OBJECTIONS TO LESLEY's HYPOTHESIS. 59 



stones were formed. The old grouping of the Coal Measures in Pennsyl- 

 vania answers best for comparison. It is, in ascending order — 



1. Potts ville. 



2. Lower coal group. 



3. Lower barren group. 



4. Upper coal grouj). 



5. Upper barren group or Permo-Carboniferous. 



The Pottsville conglomerate within the anthracite fields varies in thick- 

 ness, according to Ashburner,'''^ from 551 to 1,280 feet in the Southern 

 field, whereas in the Middle field, near Hazleton, it is but 262 feet, while 

 near Wilkesbarre, in the Northern field, it is but 96 feet. The variations 

 in thickness occur within short distances and are very startling. The 

 causes do not concern. us here. In the Broad Top region the thickness 

 of the Pottsviile is not far from 250 feet,t whereas on the east side of the 

 Cumberland basin, along the Baltimore and Ohio railroad, in West Vir- 

 ginia, it is 451 feet. J Its loAver plate disappears in southwest Pennsyl- 

 vania, where the total thickness is not far from 200 feet. § 



The lower coal group, || taking the Mammoth bed as its upper limit, 

 shows an extreme thickness of 500 to 437 feet in the Southern field; of 

 213 to 156 feet in the Middle field and of 476 to 257 feet in the Northern 

 field. In the semi-bituminous Broad Top field the extreme thickness is 

 barely 220 feet,^ and on the eastern side of the Cumberland basin, on 

 the Baltimore and Ohio railroad, it is only 268 feet.** These are all 

 within the strip between the Alleghany and the Kittatinny mountain — 

 the anthracite strip ; l)ut in the bituminous basins the thickness of this 

 group varies from 300 to 350 feet, as given by White, Piatt and Steven- 

 son in their several reports. 



The lower barren group in Broad To]) is about 520 feet, and about 600 

 feet on the east side of the Cumberland basin. No positive statement 

 can be made respecting its thickness in the anthracite field, as the identi- 

 fication of the Pittsburg coal-bed there is not wholly certain, but it is 

 approximately the same as in Broad Top. In the bituminous areas, 

 according to White, Piatt and Stevenson, the thickness varies from 570 

 to 610 feet. 



* Ashburner: Ann. Rep. Gool. Survey of Pennsylvania for 1885/p. 294. 



fStevenson: Seeond Geol. Survey of Pennsylvania; Geology of Bedford and Fulton Counties, 

 1882, p. G5. 



X I. C. White : Proc. Amer. Phil. Society, vol. 19, 1881, p. 445. 



§The writer long ago became convinced thai he should have placed the coal beds underlying 

 the Pottsviile conglomerate of southwest Pennsylvania with the Pottsviile instead of in the lower 

 Carboniferous. This correction he made in Am. Jour. Sei., vol. xxxiv, 1887, p. 37. 



II These figures are taken from Ashburner in the Ann. Rep. for 1885, 



K Geology of Bedford and Fulton Counties, p. 60. 



** 1. C. White : Proc. Amer. Phil. Society, vol. 19, p. 445. 



