e 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._
Yellow and gray sandstones, blue and black clays and shales,
bands of coal and ironstone, fossil plants, bivalves
and fish, occasional marine bands.

MIDDLE CARBONIFEROUS.
_Gannister beds_ or _Lower coal-measures._
_Millstone grit._ Flagstone series in Ireland.
_Yoredale beds._ Upper shale series of Ireland.

LOWER CARBONIFEROUS.
_Mountain limestone_.
_Limestone shale_.

Each of the three principal divisions has its representative in Scotland,
Belgium, and Ireland, but, unfortunately for the last-named country, the
whole of the upper coal-measures are there absent. It is from these
measures that almost all our commercial coals are obtained.

This list of beds might be further curtailed for all practical purposes
of the geologist, and the three great divisions of the system would thus
stand:--

Upper Carboniferous, or Coal-measures proper.

Millstone grit.

Lower Carboniferous, or Mountain limestone.

In short, the formation consists of masses of sandstone, shale, limestone
and coal, these also enclosing clays and ironstones, and, in the
limestone, marbles and veins of the ores of lead, zinc, and antimony, and
occasionally silver.

[Illustration: FIG. 18.--Sigillarian trunks in current-bedded sandstone.
St Etienne.]

As the most apparent of the rocks of the system are sandstone, shale,
limestone, and coal, it will be necessary to consider how these were
deposited in the waters of the carboniferous ages, an