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 swiftly than the bark. And the traveller,
setting his foot on a prostrate trunk, finds that it is a mere shell,
which breaks under his weight, and lands his foot amidst the insects,
or the reptiles, which hav