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purple or slate colour ; a feeble attempt is made in fig. 3, 
PI. LVII. to depict the edge of such a slate. 
In the case of joints* it is found that they traverse all rocks 
without any apparent reference to their hardness* and that 
they frequently divide cleanly, as if cut with a knife* even the 
hardest pebbles, which may, perchance* be in their way ; with 
cleavage, however* it is different, for when the lines of cleavage 
meet bodies which oppose a resistance too great to be overcome, 
they stop short and recommence on the other side. A very 
beautiful illustration is represented in fig. 4* which shows an 
ideal section of a hand specimen in the possession of Mr. James 
Wyatt ; this consists of a small block of Cambrian slate from 
the celebrated Penrhyn quarry, near Bangor : the centre of this; 
block contains a rounded pebble of hard diabase, about the size 
of a turkey’s egg, which has been disclosed and loosened by 
carefully splitting up the block into thin slates* so that the 
pebble may be replaced in its position as before, with the slates 
around it so naturally, that its existence would not even be 
suspected. The cleavage planes* although interrupted by the 
pebble, are nevertheless continuous around it on all sides. 
When, however, such foreign bodies are not sufficiently hard 
to resist the effects of cleavage, they are seen, as in the case of 
the yellow concretions so common in the purple slates of North 
Wales* to be compressed and elongated in the direction of 
the planes of cleavage, which pass uninterruptedly through 
them, quite ignoring their presence, as is shown in fig. 5, PI. 
LVII. It has further been observed* that when hard substances, 
incapable of receiving cleavage structure, occur in cleaved rocks,, 
that they are, as a rule, arranged with their long axis parallel 
to the planes of cleavage, as seen in fig. 4, PI. LVII. This is a 
very common occurrence, and a woodcut, page 145 of Professor 
Ramsay’s report on the geology of North Wales, shows how in 
the slate conglomerate near Lynn Padarn, the slate pebbles are 
all arranged with their major axis nearly upright along the lines 
of cleavage, instead of having their flat sides corresponding to 
the planes of stratification, as under ordinary circumstances they 
would naturally lie. 
After that the labours of Professor Sedgwick had proved 
conclusively that cleavage and stratification were two distinct 
structures, much attention was directed to their mutual rela- 
tions. Horizontal cleavage is not of common occurrence, but 
has been observed in Wales, Cornwall, and Devon ; the angle or 
dip of the cleavage is usually much higher than that of the bed- 
ding, and the two are seldom found coincident except in cases of 
nearly vertical stratification. The lines of cleavage and strati- 
fication do not even necessarily dip in the same direction, for, 
as pointed out by Sedgwick, the cleavage planes in the gorge of 
