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.
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