402 
ELEMENTS OF GEOLOGY. 
B C. 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 re¬ 
gion D A B to be thicker than the two distinct seams E and 
C, and no doubt there would actually be a slight excess rep¬ 
resenting 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 through- 
out the area DAB 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 SigUlarice 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. Yet the coal itself, or al¬ 
tered 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 herb¬ 
age 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 centu¬ 
ries, forming coal if the climate be favorable. 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 un¬ 
able to carry in any sediment within the outer boundaries 
of the morass, so dense is the marginal belt of reeds and 
brush-wood. It may be affirmed that generally, in the cy¬ 
press swamps” of the Mississippi, no sediment mingles with 
the vegetable matter accumulated there from the decay of 
trees and semi-aquatic qDlants. As a singular pi'oof of this 
