Mr. Hopkins on the Mechanism of Glacial Motion. 243 



overcome before any discontinuity can be produced between 

 the two elements, by the one sliding past the other. I know 

 of no experiments which determine the relative magnitudes of 

 Fj and Fg under an assigned pressure. If, however, the co- 

 hesive power be such as it must be in glacial ice, a hard 

 crystalline substance, and the pressure be small, Fj will neces- 

 sarily be small compared with P'g » ^"^ in no case will it be 

 contended, I imagine, that Fg can be small compared with Fj, 

 except perhaps where the pressure is sufficient to crush the 

 mass, and thus to change altogether its molecular constitution. 

 Now, if it be allowed that Fg is not small compared with Fj 

 (which is the condition above alluded to), I conceive it to be 

 mechanically impossible, as in the case just considered, that a 

 system of parallel planes of discontinuity, at distances from 

 each other varying from the fraction of an inch to two or 

 three inches, should be produced by such forces as those which 

 are brought into action by the unequal motion of different 

 parts of the mass. For suppose the continuity of the mass to 

 be destroyed along p q (fig. 3), the greatest tangential force 

 which could be exerted along that line would be Fj, and, 

 therefore, the greatest tangential force v/hich could be exerted 

 along the opposite side of the element bounded by p q, could 

 not exceed Fj, as has been before proved. But the mass is 

 capable, by hypothesis, of resisting the force Fj + Fg before 

 its cohesion can be destroyed by the mechanical action here 

 contemplated. Hence no second plane of discontinuity can 

 be formed parallel and very near to the first, by forces such as 

 we are here considering, where Fg is supposed to be of consi- 

 derable magnitude. 



That longitudinal planes of discontinuity may be formed by 

 internal constraint at irregular and considerable distances, is 

 very possible, but that a system of such planes should be 

 formed of such perfect regularity, and with such minute inter- 

 vening spaces, is, as I conceive, mechanically impossible. 



The above considerations appear to me to involve most 

 serious objections against Prof. Forbes's mechanical theory of 

 the laminated structure*. I am far, however, from supposing 

 that the origin of this structure is unconnected with the motion 

 of the glacier. The fact of the bands, according to Prof. 

 Forbes's statement of ihe law of their arrangement, being 

 in directions coincident with that of greatest distension and 



* Prof. Forbes has stated his theory in the following passage (Travels, 

 p. ^77) : — "The crevices formed by the forced separation of a half rigid 

 mass, whose parts are compelled to move with different velocities, becoming 

 infiltrated with water, and frozen during winter, produce the bands we have 

 described." 



