Chap. II.] 
SLATY CLEAVAGE. 
33 
sedimentary rocks ; whilst the North-Welsh strata of the same age have 
been so affected by a distinct rearrangement of the fine, particles of mud 
and sand of which they were originally composed, that they will only split in 
a direction mostly transverse to the original bedding, cleaving, in fact, into 
countless, thin, hard plates, all precisely parallel to each other, — in other 
words, the finest roofing slates. This diagram will explain the distinction 
between original bedding, as indicated by the stronger curvilinear lines, 
and the finer slanting lines of the superinduced slaty cleavage, to which 
the weathered and the jagged edges of some of the mountains of Wales 
are due. 
To whatever force you subject hand-specimens (c) of the rock so affected, 
they will only break in the direction of the fine plates, or, in other words, 
along the lines of slaty cleavage. Whilst, to the geologist, the original 
layers of deposit are apparent, as marked by grey, purple, and greenish- 
coloured different laminae, the unpractised observer is easily deceived (like 
many geologists of the old school), and often mistakes these slaty plates for 
lines of stratification. 
To Professor Sedgwick is justly assigned the merit of having first 
taught the true distinction between slaty cleavage and stratification ; and 
his valuable observations have had a wide application. For this slaty 
impress has affected not only the oldest rocks of Carnarvonshire, where the 
finest slates are obtained, but also most of the overlying Silurian forma- 
tions in Wales ; whilst in the original Silurian region the same rocks are, 
as before said, exempt from it. Slaty cleavage prevails also in the Devo- 
nian schists of the Rhine and Devonshire, and in the Lowest Carboniferous 
strata of Devonshire and Ireland. In short, although this structural 
arrangement was, in the early days of geology, looked upon as an indica- 
tion of the great antiquity of the rocks in which it prevailed, and thus led 
to serious mistakes, the phenomenon is now known to be one which in 
Britain reoccurs throughout the Palaeozoic rocks up to the Carboniferous 
inclusive. 
Those who wish to be better acquainted with this striking phenomenon 
will do well to consult, in the first instance, the excellent memoir of Sedg- 
wick, who, distinguishing, as before said, cleavage from stratification, 
referred this mighty change in the strata to some grand operation in which 
heat and electricity were probably combined ; or, in the words of that 
author, " crystalline or polar forces have re-arranged whole mountain- 
masses, producing a beautiful crystalline cleavage passing alike through 
all the strata " *. 
Subsequently Professor John Phillips f and Mr. Daniel Sharpe $ led the 
way in propounding another theory, the chief feature of which has been 
ably sustained and illustrated. Observing that the fossil shells entombed 
in certain slaty rocks of Wales, Westmoreland, and Devonshire were dis- 
* Trans. Geol. Soc. Lond. 2nd Ser. vol. iii. p. 477. I Quart. Journ. G-eol. Soe. Lond. vol. iii. p. 74 ; 
t Rep. Brit. Assoc. 1843, Sect. p. 60. and vol. v. p. 111. 
