e Yoredale
Rocks, which are highly developed in the moorland region between
Lancashire and the north side of Yorkshire. These are also called the
Upper Limestone Shale, a similar group being found in places below the
limestone, and called the Lower Limestone Shale, or, in the north of
England, the Tuedian group. Going northward the beds of limestone
diminish in thickness, with a proportional increase in the intercalated
sandstones and shales, until in Scotland they are entirely subordinate
to a mass of coal-bearing strata, which forms the most productive
members of the Scotch coalfields. The next member of the series is a
mass of coarse sandstones, with some slates and a few thin coals, known
as the Millstone Grit, which is about equally developed in England and
in Scotland. In the southern coalfields it is usually known by the
miners' name of "Farewell rock," from its marking the lower limit of
possible coal working. The Coal Measures, forming the third great member
of the Carboniferous series, consist of alternations of shales and
sandstones, with beds of coal and nodular ironstones, which together
make up a thickness of many thousands of feet--from 12,000 to 14,000 ft.
when at the maximum of development. They are divisible into three parts,
the Lower Coal Measures, the middle or Pennant, a mass of sandstone
containing some coals, and the Upper Coal Measures, also containing
workable coal. The latter member is marked by a thin limestone band near
the top, containing _Spirorbis carbonarius_, a small marine univalve.

The uppermost portion of the Coal Measures consists of red sandstone so
closely resembling that of the Permian group, which are next in
geological sequence, that it is often difficult to decide upon the true
line of demarcation between the two formations. These are not, however,
always found together, the Coal Measures being often covered by strata
belonging to the Trias or Upper New Red Sandstone series.

The areas containing productive coal measures are usually known as
coalfields or basins, within which coal occurs in more or less regular
beds, also called seams or veins, which can often be followed over a
considerable length of country without change of character, although,
like all stratified rocks, their continuity may be interrupted by faults
or dislocations, also known as slips, hitches, heaves or troubles.

The thickness of coal seams varies in Great Britain from a mere film to
35 or 40 ft.; but