specimens of British coal, from various localities, which I have
examined, tell one tale as to the predominance of the spore and
sporangium element in their composition; and as it is exactly in the
finest and purest coals, such as the "Better-Bed" coal of Lowmoor,
that the spores and sporangia obviously constitute almost the entire
mass of the deposit.

Coal, such as that which has been described, is always found in
sheets, or "seams," varying from a fraction of an inch to many feet
in thickness, enclosed in the substance of the earth at very various
depths, between beds of rock of different kinds. As a rule, every seam
of coal rests upon a thicker, or thinner, bed of clay, which is known
as "under-clay." These alternations of beds of coal, clay, and rock
may be repeated many times, and are known as the "coal-measures;"
and in some regions, as in South Wales and in Nova Scotia, the
coal-measures attain a thickness of twelve or fourteen thousand
feet, and enclose eighty or a hundred seams of coal, each with its
under-clay, and separated from those above and below by beds of
sandstone and shale.

The position of the beds which constitute the coal-measures is
infinitely diverse. Sometimes they are tilted up vertically, sometimes
they are horizontal, sometimes curved into great basins; sometimes
they come to the surface, sometimes they are covered up by thousands
of feet of rock. But, whatever their present position, there is
abundant and conclusive evidence that every under-clay was once a
surface soil. Not only do carbonized root-fibres frequently abound in
these under-clays; but the stools of trees, the trunks of which are
broken off and confounded with the bed of coal, have been repeatedly
found passing into radiating roots, still embedded in the under-clay.