This formation is composed of a succession of beds, of various
thicknesses, consisting of sandstones or gritstones, of clays and
shales, sometimes so bituminous as to be inflammable--and passing, in
short, into an imperfect kind of _coal_. These rocks are interstratified
with each other in such a manner that they may consist of many
alterations. Carbonate of protoxide of iron (clay-ironstone) may also
be considered a constituent of this formation; its extensive
dissemination in connection with coal in some parts of Great Britain has
been of immense advantage to the ironworks of this country, in many
parts of which blast-furnaces for the manufacture of iron rise by
hundreds alongside of the coal-pits from which they are fed. In France,
as is frequently the case in England, this argillaceous iron-ore only
occurs in nodules or lenticular masses, much interrupted; so that it
becomes necessary in that country, as in this, to find other ores of
iron to supply the wants of the foundries. Fig. 70 gives an idea of the
ordinary arrangement of the coal-beds, one of which is seen
interstratified between two parallel and nearly horizontal beds of
argillaceous shale, containing nodules of clay iron-ore--a disposition
very common in English collieries. The coal-basin of Aveyron, in France,
presents an analogous mode of occurrence.
[Illustration: Fig. 70.--Stratification of coal-beds.]
The frequent presence of carbonate of iron in the coal-measures is a
most fortunate circumstance for mining industry. When the miner finds,
in the same spot, the ore of iron and the fuel required for smelting it,
arrangements for working them can be established under the most
favourable conditions. Such is the case in the coal-fields of Great
Britain, and also in France to a less extent--that is to say, only at
Saint-Etienne and Alais.
The extent of the Coal-measures, in various parts of the world, may be
briefly and approximately stated as follows:--