Mr. HOPKINS, ON THE MOTION OF GLACIERS. 



161 



that no such motion would take place in such a glacier, for instance as that of the Lower Aar, 

 even if it were perfectly stratified, and there were no adhesion whatever between the strata. Much 

 more then is such motion impossible in the actual case of a glacier in which there is little or no 

 indication of stratification, and none whatever of the want of powerful cohesion between two 

 contiguous portions separated by any nearly horizontal plane, such as MN. If, then, any mo- 

 tion of the upper portion take place by its sliding over a lower portion, it must be at a depth at 

 which the hard and crystalline structure of the ice is destroyed*. This brings us to the second of 

 the cases above specified, as possible modes in which glacial motion may take place. 



3. There are three causes, I conceive, which may tend to destroy the crystalline structure 

 of the mass — temperature, moisture, and pressure. With respect to the first we may observe, 

 that during the summer the interior temperature, except at points very near the lower surface, 

 must necessarily be lower than that immediately beneath the upper surface, where however, there 

 is no such disintegration of the ice as we are now contemplating. Consequently there can be no 

 such disintegration, as the result of temperature, in the interior of the glacier. Similarly with 

 respect to moisture, if no sensible disintegration result from it near the upper surface where it is 

 most completely disseminated by immediate infiltration, it is not to be supposed that any such 

 effect will be produced in the interior of the mass, except at points so near its lower surface as 

 to be within the influence of the sub-glacial reservoirs and currents. 



It would seem then that the only cause to which we can refer any disintegration of the mass, 

 except at points very near the lower surface, must be the pressure of the superincumbent portion. 

 And this must be allowed to be a possible cause of such an effect, for it cannot be doubted that 

 if ice formed under a small pressure were exposed to a very great pressure, its crystalline structure 

 would be effectively destroyed. Still it does not follow that we can assert it to be probable 

 that such is actually the case in existing glaciers ; for the hard crystalline structure of glacial 

 ice is doubtless acquired gradually, and probably, in its ultimate degree, under a pressure which 

 bears a considerable ratio to the greatest pressure to which it afterwards becomes subjected ; and 

 on this account I should deem it the more probable hypothesis that no part of a glacier becomes 

 disintegrated merely by the pressure which it sustains. Without dwelling, however, on the 

 assertion of probabilities, we may, to a certain extent, appeal to observation. M. Agassiz has 

 descended a vertical fissure to the depth of nearly 200 feet, but we hear of no appearance of a 

 change of structure in the ice, such as here supposed, and which, had it existed, could hardly 

 have escaped his observation. But more conclusive evidence is found in the bore which 

 M. Agassiz sunk to the depth of nearly 200 feet. At the bottom of it the ice was found to be 

 excessively hard, and so little had its structure yielded to the pressure which it sustained, that 

 its specific gravity could scarcely have exceeded that of the superficial ice, as proved by the 

 facility with which the broken fragments rose from the bottom of the bore to the surface when 

 the bore was filled with water. At the depth of the bore, then, we may assert the absence of 

 even the smallest tendency to disintegration, and therefore we are justified in concluding by in- 

 duction, that no very sensible effect of that kind existed at considerably greater depths, as for 

 instance, at the depth of 300 feet or upwards. 



4. Nor does it appear to me possible that glacial ice, retaining its crystalline structure, 

 should possess a degree of plasticity sufficient to admit of a motion of the kind above specified in 

 (.S). It may be conceived to be possible that the elementary particles of a fluid mass should change 

 their form indefinitely, and that a continuous motion might result from such change ; hut solid 

 bodies are susceptible of a relative motion of their parts, from this change of form, by the action 



• It was stated by .M. AgasBiz, in hiH Etudes hut les Glaciers^ 

 on the authority of M. HuK', that the upper portion of glaciers 

 may be observed in deep transvcrHe HsKurcs to project in strata 



over the lower portion, so as to itulicate the relative motion above 

 described. It is now well known tilat there is a rcniarkable absence 

 of such appearances. 



