;54 
SILURIA. 
[Chap. II. 
torted in directions athwart the original layers of deposit, they inferred 
that the planes of cleavage which had so deformed the shells had mainly 
resulted from mechanical forces compressing the sediment at right angles 
to the lines of cleavage. Mr. Sharpe also, in further illustrating this sub- 
ject, agreed with Sedgwick in regarding the phenomena of cleavage as being 
partly due to the action of some unknown and peculiar crystalline force. 
Sorby, adopting the mechanical portion of this view, demonstrated its 
truthfulness * by a series of ingenious experiments, clearly showing how 
the numerous flattish unequiaxed particles of the ancient mud and sand, 
out of which slate has been formed, had in all cases aided the great cause, 
or lateral compressing force, in producing cleavage. 
Adhering exclusively to the mechanical theory, Tyndall f further deve- 
loped the subject. He added the fact, that the production of cleavage was 
aided by the extension, under pressure, of the minute interstitial cavities 
which must exist in even the most finely levigated muds tones. 
Passing over certain slight differences, these authors, as well as Professor 
Phillips, agree essentially in affirming that cleavage is due to mechanical 
causes, or that slates are highly compressed strata. 
Whilst it is beyond my present object to dilate upon the forces that pro- 
duced a phenomenon which, however it may be explained, throws no light 
upon the order of the strata, I might be permitted to suggest that the 
geologist may, with profit, follow Sharpe ; and, in accepting the suggestions 
of Sedgwick, he may endeavour to combine them with the purely mechanical 
theory of Sorby and Tyndall. 
If the last-mentioned authors have succeeded (as I think they have) in 
demonstrating that powerful lateral pressure was a main cause in the for- 
mation of slates, I am still disposed to agree with Sharpe, that this cause 
alone could scarcely have produced such results as we see in the wonder- 
fully minute parallel cleavages of a whole mountain-chain, had not the 
rocks at the same time been cogently affected, as Sedgwick supposed, by 
such forces as heat and electricity, combined with vapour, mineral waters, 
and great lateral pressure J. 
Not dwelling longer, however, upon this physical problem, I would re- 
mind the reader that, once having obtained a clear insight into the distinc- 
tions between cleavage and stratification, the geologists who have long and 
patiently laboured in the Welsh mountains have been enabled to trace the 
symmetry of deposits, whose slaty composition, and frequent perforation by 
igneous rocks of various characters, had veiled them in an obscurity from 
which their equivalents in Shropshire, Herefordshire, Radnorshire, &c, or 
the unaltered region of Siluria, are almost entirely exempt. 
* Edinburgh New Phil. Journ. vol. lv. p. 137; some other important works, viz. 'Geological Ob- 
Phil. Mag. 4th Ser. vol. xi. p. 20; ibid. vol. xii. serrations in South America,' p. 163, by C. Darwin, 
p. 127. and D. Sharpe's 'Eesearehes on the Symmetry in 
t Phil. Mag. 4th Ser. vol. xii. p. 35 ; ibid. p. 129. the Arrangement of the Planes of Cleavage over 
1 The reader who desires to be acquainted with large areas,' Phil. Trans. 1852, p. 445 ; Journ. Geo!. 
Blithe phenomena relating to thisdimcult subject, Soc. vol. iii. p. 87 ; and Phillips's Keport on Clea- 
including the foliation of rocks, must also consult vage, Brit. Assoc. Kep. 1857. 
