It is called by the miners the "Farewell Rock," as when
they reach it they have no longer any hopes of obtaining coal at a greater depth
in the same district. In the central and northern coal-fields of England this
same grit, including quartz pebbles, with some accompanying sandstones and
shales containing coal plants, acquires a thickness of several thousand feet,
lying beneath the productive coal-measures, which are nearly 10,000 feet thick.
Below the Millstone Grit is a continuation of similar sandstones and shales
called by Professor Phillips the Yoredale series, from Yoredale, in Yorkshire,
where they attain a thickness of from 800 to 1000 feet. At several intervals
bands of limestone divide this part of the series, one of which, called the Main
Limestone or Upper Scar Limestone, composed in great part of encrinites, is 70
feet thick. Thin seams of coal also occur in these lower Yoredale beds in
Yorkshire, showing that in the same region there were great alternations in the
state of the surface. For at successive periods in the same area there prevailed
first terrestrial conditions favourable to the growth of pure coal, secondly, a
sea of some depth suited to the formation of Carboniferous Limestone, and,
thirdly, a supply of muddy sediment and sand, furnishing the materials for
sandstone and shale.
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