5) 'Petrefacta Germaniae.' Goldfuss.
(36) 'Versteinerungen der Grauwacken-formation.' &c. Geinitz.
(37) 'Beitrag zur Palaeontologie des Thueringer-Waldes.' Richter and
Unger.
(38) 'Ueber die Placodermen der Devonischen System.' Pander.
(39) 'Die Gattungen der Fossilen Pflanzen.' Goeppert.
(40) 'Genera et Species Plantarum Fossilium.' Unger.

CHAPTER XII.

THE CARBONIFEROUS PERIOD.

Overlying the Devonian formation is the great and important series
of the _Carboniferous Rocks_, so called because workable beds
of coal are more commonly and more largely developed in this
formation than in any other. Workable coal-seams, however, occur
in various other formations (Jurassic, Cretaceous, Tertiary), so
that coal is not an exclusively Carboniferous product; whilst
even in the Coal-measures themselves the coal bears but a very
small proportion to the total thickness of strata, occurring
only in comparatively thin beds intercalated in a great series
of sandstones, shales, and other genuine aqueous sediments.

Stratigraphically, the Carboniferous rocks usually repose conformably
upon the highest Devonian beds, so that the line of demarcation
between the Carboniferous and Devonian formations is principally
a palaeontological one, founded on the observed differences in
the fossils of the two groups. On the other hand, the close of
the Carboniferous period seems to have been generally, though
not universally, signalised by movements of the crust of the
earth, so that the succeeding Permian beds often lie unconformably
upon the Carboniferous sediments.

Strata of Carboniferous age have been discovered in almost every
large land-area which has been sufficiently investigated; but
they are especially largely developed in Britain, in various
parts of the continent of Europe, and in North America. Their
general composition, however, is, comparatively speaking, so
uniform, that it will suffice to take a comprehensive view of
the formation without considering any one area in detail, though
in each region the subdivisions of the formation are known by
distinctive local names. Taking such a comprehensive view, it is
found that the Carboniferous series is generally divisible into a
_Lower_ and essentially calcareous group (the "Sub-Carboniferous" or
"Carboniferous Limestone"); a _Middle_ and principally arenaceous
group (the "Millstone Grit"); and an Upper group, of alternating
shales and sandstones, with workable seams of c