Yet the coal itself, or altered vegetable
matter, remained all the while unsoiled by earthy particles. This enigma,
however perplexing at first sight, may, I think, be solved by attending to what
is now taking place in deltas. The dense growth of reeds and herbage which
encompasses the margins of forest-covered swamps in the valley and delta of the
Mississippi is such that the fluviatile waters, in passing through them, are
filtered and made to clear themselves entirely before they reach the areas in
which vegetable matter may accumulate for centuries, forming coal if the climate
be favourable. There is no possibility of the least intermixture of earthy
matter in such cases. Thus in the large submerged tract called the "Sunk
Country," near New Madrid, forming part of the western side of the valley of the
Mississippi, erect trees have been standing ever since the year 1811-12, killed
by the great earthquake of that date; lacustrine and swamp plants have been
growing there in the shallows, and several rivers have annually inundated the
whole space, and yet have been unable to carry in any sediment within the outer
boundaries of the morass, so dense is the marginal belt of reeds and brush-wood.
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