considered probable, from similar vegetation being now found in such
situations within the tropics. With regard to the circumstances
under which the masses of vegetable matter were transformed into
successive coal strata, geologists are divided. From examples seen
at the present day, at the mouths of such rivers as the Mississippi,
which traverse extensive sylvan regions, and from other circumstances
to be adverted to, it is held likely by some that the vegetable
matter, the rubbish of decayed forests, was carried by rivers into
estuaries, and there accumulated in vast natural rafts, until it sunk
to the bottom, where an overlayer of sand or mud would prepare it for
becoming a stratum of coal. Others conceive that the vegetation
first went into the condition of a peat moss, that a sink in the
level then exposed it to be overrun by the sea, and covered with a
layer of sand or mud; that a subsequent uprise made the mud dry land,
and fitted it to bear a new forest, which afterwards, like its
predecessor, became a bed of peat; that, in short, by repetitions of
this process, the alternate layers of coal, sandstone, and shale,
constituting the carboniferous group, were formed. It is favourable
to this last view that marine fossils are scarcely found in the body
of the coal itself, though abundant in the shale layers above and
below it; also that in several places erect stems of trees are found
with their roots still fixed in the shale beds, and crossing the
sandstone beds at almost right angles, shewing that these, at least,
had not been drifted from their original situations. On the other
hand, it is not easy to admit such repeated risings and sinkings of
surface as would be required, on this hypothesis, to form a series of
coal strata. Perhaps we may most safely rest at present with the
supposition that coal has been formed under both classes of
circumstances, though in the latter only as an exception to the
former.