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rations also the presence of such joints greatly assists the progress 
of the miner in his subterranean explorations, besides reducing 
the expense of driving his galleries and shafts, which would be 
found to cost vastly more in the event of the rock being a solid 
and compact mass, devoid of such natural fissures. 
Cleavage . — Although this structure had been noticed as far 
back as 1820 by Otley and Bakewell, the latter of whom 
pointed out that previous observers in the Alps had, in more 
than one instance, confounded cleavage with stratification, it 
seems quite to have escaped the attention of foreign geologists, 
and it was not until 1835 that, in his valuable “ Essay on the 
Structure of large Mineral Masses,” the true character of cleav- 
age structure and its relations to jointing and stratification was 
clearly established by Professor Sedgwick, who first pointed out 
that the lines of cleavage are often continuous over long ranges 
of country, even across bent and contorted strata, and that they 
were quite independent of those of stratification. 
In order to comprehend the phenomena of cleavage, it is ab- 
solutely necessary, in the first place, to understand its relations 
to stratification and jointing, which is best done by appealing 
to the eye. In fig. 13, PI. LVII. a sketch is given, taken from 
Murchison’s Silurian system, which shows all these three struc- 
tures presenting themselves in the same quarry ; we have, in 
the first place, a number of sedimentary beds of rock (b b b b) 
superposed one upon the other, which are traversed by a series 
of parallel joints (j j j j), whilst at the same time the beds are 
seen to be cut through at a totally different angle, by another, 
and much more numerous, series of perfectly parallel divisional 
planes (c c c c ), which is the cleavage. 
It is not at all difficult, however, to procure even hand 
specimens which illustrate the above relations in the most clear 
manner; in fig. 14, PI. LVII. is seen figured a fragment of 
ordinary purple roofing slate of the Cambrian formation, from 
the quarries of Mr. Assheton Smith at Llanberris in North 
AValcs. In this small fragment, which is only about four inches 
long by two inches broad and one inch in thickness, the front 
face and its corresponding one behind are due to cleavage, whilst 
the side faces are bounded by two smooth parallel joints, which 
thus divide the slate rock into lozenge-shaped fragments ; the 
stratification, on the contrary, is represented by the alternating 
bands of greenish and purple-coloured slate, the direction and 
angle of which are seen at a glance to be quite unconnected with 
those of either the cleavage or jointing; the specimen can be 
split ad infinitum parallel to the front face, i.e. that of cleavage, 
but possesses no divisional planes between the two joints, nor 
can it !e parted along the coloured Lands, or lines of original 
stratification, which, in uncleaved slate, would have been the 
