THE STRUCTURE OF ROCK MASSES. 
125 
have, as a rule, their flat sides or long axes parallel to the 
planes of cleavage, and not to those of stratification, as is the 
case in unaltered sedimentary deposits. That this effect was 
due to the same mechanical force which produced the cleavage, 
was demonstrated by Mr. Sorby by the following simple ex- 
periment : — 
A quantity of scales of oxide of iron was mixed promiscuously, 
so as to distribute them in all directions throughout pipe-clay; 
after drying, a section of the mass presented the appearance 
shown in fig. 7 a, PI. LVII. Upon submitting this mass to a 
pressure sufficient to change its dimensions to the same extent 
that is supposed to have taken place in natural slate rocks, a 
section at right angles to the diameter of the applied pressure 
(which, consequently, would correspond to the dip of the cleavage 
planes in roofing slate) now presented the appearance seen in 
fig. 7 6, rendering it apparent that the effect of the pressure 
had caused the scales to re-arrange themselves in nearly parallel 
lines, perpendicular to the pressure, along which the mass now 
admitted of easy division into thin plates, whereas it did not do 
so in other directions. 
Ordinary clay, which contains but few flat particles, when 
submitted to pressure was also found to receive a most distinct 
laminated structure, but did not cleave perfectly, so that it is 
probable that the perfection of the cleavage in roofing slates 
depends, in part at least, upon the presence of the numerous 
flat particles of mica, chlorite, &c., which, by their re-arrange- 
ment, also contribute to determine the easy splitting qualities 
of the rock. That rocks may receive a comparatively well- 
developed cleavage structure, even in the absence of such par- 
ticles, is evident, however, from the descriptions given by Sorby 
of the cleaved limestones of Devonshire ; and the subsequent 
experiments of Tyndall showed also that a most perfect cleavage 
could be induced in pure white wax, by the application of mere 
| pressure. 
The consideration of the above, and numerous similar data 
| connected with the subject, led Mr. Sorby to the conclusion 
! that rocks possessing a perfect cleavage structure, such as the 
Welsh roofing slate, had experienced a diminution in bulk by 
! compression to about one-half of their original volume (conse- 
quently would, after cleavage, increase so much more in density), 
and to advocate a purely mechanical theory of cleavage, such as 
explained above. 
EXPLANATION OF FLATE LVII. 
Fig. 1 . Sketch section across Llwyd-Mawr, Carnarvonshire : (a) quartz 
porphyrite on which rests the purple Cambrian slate rocks ; ( d ) 
