o~ 
ON SLATY CLEAVAGE AND ALLIED ROCK-STRUCTURES. 821 
irregular shapes, to an almond-like or ellipsoidal form. Observing with a 
glass the fine-grained slates, he was of opinion that they too presented 
the same structure on a small scale, their minute constituent fragments 
tending to the general form of ellipsoids with their shortest axes perpen- 
dicular to the cleavage-planes and their longest axes along the cleavage- 
dip. He pointed out that a rock having this constitution would, in 
consequence, split most readily in a direction perpendicular to the shortest 
axes of the ellipsoids, and so to the greatest compression of the rock- 
mass, for such a surface of fracture would run along the flattest faces of 
the fragments and meet the smallest number of them. The cleavage 
perpendicular to the direction of greatest compression in the rock was 
thus accounted for. Mr. Sharpe also maintained that there would be a 
second, though less perfect, cleavage perpendicular to the strike of the 
first, and so parallel to what the workmen call the ‘side’ of the slate ; 
but we shall see that this could not be called a direction of cleavage in 
the strict sense of the word. 
The author quoted considered, then, that the distortion of the rock- 
mass was shared by the ultimate fragments composing it. Dr. Sorby, 
on the other hand, held that the yielding of the rock was effected by a 
sliding of the originally flat or linear fragments over one another and a 
re-arrangement of them approximately perpendicular to the greatest 
compression of the rock. He supposed that the rocks which now form 
slates were originally composed, to a large extent, of flat and linear 
elements: in the slates of Penrhyn, of Llanberis,! about half the bulk of 
the rock consists of minute flakes of mica averaging ,,),, inch in length 
and +54 op inch in thickness; in the Devonian limestones of Devonshire ? 
fragments of crinoids and corals play a similar part. In certain uncleaved 
rocks of like composition the fragments lie in all directions at random, 
and we may suppose that the slate-rocks and limestones in question had 
originally a similar constitution. When the rock experienced a lateral 
compression and an expansion in a direction perpendicular to it, the 
fragments moved with the mass; there was thus a tendency in the flat 
constituents to set themselves perpendicular to the direction of the 
greatest compression, and in the linear fragments to arrange themselves 
not only approximately perpendicular to the compression, but also roughly 
parallel to the direction of expansion. In this way a structure would 
be set up in the rock effectively the same as that supposed by Mr. 
Sharpe, but arising in a different way. Premising that the two theories 
are by no means mutually exclusive, we may go on to consider that of 
Dr. Sorby more closely. 
Conceive a number of planes traced in the rock, having the same 
‘strike’ as the cleavage, and arranged, previously to the distortion, at 
_ equal angular intervals: fig. 3 shows the traces of such planes on a plane 
perpendicular to the cleavage-strike. After the distortion of the whole 
it is clear that all the planes will have been turned so as to make smaller 
angles with the plane perpendicular to the greatest compression, or, in 
other words, the principal diametral plane of the strain-ellipsoid ; and, 
further, the planes in the neighbourhood of that plane will be more closely 
packed together than those more remote (fig. 4). In fact, if @ be the 
angle made before distortion by any one of these planes with the plane 
1 Edinb. New Phil. Jowrn., vol. lv. p. 187 (1853). Phil. Mag., 4th ser., vol. xii. 
p. 127 (1856). 
? Phil. Magq., vol. xi. p. 20 (1856). 
