THE STEUCTUEE AND MOTION OF GLACIEES. 
337 
magnetism, he was fortunate enough to discover that in white wax, and other bodies, a 
cleavage of surpassing fineness may be developed by pressure, and he afterwards endea- 
voured, in a short paper*, to show the application of this result, both to slaty cleavage 
and to a number of other apparently unrelated phenomena. The theory propounded 
in this paper may be thus briefly stated. If a piece of clay, wax, marble 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 exposing these facets, in some cases crystalline, in others purely 
mechanical, 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 quahty must, in an eminent degree, have been possessed by the mud 
of which slate-rocks are composed, after the water with which the mud had at flrst 
been saturated had drained away; and the result of the apphcation of pressure to 
such a mass would be, to develope in it a lamination similar to that so perfectly pro- 
duced on a small scale in white wax. Thus one cause of cleavage may be stated, in 
general terms, to be the conversion by pressure of irregularly-formed surfaces of weak 
cohesion into parallel planes. To produce lamination in a compact body such as wax, 
it is manifest that while it yields to the compression in one direction, it must have an 
opportrmity of expanding in a dunction at right angles to that in which the pressure 
is exerted ; a second cause is the lateral sliding of the particles which thus takes place, 
and which may be very influential in producing the cleavage f. 
Before attempting to show the connexion between this theoiy and the case at present 
under consideration, a mode of experiment may be described which was found to assist 
in forming a conception of the mechanical conditions of a glacier, and which has ah’eady 
been resorted to by Professor Foebes in demonstration of the viscous theory. Owing to 
* Proceedings of the Eoyal Institution, June 1856 ; Philosophical Magazine for July 1856. 
t Three principal causes may operate in producing cleavage : — 1st, the reducing of surfaces of weat 
cohesion to parallel planes ; 2nd, the flattening of minute cavities ; and 3rd, the weakening of cohesion by 
tangential action. The third action is exemplified by the state of the rails near a station where the break is 
applied. In this case, while the weight of the train presses vertically, its motion tends to cause longitudinal 
sliding of the particles of the rail. Tangential action does not however necessarily imply a force of the 
latter kind. AVhen a solid cylinder, an inch in height, is squeezed by vertical pressure to a cake a quarter 
of an inch in height, it is impossible, physically speaking, that the particles situated in the same vertical 
line shall move laterally with the same velocity ; but if they do not, the cohesion between them will be weak- 
ened or ruptured The pressure wiU produce new contact, and if the new contact have a cohesive value equal 
to that of the old, no cleavage from this cause can arise. The relative capacities of different substances for 
cleavage, appears to depend in a great measure upon their different properties in this respect. In butter, for 
example, the new attachments are equal, or nearly so, to the old, and the cleavage is consequently indistinct ; 
in wax this does not appear to be the case, and hence may arise in a great degree the perfection of its cleavage 
The further examination of this subject promises interesting results. 
MDCCCLVII. 2 Y 
