244 Mr. Hopkins on the. Mechanism of Glacial Motion. 



perpendicular to that of greatest pressure, would seem to 

 establish a relation between the laminar structure and the 

 internal forces of constraint, which may possibly afford the 

 key to a satisfactory physical explanation of the bands in 

 question. I do not profess however to offer any theory on the 

 subject. 



5. There is another point, connected with Professor Forbes's 

 theory of the mode of progression of glaciers, which calls for 

 a few remarks. I mean \hefro7ital dip of the structural sur- 

 faces, which Prof. F'orbes accounts for as follows : — ". . . . any 

 particle in a fluid or semifluid mass, urged by a force from 

 above, does not necessarily move in the direction in which the 

 force impels it, it moves diagonally ; forwards, in consequence 

 of the impulse ; upwards, in consequence of the resistance in 

 front. Hence a series of surfaces of separation, shaped (to 

 use a familiar illustration) somewhat like the mouth of a coal- 

 scuttle or sugar-scoop, will rise toward the surface, varied in 

 curvature by the law of velocity of the different layers of the 

 glacier. Near the head or origin of the glacier, where the 

 resistance in front is enormous^ the tendency of the separation 

 platies, which are those of apparent cleavage, will be very 

 highly inclined. As the lower end of the glacier is approached, 

 the resistance continually diminishes, the line of least resist- 

 ance becomes more and more nearly horizontal ; and finally, 

 when the lower end of the glacier is reached, the planes fall 

 away altogether, and the upper layers roll over the lower ones, 

 now wholly unsupported." 



In the first place, I would remark that whatever validity 

 this mechanical reasoning may have, depends on the hypo- 

 thesis of the mass being " urged by a force from above ;" in 

 other words, the glacier must be generally in a state o( longi- 

 tudinal compression. Now I maintain that the existence of a 

 number of transverse fissures in the central portion of a glacier, 

 whose directions are not reniote from perpendicularity to the 

 axis of the glacier, as is the case in the glaciers of the Aar and 

 of the Mer de Glace, is altogether inconsistent with the hy- 

 pothesis of a general longitudinal compression, though such 

 compression may doubtless exist in particular localities. I 

 should think it impossible for any one to inspect more espe- 

 cially the lower portions of these and many other glaciers, and 

 to conclude them to be in that state, at least in the summer 

 months, when the motion is greatest. In fact, Professor 

 Forbes himself appears to be of the same opinion, for he ap- 

 peals to " M, Elie de Beaumont's acute observation, that a 

 glacier does not so much resemble a body thrust or pushed 

 forward, as one dragged down and pulled." (Travels, p. 178.) 



