less than 294 feet of coal. From this it is possible to form a faint idea
of the enormous growths of vegetation required to form some of our
representative coal beds. But the coal is not found in one continuous
bed. These numerous seams of coal are interspersed between many thousands
of feet of sedimentary deposits, the whole of which form the
"coal-measures." Now, each of these seams represents the growth of a
forest, and to explain the whole series it is necessary to suppose that
between each deposit the land became overwhelmed by the waters of the sea
or lake, and after a long subaqueous period, was again raised into dry
land, ready to become the birth-place of another forest, which would
again beget, under similarly repeated conditions, another seam of coal.
Of the conditions necessary to bring these changes about we will speak
later on, but this instance is sufficient to show how inadequate the
quantity of fuel would be, were we dependent entirely on our own existing
forest growths.

However, we will leave for the present the fascinating pursuit of
theorising as to the how and wherefore of these vast beds of coal,
relegating the geological part of the study of the carboniferous system
to a future chapter, where will be found some more detailed account of
the position of the coal-seams in the strata which contain them. At
present the actual details of the coal itself will demand our attention.

Coal is the mineral which has resulted, after the lapse of thousands of
thousands of years, from the accumulations of vegetable material, caused
by the steady yearly shedding of leaves, fronds and spores, from forests
which existed in an early age; these accumulated where the trees grew
that bore them, and formed in the first place, perhaps, beds of peat; the
beds have since been subjected to an ever-increasing pressure of
accumulating strata above them, compressing the sheddings of a whole