It may be objected that the uninterrupted growth of plants during the
interval of time required for the filling up of the lagoon will have caused the
vegetable matter in the region D-A-B to be thicker than the two distinct seams E
and C, and no doubt there would actually be a slight excess representing one or
more generation of trees and plants forming the undergrowth; but this excess of
vegetable matter, when compressed into coal, would be so insignificant in
thickness that the miner might still affirm that the seam D-A throughout the
area D-A-B was equal to the two seams C and E.
CAUSE OF THE PURITY OF COAL.
The purity of the coal itself, or the absence in it of earthy particles and
sand, throughout areas of vast extent, is a fact which appears very difficult to
explain when we attribute each coal-seam to a vegetation growing in swamps. It
has been asked how, during river inundations capable of sweeping away the leaves
of ferns and the stems and roots of Sigillariae and other trees, could the
waters fail to transport some fine mud into the swamps? One generation after
another of tall trees grew with their roots in mud, and their leaves and
prostrate trunks formed layers of vegetable matter, which was afterwards covered
with mud since turned to shale.
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