Ch. XXIV.] PURITY OF THE COAL. 491 



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 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 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 trees and semi-aquatic plants. As a singular 

 proof of this fact, I may mention that whenever any part of a swamp 

 in Louisiana is dried up, during an unusually hot season, and the wood 

 set on fire, pits are burnt into the ground many feet deep, or as far down 

 as the fire can descend, without meeting with water, and it is then found 

 that scarcely any residuum or earthy matter is left.* At the bottom 

 of all these " cypress swamps " a bed of clay is found, with roots of the 

 tall cypress (Taxodium distichum), just as the underclays of the coal 

 are filled with Stigmaria. 



It has been already stated that the carboniferous strata at the South 

 Joggins, in Nova Scotia, are nearly three miles thick, and the coal- 

 measures are ascertained to be of vast thickness near Pictou, more than 

 100 miles to the eastward. If, therefore, we speculate on the prob- 

 able volume of solid matter contained in the Nova Scotia coal-fields, 

 there appears little danger of erring on the side of excess if we take the 

 average thickness of the beds at 7500 feet, or about half that ascer- 

 tained to exist in one carefully-measured section. As to the area of 

 the coal-field, it includes a la^ge part of New Brunswick to the west, 

 and extends north to Prince Edward's Island, and probably to the 

 Magdalen Isles. When we add the Cape Breton beds, and the con- 

 necting strata, which must have been denuded or are still concealed 

 beneath the waters of the Gulf of St. Lawrence, we obtain an area 

 comprising about 36,000 square miles. This, with the thickness of 

 7500 feet before assumed, will give 51,000 cubic miles of solid matter 

 as the volume of the carboniferous rocks. 



According to the latest estimate of the annual discharge of water by 

 the Mississippi, and the proportion of sediment held in suspension in 

 its waters at different seasons of the year, after making due allowance 



* Lyell's Second Yisit to the TJ. S., toI. ii. p. 245 ; and American Journ. of Sci- 

 ence, Second Series, vol. v. p. 17. 



