THE CARBONIFEROUS PERIODS 499 



Coal 



Coal occurs in very thin beds in the Devonian and in thicker beds 

 in the Mississippian, but it is in the Pennsylvanian or Coal Measures 

 that it occurs for the first time in beds or seams thick enough to be of 

 commercial value. The thickness and purity of the coal beds of this 

 period are such as to make it the most important of all coal-bearing 

 systems. 



Mode of Occurrence. — The total thickness of the Pennsylvanian 

 or Coal Measures is 4000 to 5000 feet in the Appalachian Mountains 

 and 18,000 feet in Arkansas, but of this great accumulation of sedi- 

 ment seldom more than two per cent, is coal, the remainder being 

 sandstone, shale, limestone, and iron ore. In the section shown in 

 Fig. 449, p. 473, it is apparent that there is no regular order of 

 succession of the beds, except that often a bed of fire clay immediately 

 underlies a coal seam. It is also usual to find shale immediately 

 overlying the coal, although this does not invariably happen. In 

 different portions of the same field the same order is usually found, but 

 in separate basins the order may vary greatly and is probably never 

 the same in all particulars. 



Origin of Coal. — Coal is of vegetable origin, as is proved (1) by 

 a microscopic examination which, even in dense anthracite, shows 

 the cellular structure of plant tissue, and (2) by stumps of trees with 

 their roots penetrating the underclay which sometimes underlies the 

 coal seam. In South Wales, for example, there are 100 coal seams 

 in which such stumps are embedded. In Nova Scotia, of j6 coal 

 seams 20 have upright stumps with spreading roots penetrating the 

 clay ; in the United States few such occurrences are known. (3) Leaves 

 are often beautifully preserved in the shale immediately overlying the 

 coal. (4) Fire clay often, although not invariably, underlies coal 

 beds. The character which a fire clay possesses of withstanding in- 

 tense heat is due to the absence of alkalies, such as potash and soda, 

 whose withdrawal was brought about by the plants whose roots re- 

 moved the soluble salts which they required for food or which were 

 removed by the leaching action of the water in the lakes or lagoons. 



It is evident, therefore, that coal is compressed bituminized or min- 

 eralized vegetable matter. 



Necessary Conditions for Coal Formation. (1) How Vegetable 

 Tissue Accumulated. — It is generally believed that coal originated, 

 for the most part, from vegetation that grew in swampy or marshy 



