382 PURITY OF THE COAL. [Ch. XXIV. 



ture, so as to give rise, by their mixture, to the condensation of aqueous 

 vapor. 



The more closely the strata productive of coal have been studied 

 the greater has become the force of the evidence in favor of their 

 having originated in the manner of modern deltas. They display a 

 vast thickness of stratified mud and fine sand without pebbles, and in 

 them are seen countless stems, leaves, and roots of terrestrial plants, free 

 for the most part from all intermixture of marine remains, — circumstances 

 which imply the persistency in the same region of a vast body of fresh 

 water. This water was also charged, like that of a great river, with an 

 inexhaustible supply of sediment, which seems to have been transported 

 over alluvial plains so far from the higher grounds that all coarser parti- 

 cles and gravel were left behind. Such phenomena imply the drainage 

 and denudation of a continent or large island, having within it one or 

 more ranges of mountains. The partial intercalation of brackish-water 

 beds at certain points is equally consistent with the theory of a delta, the 

 lower parts of which are always exposed to be overflowed by the sea even 

 where no oscillations of level are experienced. 



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 

 Sigillarioe 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 altered vegetable matter remained 

 all the while unsoiled by earthy particles. This enigma, however per- 

 plexing 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 favorable. There is no pos- 

 sibility 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 brushwood. It may be affirmed 

 that generally in the " cypress swamps" of the Mississippi no sediment 

 mingles with the vegetable matter accumulated there from the decay of 



