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[ex]ample of the stratification (or, as some will have it,
rhomboidal crystallization) of these rocks. The cleavage is here
nearly perpendicular; and the strata, being from one foot to five
in thickness, dip to the south-east at an angle of about thirty
degrees. In some districts in Yorkshire the layers are so much
diminished in thickness, that slates and tables are formed in the
plane of the stratification, instead of the cleavage; and this
has probably given rise to the notion of two distinct cleavages
crossing each other under a certain angle. Roofing slate (called
black slate, to distinguish it from the pale-blue or green of the
second division) is manufactured in large quantities near Kirkby
Ireleth, from whence it is now taken by railway to Barrow, where
it is shipped.
The preference given to the slates from certain quarries as
requiring less weight, for the covering of a roof of given
dimensions, depends not so much upon the specific gravity (which
varies at most from 2749 to 2800, or one part in 55) as upon the
fineness of grain, which enables it to bear splitting thinner.
All the rocks of this division effervesce more or less with
acids; they contain some calcareous spar and pyrites; but little
metallic ore, except a small quantity of galena, with green and
yellow phosphate of lead, which was formerly got near Staveley,
and a little yellow copper in Skelwith; and recently, lead-ore
has been discovered between Winster and Crosthwaite.
Although little notice has hitherto been taken, by authors, of
the difference between the roofing slates of these three
divisions, yet a workman of moderate experience will readily
distinguish them: and I have endeavoured so to describe the
peculiarities of each, that those
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