevasses at right angles, as stated in the second extract 

 given at the commencement of this letter. This statement of 

 Prof. Forbes's theory is independent of the modification in- 

 troduced into it by the hypothesis that the upper surface of 

 the glacier moves faster than the lower one, a point in his 

 theory to which I shall allude in the sequel. 



If we suppose the cohesion of a glacier to be uniform, or, 

 more generally, to vary according to aify continuous law*, it 

 is manifest that the planes of discontinuity above mentioned 

 will be formed, if formed at all, in that direction in which 

 there is the greatest tendency in one particle to slide past a 

 contiguous one. Prof. Forbes has concluded that this direc- 

 tion is perpendicular to the transverse fissures, because that 

 is the direction of greatest distension and maximum tensionf. 



* See article 19, Second Letter. 



t See note, p. 377 of Travels in the Alps ; also Prof. Forbes's Seventh 

 Letter on Glaciers, p. 14. 



