Week 24
The next thing we have to account for is the bed of shale or
hardened clay covering over the coal. Now we know that from time
to time land has gone slowly up and down on our globe so as in
some places to carry the dry ground under the sea, and in others
to raise the sea-bed above the water. Let us suppose, then, that
the great Dismal Swamp was gradually to sink down so that the sea
washed over it and killed the reeds and shrubs. Then the streams
from the west would not be sifted any longer but would bring down
mud, and leave it, as in the delta of the Nile or Mississippi, to
make a layer over the dead plants. You will easily understand
that this mud would have many pieces of dead trees and plants in
it, which were stifled and died as it covered them over; and thus
the remains would be preserved like those which we find now in
the roof of the coal-galleries.
But still there are the thick sandstones in the coal-mine to be
explained. How did they come there? To explain them, we must
suppose that the ground went on sinking till the sea covered the
whole place where once the swamp had been, and then sea-sand
would be thrown down over the clay and gradually pressed down by
the weight of new sand above, till it formed solid sandstone and
our coal-bed became buried deeper and deeper in the earth.
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