De la Beche that there are ten principal
masses of sandstone. One of these is 500 feet thick, and the whole of them make
together a thickness of 2125 feet. They are separated by masses of shale,
varying in thickness from 10 to 50 feet. The intercalated coal-beds, sixteen in
number, are generally from one to five feet thick, one of them, which has two or
three layers of clay interposed, attaining nine feet. At other points in the
same coal-field the shales predominate over the sandstones. Great as is the
diversity in the horizontal extent of individual coal-seams, they all present
one characteristic feature, in having, each of them, what is called its
UNDERCLAY. These underclays, co-extensive with every layer of coal, consist of
arenaceous shale, sometimes called fire-stone, because it can be made into
bricks which stand the fire of a furnace. They vary in thickness from six inches
to more than ten feet; and Sir William Logan first announced to the scientific
world in 1841 that they were regarded by the colliers in South Wales as an
essential accompaniment of each of the eighty or more seams of coal met with in
their coal-field.
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