1848.] MECHANICAL ORIGIN OF CREASES OR WRINKLES. 215 



Mer de Glace ; and that the season was that of the disappear- 

 ance of the winter snow, which lay in wreaths between the 

 ridges, thus perfectly defining their contour. 



With respect to the origin of these wrinkles in the mud- 

 slide, Mr. Milward has, I think, very justly rejected the expla- 

 nation of them formed on a supposed alternation of beds or strata 

 of different texture, the existence of such beds being entirely 

 hypothetical, and, considering the manner in which such a mud- 

 slide is formed, utterly improbable. In looking for an explana- 

 tion of the analogous phenomena in glaciers, it would, therefore, 

 be wise to try to find one which should hold good in a mass of 

 uniform consistence, and rather to look for the less compact 

 structure of the ice beneath the dirt-bands as an effect of the 

 same cause which produces the wrinkles, than as the cause 

 itself. I believe that the phenomena of ridges or wrinkles is a 

 general one, depending on the toughness of a semifluid or semi- 

 solid mass forcibly compelled to advance or extend itself; that 

 the periodicity, or repetition of the wrinkles at nearly regular 

 intervals, is due to mechanical causes alone, and to no variation 

 of internal consistence. 



Having successfully imitated these wrinkles in my experi- 

 ments, I think that I am able so far to account for them. Although 

 neither mud nor plaster is capable of retaining the internal veined 

 structure of the frontal dip, which bears evidence of the direc- 

 tion of the slide being such as I have stated, that evidence is 

 to be found in the glaciers themselves, in certain cases of lava 

 streams, which I have elsewhere described,* and in Professor 

 Gordon's beautiful experiment with brittle pitch in motion.f 



A body soft enough to convey a pressure partly hydro- 

 static, or one acting in any direction, if it be tenacious enough, 

 always tends to crease, or have its surface near the point of 

 pressure pushed upwards and forwards relatively to the surface 

 farther off. A heavy weight laid on a tough paste will raise a 

 wrinkle round it, but at some distance ; farther off in proportion 

 to its toughness. So railway embankments raised on a boggy 



* Phil. Trans. 1836. f phil - Mag., 1845. vol. xxvi. p. 206. 



