820 FLORAL ZONES OF THE POTTSVILLE FORMATION. 



connection it should be recollected that the flora of the lo\v(>r portion 

 of our Lower Coal Measures (Allegheny series), as, for example, at 

 Mazon Creek. Illinois, or Henry County. Missouri, is gvnerally recog- 

 nized as referal:>le to the Middle Coal Measures, or as near the base of 

 the Upper Coal Measures of Europe. 



RELATIVE HORIZONS OF THE RASAL BEDS OF THE THIN SEC- 

 TIONS AS COMPARED TO THE THICK EASTERN SECTIONS. 



So far as the examination of the phytiferous beds in the Pottsville 

 formation has (extended, it appears that only the lowest beds of the 

 formation in the Virginia region, where the formation attains a thick- 

 ness of perhaps 2,500 feet, contain plants of greater antiquity than 

 those of the lower beds in the Lower Lykens division of the type sec- 

 tion. It is even possible that the oldest plants in the basal portion of 

 the formation of Virginia are not of earlier date than those in the top- 

 most red shale at the Pottsville and "Westwood gaps. Further study 

 is needed to determine this question. The oldest plants that have yet 

 been found in the Lookout sandstone seem to be of later age than the 

 Pocahontas coal, and to be referable to the Mariopteris jwUsvillea 

 zone. Beds possibly referal)le to the same zone appear to lie close to 

 the red shale in the Pickens formation, in the Buckhannon quad- 

 rangle, in central West Virginia; and the lowest beds of the Potts- 

 ville, which is 270 feet in thickness,' at Hanging Rock, near Ironton. 

 on the Ohio Kiver, at the western margin of the Appalachian trough, 

 are perhaps referable to the uppermost part of the same zone. 

 North of these points the sections of the Pottsville formation seldom 

 exceed 400 feet, and throughout the bituminous sections of Pennsyl- 

 vania they are in most cases less than 300 feet in thickness, while in 

 some cases they are less than 150 feet. In none of these sections 

 have I yet found plants older than the upper beds of the Quinnimont 

 formation. No phytiferous ])eds so low as the Lykens coal No. 4, in 

 the M(irhj2Jteru 2)(>iti<v(U(<i zon(\ have yet come to light. On the other 

 hand, the flora of the Sharon coal, which, along the northern mar- 

 gin of the Appalachian trough, is separated from the marine Lower 

 Carboniferous by a conglomerate, and which at a few points seems to 

 rest directly upon the Shenango shales, is, as has already been pointed 

 out, accompanied ))y the Sewanee flora. The base of the Pottsville at 

 Hernice, Sullivan County, Peimsylvania, where the formation is prob 

 a))ly less than 100 feet thick, is apparently higher than the Sharon 

 coal, while the lowest coal, Avithin 20 feet of the red shale in the 

 Mehoopany Basin in Wyoming Coimty, Pennsylvania, is clearly 

 within the Sewanee zone. 



The measurements of the Pottsville at Campbell Ledge, in the 



' 1. C. Wliite, Bull. U. S. Geol. Survey No. as. p. YX\. 



