14 ON THE STRUCTURE AND MOTION OF GLACIERS 
—‘“The experiment,” he says, “is one which the boldest philosopher 
would be puzzled to repeat in his laboratory ; it probably requires 
acres for its scope, and years for its accomplishment.” 
While one of us was engaged in 1855 in examining the influence 
of pressure upon 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 endeavoured, 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 quality 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 first been 
saturated had drained away; and the result of the application of 
pressure to such a mass would be, to develop in it a lamination similar 
to that so perfectly produced 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 opportunity of expanding in a direction 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. 
1 Proceedings of the Royal Institution, June 1856; Philosophical Magazine for 
July 1856. 
? Three principal causes may operate in producing cleavage:—1st, the reducing of 
surfaces of weak 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. When 
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 
