TABLE 23.1.
In the south-western part of our island, in Somersetshire and South Wales, the
three divisions usually spoken of are:
1. Coal-measures: strata of shale, sandstone, and grit, from 600 to 12,000 feet
thick, with occasional seams of coal.
2. Millstone grit: a coarse quartzose sandstone passing into a conglomerate,
sometimes used for millstones, with beds of shale; usually devoid of coal;
occasionally above 600 feet thick.
3. Mountain or Carboniferous Limestone: a calcareous rock containing marine
shells, corals, and encrinites; devoid of coal; thickness variable, sometimes
more than 1500 feet.
If the reader will refer to the section in Figure 85, he will see that the Upper
and Lower Coal-measures of the coal-field near Bristol are divided by a
micaceous flaggy sandstone called the Pennant Rock. The Lower Coal-measures of
the same section rest sometimes, especially in the north part of the basin, on a
base of coarse grit called the Millstone Grit (No. 2 of the above Table 23.1.)
In the South Welsh coal-field Millstone Grit occurs in like manner at the base
of the productive coal.
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