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nearly in the line of cleavage, as in those perpendicular to it. 
I have observed similar facts in all the contortions in slate 
rocks I have yet seen. It will also be remarked that the 
cleavage lies in the line of greatest thickness of the beds. 
I have found that this is the case in every instance, whatever 
be its inclination, and that, though it may vary as much as a 
right angle in contortions at no great distance from one 
another, yet there the cleavage does not pass evenly through 
them, as would most probably be the case if it was not due 
to a mechanical cause, but coincides with the line of maxi- 
mum elongation. The fan-shaped arrangement of the 
cleavage in the contorted bed is what occurs in such cases 
in coarse-grained slates, but in fine-grained it is very slight, 
or even absent. 
These changes of dimension appear to result both from 
the absolute compression of the rock by forcing more closely 
together the ultimate particles, and from an elongation in the 
line of dip, that is to say, upwards, where nothing but the 
weight of the mass opposed it. This is indeed rendered 
almost certain by an examination of the green spots so often 
seen in some slates. I am persuaded that they have been a 
kind of concretion, formed round bodies lying in the plane 
of bedding : and in rocks not having cleavage, such spots are 
spherical or somewhat elongated in the plane of bedding. 
In slate rocks, however, they have been greatly changed in 
their form and dimensions. When seen in a section perpen- 
dicular to cleavage in the line of dip, they are much longer 
in the direction of cleavage than perpendicular to it, the 
mean of very numerous measurements of such symmetrical 
ones as must have resulted from spots originally nearly 
spherical, in the slate of Penrhyn and Llanberis, shewing 
that the proportion was as 6 : 1. In the plane of cleavage 
they are seen to be elongated in the line of dip, the mean 
proportion between the length in that direction and that of 
