58 J. J. STEVENSON — PENNSYLVANIA ANTHRACITE. 



their presence is necessary to the hypothesis that the coal is meta- 

 morphic, and they are supposed to be at no considerable distance below 

 the surface. 



OBJECTIONS TO LESLEY'S HYPOTHESIS. 



Increased Rock-covering not productive of increased Heat. — Professor 

 Lesley suggests that the increased thickne ^s of overlying rock might lead 

 to increased heat in the lower beds. He illustrates his point by com- 

 parison of analyses of coal from the Washington and the Sharon beds, 

 the two available extremes of the column in western Penns3dvania ; but 

 the comparison is insufficient, since comparison of the Washington coal 

 with coal from the Mercer and Quakertown beds, belonging at approxi- 

 mately the same horizon with the Sharon, gives a contrary result.* 



No increased Rock-covering in Anthracite Region. — Professor Lesley, 

 accepting this suggestion as a possible explanation of varying percentages 

 of volatile combustible matter, applies it to explain debituminization of 

 the coals of eastern Pennsylvania. He maintains that as the Paleozoic 

 groups thicken toward the east, there is every reason to suppose that the 

 Permo-Carboniferous, existing in southwest Pennsylvania and in West 

 Virginia, must have extended into the anthracite region with constantly 

 increasing thickness, so that one should expect to find, as he does find, 

 the coal with very much less volatile there than in the region west from 

 the Alleghany mountain. 



The supposition that the coal groups thicken eastward toward the 

 anthracite region is hardly in accordance with the facts, as recorded in 

 the reports of the Pennsylvania survey. f It is altogether true that the 

 Devonian and lower rocks have their greatest thickness at the east, and 

 that they do decrease with great rapidity westward, even within the lim- 

 its of Pennsylvania; but this is not altogether true of the Coal Measures 

 groups, especially of the higher groups, which lose thickness as they re- 

 cede from southwest Pennsylvania, north, east and west. This general 

 statement is necessary, for the old conception still prevails too widely 

 that the same law of decrease holds good for the Coal Measures as for 

 the lower groups ; but the conditions had changed at the end of the De- 

 vonian, so that the Cincinnati arch at that time, whatever it may have 

 been during the Devonian, had become more than a bar, had become a 

 low upland, with drainage enough to bring down not very coarse mate- 

 rial for the sandstones of the Coal Measures. The Coal Measures were 

 deposited in an almost land-locked basin, along whose central strip lime- 



* Second Report of Progress in the Laboratory, etc, 1879, pp. liG-l-iT. 



•j-Many of these were published after Professcy Lesley's discussion was prepared. 



