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strike, in the above named localities, being 1.6 : 1. Now 
if they had been originally spherical, and there had not been 
any absolute condensation, the ratio in the plane perpen- 
dicular to cleavage in the line of dip, calculated from that 
in the plane of cleavage, should have been as its square, 
viz., 2.56 : 1, instead of 6:1, and therefore there must 
have been an absolute compression from 100 to about 43, 
or, considering the nature of the probable errors, to about 
one-half of the original volume ; and the close packing of 
the particles, as seen in thin sections, agrees well with this 
supposition. 
I have found in some slate rocks rounded grains of mica, 
which have been broken up in such a manner as to indicate 
the same kind of change of dimension. But not to occupy 
too much time, I may state my firm conviction that there is 
abundance of facts to prove that slate rocks have undergone 
a very considerable change of dimensions, to which the 
cleavage is distinctly related, having been greatly com- 
pressed in a line perpendicular to its plane, and elongated 
in that of its dip. 
The power most generally useful in examining slate rocks 
is about 400 linear, but higher and lower are of course 
valuable for particular purposes. It is almost indispensable 
to use a polarizing microscope, and there should be such 
contrivances as to give a good bright polarised light with 
high powers. The physical structure and optical properties 
of the minerals found in them are such, that they can be 
identified with great certainty, when in grains of even less 
than inooth of an inch in diameter. 
Some slate rocks, as, for instance, the pencil slate of Shap, 
consist almost entirely of rounded grains and minute flakes 
and granules of mica, varying from about i^oth to lo^oooth 
of an inch in diameter, but chiefly under loWth. I do not 
believe that this is in the least due to metamorphism, but has 
