and Motion o/Glaciei's. 377 



theory, by computing, from the amount of the distortion of fos- 

 sils, the magnitude of the change which cleaved rocks have un- 

 dergone*. By the united testimony of these and other observers, 

 whose researches have been carried out in different places, the 

 association of cleavage and compression has been established in 

 the most unequivocal manner ; and hence the question naturally 

 arises, "Is the pressure sufficient to produce the cleavage?" 

 Sharpe appears to have despaired of an experimental answer to 

 this question. " If," says he, " to this conclusion it should be 

 objected, that no similar results can be produced by experiment, 

 I reply, that we have never tried the experiment with a power at 

 all to be compared with that employed ; and that this may be 

 one of the many cases where our attempts to imitate the opera- 

 tions of nature fail, owing to the feebleness of our means, and 

 the shortness of the period during which we can employ them." 

 The same opinion appears to have been entertained by Professor 

 Forbes : — " The experiment," he says, " is one which the boldest 

 philosopher would be puzzled to repeat in his laboratory ; it pro- 

 bably requires acres for its scope, and years for its accomplish- 

 ment." 



While one of us was engaged in 1855 in examining the influ- 

 ence of pressure upon magnetism, he was fortunate enough to 

 discover that in white wax, and other bodies, a cleavage of sur- 

 passing fineness may be developed by pressure, and he afterwards 

 endeavoured, in a short paper f, to show the application of this 

 result, both to slaty cleavage and to a number of other appa- 

 rently unrelated phsenomena. The theory propounded in this 

 paper may be thus briefly stated. If a piece of clay, wax, mar- 

 ble or iron be broken, the surface of fracture will not be a plane 

 surface, nor will it be a surface dependent only on the form of 

 the body, and the strain to which it has been subjected; the 

 fracture will be composed of innumerable indentations, or small 

 facets, each of which marks a surface of weak cohesion. The 

 body has yielded, where it could yield, most easily, and in expo- 

 sing these facets, in some cases crystalline, in others purely me- 

 chanical, wherever the mass is broken, it is shown to be composed 

 of an aggregate of irregularly shaped parts, which are separated 

 from each other by surfaces of weak cohesion. Such a quality 

 must, in an eminent degree, have been possessed by the mud of 

 which slate-rocks are com|)osed, after the water with which the 

 mud had at first been saturated had drained away; and the 

 result of the application of pressure to such a mass would be, to 

 develope in it a lamination similar to that so perfectly produced 

 on a small scale in white wax. Thus one cause of cleavage may 



* Philosophical Magazine for December, 1856. 

 t Ibid, for July, 185(;. 

 Phil. Mag. S. 4. Vol. 15. No. 101. May 1858. 2 C 



