lated on their edges. The striations on the slopes are
very similar to those on the conical ironstone; and
though the differences are in other respects great, they
both probably depend on some general law of concretionary
action, modified in operation by the nature of
the substances acted on: but we are quite ignorant of
the circumstances which determine this peculiar structure
in coal.
Succession and Thickness of Strata.
Considered in its greatest generality, and with reference
to countries where the masses appear in the greatest
simplicity (as in the south of England), the carboniferous
system consists of three formations: viz.—
Coal formation. A mass, 1000 yards or more in thickness, consisting of indefinite alternations of shales and sandstones of different kinds, with about 50 feet of coal in many beds, some ironstone layers, and (very rarely) thin layers of limestone.
Millstone grit. A variable mass of cherty rock, or hard gritstone, with some shales, incompletely distinguished from the coal series above and the limestone series below.
Mountain limestone. A mass of calcareous rocks, with very few partings of argillaceous matter, almost no grits, no coal, some chert nodules, and occasionally layers of red oxide of iron 500 to 1500 feet in thickness.
This triple system becomes modified in the north of
England, so as to constitute, in Derbyshire, a quadruple
system, without any red sandstone, thus:—
Coal formation, 2000 feet.
Millstone grit group. A series of very pebbly quartzose
and felspathic gritstones, with other sandstones and shales, and some thin bad coal, several hundred feet.
Limestone shale. A nearly uniform series of laminated shales or plates, mostly bituminous, with some ironstone and thin black limestones, but no coal 1000 feet or more.
