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. On many parts of the coast of England,
what are commonly known as "submarine forests" are to be seen at low
water. They consist, for the most part, of short stools of oak, beech,
and fir-trees, still fixed by their long roots in the bed of blue clay in
which they originally grew. If one of these submarine forest beds should
be gradually depressed and covered up by new deposits, it would present
just the same characters as an under-clay of the coal, if the
_Sigillaria_ and _Lepidodendron_ of the ancient world were substituted
for the oak, or the beech, of our own times.

In a tropical forest, at the present day, the trunks of fallen trees, and
the stools of such trees as may have been broken by the violence of
storms, remain entire for but a short time. Contrary to what might be
expected, the dense wood of the tree decays, and suffers from the ravages
of insects, more s