rious parts of North
America, on the other hand, the Coal-measures, in addition to
sandstones, shales, coal-seams, and bands of clay-ironstone,
commonly include beds of limestone, charged with marine remains,
and indicating marine conditions. The subjoined section (fig. 107)
gives, in a generalised form, the succession of the Carboniferous
strata in such a British area as the north of England, where
the series is developed in a typical form.

As regards the _life_ of the Carboniferous period, we naturally
find, as has been previously noticed, great differences in different
parts of the entire series, corresponding to the different mode of
origin of the beds. Speaking generally, the Lower Carboniferous
(or the Sub-Carboniferous) is characterised by the remains of
marine animals; whilst the Upper Carboniferous (or Coal-measures)
is characterised by the remains of plants and terrestrial animals.
In all those cases, however, in which marine beds are found in
the series of the Coal-measures, as is common in America, then
we find that the fossils agree in their general characters with
those of the older marine deposits of the period.

[Illustration: Fig. 107. GENERALIZED SECTION OF THE CARBONIFEROUS
STRATA OF THE NORTH OF ENGLAND.]

Owing to the fact that coal is simply compressed and otherwise
altered vegetable matter, and that it is of the highest economic
value to man, the Coal-measures have been more thoroughly explored
than any other group of strata of equivalent thickness in the
entire geological series. Hence we have already a very extensive
acquaintance with the _plants_ of the Carboniferous period; and
our knowledge on this subject is daily undergoing increase. It
is not to be supposed, however, that the remains of plants are
found solely in Coal-measures; for though most abundant towards
the summit, they are found in less numbers in all parts of the
series. Wherever found, they belong to the same great types of
vegetation; but, before reviewing these, a few words must be
said as to the origin and mode of formation of _coal_.

The coal-beds, as before mentioned, occur interstratified with
shales, sandstones, and sometimes limestones; and there may,
within the limits of a single coal-field, be as many as 80 or
100 of such beds, placed one above the other at different levels,
and varying in thickness from a few inches up to 20 or 30 feet.
As a general rule, each bed of coal rests upon a bed of shale or
clay, whi