strata.

In South Wales the coal-bearing strata have been estimated at between
11,000 and 12,000 feet, yet amongst this enormous thickness of strata,
the whole of the various coal-seams, if taken together, probably does not
amount to more than 120 feet. This great disproportion between the total
thickness and the thickness of coal itself shows itself in every
coal-field that has been worked, and when a single seam of coal is
discovered attaining a thickness of 9 or 10 feet, it is so unusual a
thing in Great Britain as to cause it to be known as the "nine" or
"ten-foot seam," as the case may be. Although abroad many seams are found
which are of greater thicknesses, yet similarly the other portions of the
formation are proportionately greater.

It is not possible therefore to realise completely the significance of
the coal-beds themselves unless there is also a knowledge of the
remaining constituents of the whole formation. The strata found in the
various coal-fields differ considerably amongst themselves in character.
There are, however, certain well-defined characteristics which find
representation in most of the principal coal-fields, whether British or
European. Professor Hull classifies these carboniferous beds as
follows:--

UPPER CARBONIFEROUS.
_Upper coal-measures._
Reddish and purple sandstones, red and grey clays and shales,
thin bands of coal, ironstone and limestone, with _spirorbis_
and fish.

_Middle coal-measures._