r estuarine sediment over the beds of vegetable
remains. The seams of coal, varying in thickness from a few inches to
three or four feet, always rest on a bed of clay, known technically as the
"underclay," which represents the soil on which the plants originally
grew. In some instances the seams of coal with their thin "partings" of
clay reach an aggregate thickness of twenty to thirty feet. In many cases
the very roots of the trees are found upright in a fossilized condition in
the underclay, and can be traced upwards into the overlying coal beds; or
the completely carbonized trunk is found erect in the position in which
the tree lived and died (see Fig. 2).

[Illustration: FIG. 1.--Section through Carboniferous strata showing seams
of coal. Dislocations, or "faults," so common in the Coal Measures, are
shown at H, T, and F. Intrusions of igneous rock are shown at D. At B is
shown the coalescence of two seams, and at N the local thinning of the
seam. The vertical lines indicate the shafts of coal mines.]

[Illustration: FIG. 2.--Section showing coal seams and upright trunks
attached to roots _in situ_. A', A'', A''', beds of shale. B, coal seams.
C, underclay. D, sandstone.]

Owing to the chemical and mechanical forces to which the original
vegetable deposit has been subjected, the organic structure of coal has
for the most part been lost. Occasionally, however, portions of leaves,
stems, and the structure of woody fibre can be detected, and thin sections
often show the presence of spore-cases of club-mosses in such numbers that
certain kinds of coal appear to be entirely composed of such remains. But
although coal itself now furnishes but little direct evidence of its
vegetable origin, the interstratified clays, shales, and other deposits
often abound with fossilized plant remains in every state of
preservation, from the most delicate fern frond to the prostrate tree
trunk many yards in length. It is from such evidence that our knowledge of
the Carboniferous flora has been chiefly derived.

Now this carbonized vegetation of a past age, the history of which has
been briefly sketched in the foregoing pages, is one of the chief sources
of our industrial supremacy as a nation. We use it as fuel for generating
the steam which drives our engines, or for the production of heat wherever
heat is wanted. In metallurgical operations we consume enormous quantities
of coal for extracting metals from their ores, this consumpt