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remotely connected with the bedding, and often so much 
more decided, that they were at first confounded, and the 
stratification overlooked, until the true nature of cleavage was 
pointed out by Professor Sedgwick. In most districts possess- 
ing cleavage, the stratified beds are much contorted, and dip 
at all angles, whilst the cleavage passes through them with 
more or less regularity, in some cases quite independent of the 
bedding. I should occupy far too much time if I were to 
describe one-half of the facts connected with this very ex- 
tensive subject, and shall therefore confine myself to but few. 
It is most important to bear in mind that cleavage is quite 
distinct from jointed structure. Joints are mere cracks and 
fractures, whereas cleavage is a structural weakness affecting 
the whole mass of rock. The question, therefore, is, how 
this structural weakness was developed? Mr. Fox, of Corn- 
wall, found that a kind of laminated structure was produced 
by passing an electric current through clay, and Hunt has 
since repeated, confirmed, and extended his observations. 
I, however, think this electrical theory is quite inadequate 
to explain the phenomena, which in my opinion are chiefly, 
if not entirely, due to a mechanical cause. 
It is now several years since a very distinct relation was 
observed to exist between the direction of cleavage and the 
axis of elevation of the cleaved district ; and several ob- 
servers, chiefly Professors Phillips and Sharpe, have shown 
that the organic remains found in slate rocks indicate a con- 
siderable change in their dimensions, being so altered in 
form as to shew that they had been greatly compressed in a 
line perpendicular to the plane of cleavage. The latter 
geologist, in memoirs communicated to the Geological So- 
ciety, strongly advocated the supposition of the cleavage 
being produced by this pressure. 
Having for some time been occupied with studying the 
microscopical structure of rocks, and being persuaded that 
cleavage must be due to some peculiarity in the arrangement 
