168 



Mr. HOPKINS, ON THE MOTION OF GLACIERS, 



longitudinal fissures may result from their action ; it is to the molecular forces that I am disposed 

 to attribute the veined or riband structure, their action being modified in some unknown manner 

 by the general conditions under which the glacier exists. In expressing this opinion I am offer- 

 ing no theory of the curious structure in question, but only meeting the theoretical difficulty which 

 it presents to us by a confession of profound ignorance of the nature and action of those forces, to 

 which the peculiarity of crystalline structure is generally due. The mechanical solution of the 

 problem I conceive to be utterly hopeless, till we shall have arrived at some solution of the general 

 problem whicli crystallization presents to us*. 



12. In conclusion, I will state tlie principal objections which have been urged against the 

 sliding theory, and indicate the answers which the preceding investigations afford. In doing this, 

 I shall refer principally to the work of Professor Forbes, already mentioned, as that in which those 

 objections are most systematically stated. 



(I.) The enormous friction when a glacier moves over a bed of rock, is spoken of by all 

 opponents of this theory as an insurmountable objection to it. My experiments shew that the 

 friction, or rather the force analogous to friction, is extremely small. 



(2.) Professor Forbes remarks (p. 362), "As I understand the Gravitation theory, it supposes 

 the mass of the glacier to be a rigid one, sliding over its trough or bed in the manner of solid 

 bodies." — I am not aware that any advocate of this theory has fallen into the absurdity of con- 

 sidering a glacier as a rigid, when he has spoken of it as a solid mass. I have considered it as 

 a dislocated mass, glacial ice itself having some degree of plasticity. 



(3.) When a glacier passes out of a wider into a more contracted channel. Professor Forbes 

 says that " tlie idea of sliding, in the common legitimate sense of the word, is wholly out of the 

 i]uestion." — The term "sliding" is certainly not restricted to the motion of a rigid body; it is 

 applicable to any solid body, in the sense in which a glacier is considered to be such, and on this 

 hypothesis I have distinctly explained, in my former memoir, how it may pass from a wider into a 

 narrower channel. In objections of this nature the distinction between solidity and rigidity would 

 seem to be forgotten. 



( !■.) " The inclination of the bed is seldom such as to render the overcoming of such obstacles 

 as the elbows and prominences, contractions and irregularities of the beds of glaciers, even conceiv- 

 able, being, on an average of the entire Mer de Glace, only 9", a slope practicable to loaded carts ; 

 but the greater part of the surface inclines less than 5°," (p. 363.) This difficulty has arisen in an 

 imperfect conception of the enormous pressure which, according to our theory, must be thrown on 

 abrupt local obstacles-f-. 



(5.) Anotiier objection is founded on the fact that changes in the rapidity of glacial movements 

 are found to be simultaneous with changes of external temperature. " In order to reconcile this to 



• At the last meeting of the British Association at Cork, 

 ."^Ir. Phillips mentioned a curious fact, which seems calculated 

 to throw some light on one of the modes in which external con- 

 ditions may modify the action of molecular forces, assuming 

 the lamellar structure of rocks to be due to such forces. It 

 appeared that certain Trilobites were frequently found in some 

 i»f the older rocks in South Wales, so deformed as to their 

 ijeneral proportions as to present, to a casual observer, the ap. 

 pearance of dift'erent species. On comparing, however, a number 

 of cases, it became evident that the specimens had been com- 

 pressed in a direction perpendicular to the planes of structure^ 

 from which it was justly inferred, that the general mass in which 

 these remains were imbedded had probably been subjected to 

 a great pressure in the direction above mentioned. It would 

 seem to be a legitimate inference from this fact, that the posi- 

 tion of the planes of structure had probably been mainly deter- 

 mined by the direction of greatest pressure. Perhaps some of the 



facts mentioned by Professor Forbes might admit of a similar ...- 

 terpretation. 



I may here mention a curious effect of crystallization in the 

 structure of hailstones, which may possibly have some bearing on 

 the question before us. I had an opportunity of witnessing it at 

 Cambridge, on the 9th of August, 1843, during one of the most 

 desolating hail-storms ever known in this country. IVIany of the 

 hailstones were of the form of rather flat double convex lenses, 

 nearly as large as the palm of the hand, and consisted of white 

 opaque ice in the center, surrounded by a ring of dark transparent 

 ice, with an exterior ring of ice like that in the center. In some 

 cases there were two or three dark rings, the central part and the 

 exterior ring being always opaque. These successive rings (with 

 the exception of their circular form) exactly resembled the alternate 

 opaque and transparent bands in glacial ice, where the riband 

 structure is best developed. 



t Art. 15. of my former memoir. 



