1856.] Physical Geography of North America. 183 



These coal-fields, especially the four lying west of the Atlantic 

 slope, exhibit several interesting facts of gradation, which render it 

 highly probable that they were, at one time, all of them connected, 

 the vacant intervals now separating them having been denuded of 

 their coal measures by the wide-sweeping erosive action of the 

 waters of the Appalachian Sea, set in motion at the uprising 

 of this part of the continent. The first fact of such gradation 

 relates to the thickness of the formation, and that of the individual 

 coal seams in the respective coal-fields, the comparison indicating 

 a marked reduction in this respect from east to west. Thus the 

 productive coal measures of Nova Scotia have a thickness of nearly 

 3000 feet, those of the anthracite basins of Pennsylvania a depth 

 about as great, while those in the central parts of the great 

 Appalachian basin show a thickness not exceeding 2500 feet. 

 Again, in the Illinois basin the probable thickness is reduced to 1500 

 feet, while in the farthest, or Ohio and Missouri basin, it cannot 

 exceed 1000 feet. Very similar is the reduction in the number of 

 the coal seams. Those at the Joggins, in Nova Scotia, are about 

 50 ; though only five of them are of workable dimensions, being 

 equivalent to about 20 feet of coal. The deepest anthracite basin 

 of Pennsylvania, that of the Schuylkill, contains also about 50 coal 

 seams, but 25 of these have a thickness each of more than three 

 feet, and are available for mining. Further west, the great Appa- 

 lachian coal-field contains about 20 beds in all, 10 of which are 

 thick enough to be mined. Still farther onward, the broad basin of 

 Indiana and Illinois shows apparently not more than 10 or 12 beds, 

 and it is believed that only 7 of these are thick enough and pure 

 enough for mining. Northward, in the Michigan coal-field, denu- 

 dation has left only the two or three lower beds. Still further west- 

 ward the coal-field of Iowa and Missouri contains, it is believed, 

 but 3 or 4 beds of profitable size, and the total number, thick 

 and thin, does not exceed 6 or 7. A similar gradation is noticeable 

 in the general size of the individual coal seams, by far the thickest 

 being in the anthracite basins of eastern Pennsylvania. 



Parallel with this progressive reduction in the amount of land- 

 derived material in the upper coal formation, is a diminution in the 

 coarseness of the mechanical ingredients of the strata, the eastern 

 coal measures having more conglomerates and coarse sandstones, 

 the western, more tine-grained argillaceous sandstones and clay 

 beds ; and as a further indication that the first land lay to the east, 

 and the ocean to the west, of the wide coal-producing plains or 

 meadows, there is with this westward reduction of the mechanically 

 derived sediments from the land, a steady augmentation of marine 

 limestones, and other true aqueous deposits, precipitates of a shallow 

 carboniferous sea. In some of the more western coal-fields the 

 alternation of the terrestrial coal seams with super-imposed lime- 

 stones, containing marine fossils, amounts even to an occasional 

 actual contact of the two kinds of strata. 



