o be applicable over more than
limited areas. According to L. Lemiere, who has very fully reviewed the
relation of composition to origin in coal seams (_Bulletin de la Societe
de l'Industrie minerale_, 4 ser. vol. iv. pp. 851 and 1299, vol. v. p.
273), differences in composition are mainly original, the denser and
more anthracitic varieties representing plant substance which has been
more completely macerated and deprived of its putrescible constituents
before submergence, or of which the deposition had taken place in
shallow water, more readily accessible to atmospheric oxidizing
influences than the deeper areas where conditions favourable to the
elaboration of compounds richer in hydrogen prevailed.

The conditions favourable to the production of coal seem therefore to
have been--forest growth in swampy ground about the mouths of rivers,
and rapid oscillation of level, the coal produced during subsidence
being covered up by the sediment brought down by the river forming beds
of sand or clay, which, on re-elevation, formed the soil for fresh
growths, the alternation being occasionally broken by the deposit of
purely marine beds. We might therefore expect to find coal wherever
strata of estuarine origin are developed in great mass. This is actually
the case; the Carboniferous, Cretaceous and Jurassic systems (qq.v.)
contain coal-bearing strata though in unequal degrees,--the first being
known as the Coal Measures proper, while the others are of small
economic value in Great Britain, though more productive in workable
coals on the continent of Europe. The Coal Measures which form part of
the Palaeozoic or oldest of the three great geological divisions are
mainly confined to the countries north of the equator. Mesozoic coals
are more abundant in the southern hemisphere, while Tertiary coals seem
to be tolerably uniformly distributed irrespective of latitude.

Sequences of carboniferous strata.

The nature of the Coal Measures will be best understood by considering
in detail the areas within which they occur in Britain, together with
the rocks with which they are most intimately associated. The
commencement of the Carboniferous period is marked by a mass of
limestones known as the Carboniferous or Mountain Limestone, which
contains a large assemblage of marine fossils, and has a maximum
thickness in S.W. England and Wales of about 2000 ft. The upper portion
of this group consists of shales and sandstones, known as th