BITUMINOUS AND ANTHRACITIC. 439 



Besides the strata of limestone, we meet with other chemically 

 formed deposits, in the form of numerous seams of carbonate of 

 iron, and a few considerable beds of regularly stratified chert. 

 The nodular variety of the iron ore is usually imbedded in shale, 

 and lies oftenest adjacent to the coal, while the ore in bands occurs 

 more frequently in contact with the limestone. 



3. Coal, in nearly all its known varieties, including every 

 description, from the chyest and most compact anthracites, to the 

 most ^ sible and bituminous kinds of common coal. 



Such are the three great classes of strata, comprised within the 

 Appalachian coal region of the United States. If we direct our 

 attention to the manner of their distribution, we shall behold some 

 striking and instructive phenomena, susceptible of reduction to 

 regular and harmonious laws of gradation. 



Comparing, in the first place, the rocks of mechanical origin, 

 as they occur in different districts, we almost invariably find them 

 coarsest and most massive towards the southeast, and more and 

 rriore fine-grained and less arenaceous, as we pursue them across 

 the successive parallel basins northwestward. Thus in the an- 

 thracite coal-fields, which are the most southeastern of all, the 

 coal is intersti'atified with a vast thickness of rough and ponder- 

 ous grits, and coarse siliceous conglomerates ; but is associated 

 with comparatively very little soft clay slate or shale. In this re- 

 gion, the coal slates themselves, are more than ordinarily arena- 

 ceous, and bear a smaller proportion to the sandstones, than in 

 the basins more to the west. At the same time that the coal rocks, 

 viewed in the aggregate, acquire a finer texture, in going west- 

 ward, the individual strata undergo a corresponding reduction in 

 thickness, while many of them entirely thin away. I may cite, 

 as a striking instance of these changes, the great coal conglom- 

 erate itself, which forms the general base of the main or upper 

 coal measures. This massive rock is chiefly composed of large 

 quartzose pebbles, imbedded in coarse sand. Adjacent to its 

 most southeastern outcrop in Pennsylvania, that is to say, in the 

 Sharp Mountain, where it constitutes the boundary of the first or 

 Pottsville basin, it has a thickness of nearly fifteen hundred feet ; 

 but in the mountains which embrace the Wvoming coal-field. 



