Remarks oti Glacier Theories. 339 



never rectify meclianical reasoning which is demonstrably 

 erroneous. 



Again, with respect to a plastic or viscous mass, it is asserted 

 that the curves would be of unlimited Jlejcure. This would 

 doubtless be true with a sufficient degree of plasticity, but I 

 am perfectly satisfied that there is no great flexure without 

 innumerable fractures in any actual glacier. Along the flanks 

 there are invariably numerous dislocations, and in those parts 

 the relative motions of different points do not take place merely 

 by an extension of the mass, but undoubtedly by the sliding 

 also of one mass past another; and the central portion, though 

 generally less dislocated than the flanks, is still broken into 

 thousands of fragments at particular points of its course, so 

 that the elongation of its loops cannot depend solely on the 

 plasticity of the mass. There is at present a great want 

 of observations for determining the degree of discontinuity 

 in the motion of a glacier, without which it is impossible to 

 ascertain how far the elongation of the transverse curves de- 

 pends on extension or on dislocation. I have in my second 

 letter allowed the probability that considerable, possibly great 

 extension, may take place in a long period of time, and have 

 designated this extensibility by the epithet secular, in contra- 

 distinction to that property by virtue of which glacial ice may 

 admit of extension in a small time. Now if the extension 

 which may be produced in glacial ice in many years, before 

 it arrives at the point of breaking, be much greater than that 

 which it would admit of by the application of a force which 

 should bring it to that point in a few hours or a few days, the 

 distinction denoted by the above epithet is not without its 

 due significance*. Thus applied, it does not express, as Dr. 

 Whewell asserts, a necessary condition of all extensibility or 

 plasticity, because it only has reference to long periods of 

 time, which is not the case in the common applications of those 

 terms. The observation that " it is inappropriate when ap- 

 plied to a process which takes place in a few days," is itself 

 inappropriate, inasmuch as such an application of the term 

 was never contemplated. After all, the recognition or rejec- 

 tion of it is a matter of little consequence, and 1 cannot but 

 feel surprised that it should have been sufficient to provoke a 

 departure from the rules which strict courtesy, as well as the 



* It might have been more consistent with the analogy which suggested 

 the use of the term secular, and perhaps also with the strict propriety of 

 language, if I had ap|)lied the epithet to the effect produced, rather than 

 to the property to which the effect is due. I have, however, carefully 

 defined the sense in which the terra is used, and it is at least as admissible 

 an epithet to plasticity as horizontal and vertical, for which we have the 

 high authority of your correspondent. 



2 A 2 



