594 
ELEMENTS OF GEOLOGY, 
other observations, Mr. Sorby has come to the conclusion 
that the absolute condensation of the slate rocks amounts 
upon an average to about one half their original volume. 
Most of the scales of mica occurring in certain slates ex¬ 
amined by Mr. Sorby lie in the plane of cleavage; whereas 
in a similar rock not exhibiting cleavage they lie with their 
longer axes in all directions. May not their position in the 
slates have been determined by the movement of elongation 
before alluded to ? To illustrate this theory some scales of 
oxide of iron were mixed with soft pipe-clay in such a manner 
that they inclined in all directions. The dimensions of the 
mass were then changed artificially to a similar extent to 
what has occurred in slate rocks, and the pipe-clay was then 
dried and baked. When it was afterwards rubbed to a flat 
surface perpendicular to the pressure and in the line of elon¬ 
gation, or in a plane corresponding to that of the dip of 
cleavage, the particles were found to have become arranged 
in the same manner as in natural slates, and the mass admit¬ 
ted of easy fracture into thin flat pieces in the plane alluded 
to, whereas it would not yield in that perpendicular to the 
cleavage.* 
Dr. Tyndall, when commenting in 1856 on Mr, Sorby’s ex¬ 
periments, observed that pressure alone is sufficient to pro¬ 
duce cleavage, and that the intervention of plates of mica or 
scales of oxide of iron, or any other substances having flat 
surfaces, is quite unnecessary. In proof of this he showed 
experimentally that a mass of pure white wax, after hav¬ 
ing been submitted to great pressure, exhibited a cleavage 
more clean than that of any slate-rock, splitting into laminae 
of surpassing tenuity.”f He remarks that every mass of 
clay or mud is divided and subdivided by surfaces among 
which the cohesion is comparatively small. On being sub¬ 
jected to pressure, such masses yield and spread out in the 
direction of least resistance, small nodules become converted 
into laminaB separated from each other by surfaces of weak 
cohesion, and the result is that the mass cleaves at right an¬ 
gles to the line in which the pressure is exerted. In further 
illustration of this, Mr. Hughes remarks that “ concretions 
which in the undisturbed beds have their longer axes paral¬ 
lel to the bedding are, where the rock is much cleaved, fre¬ 
quently found flattened laterally, so as to have their longer 
axes parallel to the cleavage planes, and at a considerable an¬ 
gle, even right angles, to their former position.” 
Mr. Darwin attributes the lamination and fissile structure 
* Sorby, as cited above, p. 741, note. 
t Tyndall, View of the Cleavage of Crystals and Slate rocks. 
