Year: 1872

A mistaken impression is somewhat
widely prevalent that, in the coal
fields, we have the remains of ancient
forests-in other words, it Is supposed
that wherever there was a forest in
primeval times there now exists coal
field of greater or less extent. In con-
nection with this view, also, the opln-
ION is entertained that the forests now
in existence will, in the process OF time,
and after due geological changes, oe-
come the coal-beds of future ages Bur,
although, as we shall presently see,
the coal-fields are undoubtedly due to
The vegetation of former eras, it is far
from being the case that the primeval
forests became converted in a general
way into coal. Conditions of q pecu.
liar, and to some extent exceptional,
character were requisite for the foun-
dation of coal-fields. If we consider
the evidence given by the coal-fields
themselves, We shall see what these
conditions were. The beds or seams
of coal form but small portion Of the
thickness of the great geological group
of strata to which they for the most
part appertain. This group Is called
the carbOniferous, and not uncommon
ly ''the Coal" but even where coal Is
most abundant, it forms only min
ute part of the whole mass. Thus 1t
has been estimated, Sir Charles Lyell
tells us, that in South Wales the thick
ness of the carboniferous strata
amounts, in all, to between 1000 and
II. 900 feet (or more than two miles j
"but the various coal seams do not"
according to Professor Phillips. Sex
ceed in the aggregate 120 feet" or less
than one.hundredth part of the whole.
In North Lancashire the carbonifer-
Gus strata occupy a depth of more than
three miles. Here, no fewer than
eighty seams of coal have been count
ed (seventy-one having been exposed
by the action Of the sea y but these
seams are nowhere more than five feet
in thickness, and many are but q few
inches thick. Thus, it is evident that
the formation of coal can have been in
progress but a short portion of the time
during which the carboniferOus series
of strata was in process of deposition.
Throughout by far the greater portion
of that time other minerals are being
deposited. 1t is next to be noticed
that under each coal seam a stratum
of ancient soil exists, in which there
are commonly found the roots of an
cient trees, while above the coal there
is commonly layer of shale or sand
stone, in which not unfrsquently the
trunks of those trees are found either
fallen or still i'm their original position,
and only partly converted into coal.
The bark remains, but is transmitted
into coal the hollow of the trunk, de-
eaving long before the trunk gave
way, is represented by cast in sand
stone. Thus, if we try to picture to
ourselves the state of things which
existed when such a seam of coal first
began to be covered up by the next
higher deposit, we see that there must
have been trees standing erect above a
layer of vegetable matter, the roots of
the trees being imbedded in the soil
which forms the deposit next below
the coal. The vegetable layers may
probably have been two or three times
as the resulting coal seam, and were
reduced by pressure to their present
thickness but such layers can not at
any time have reached the branches
of the forest trees. Then the process
OF deposition began. This can only
have happened when some subsidenee
of the soil had caused it to be sub
merged to a greater or less depth. We
can infer from the depth of the strata
everything the coal seams that this state
of submergence continued in many
cases for long period of time; and it
is equally clear that the formation of
the vegetable layers themselves must
have been a process occupying con-
siderable time, since tall trees grew
before the next submergence took
place. So soon as submergence was
complete, the tall trees perished and
began to decay. The stout trunks
above the vegetable layer were broken
off and swept away by the sea. The
forest itself, probably so called, was for
the most part destroyed. 1t was the
decaying refuse of the forest, inter
mixed with the lower growths, which
formed the coal seam as it now exists.
Among these were the lower parts OF
the trunks of the ancient forest trees.
These became converted, like the rest
of the vegetable matter, into coal.