EXTENT OF FOLDING AND SUBSEQUENT EROSION 45d 



Clinch Mountain is replaced northeastward by East River Mountain, 

 a pronounced monoclinal ridge, with strata dipping to the southeast and 

 the eroded edges facing the Carbonic Plateau, from which it is separated 

 by a narrow valley. Its crest forms the boundary line between Virginia 

 and West Virginia in this section. The strata have been in part thrust 

 up onto the younger beds, but on the whole little disturbance is noticeable 

 within the beds of the mountain. Southward from this are a number of 

 other ridges, among which is Big Walker Mountain, part of another 

 overthrust block of these Ordovicic-Siluric strata, which here also 'lip to 

 the southeast. This latter ridge may in general be considered the con- 

 tinuation of the Bays Mountain folds, bring up essentially the con- 

 tinuation of the strata of those mountains. 



The Bays of Big Walker Mountain, Virginia. — A visit to Stevensons 

 fossil locality on Big Walker Mountain enabled me to secure a goodly 

 number of fossils from the soft, red Bays sandstones on the north side of 

 the mountain, estimated at perhaps 100 feet below the Clinch. The 

 general thickness of the Bays in this region is estimated by Campbell 

 (Pocahontas folio) to be about 250 feet, which brings the fossil iferous 

 horizon near the middle of the formation, though some greenish gray 

 argillaceous beds are exposed in the road only a few feet below the fossil- 

 bearing strata. These may be the top of the Sevier, which would make 

 the thickness of the Bays formation much less and bring the fossiliferous 

 horizon near the base. The beds are largely shale, but carry occasional 

 heavy beds of sandstone, with a dip averaging 35° to the southeast. The 

 fossiliferous bed is about 10 feet thick and fossils are abundant. Mnsi 

 of them are either external or internal molds. 



PLATYSTROPHIA PONDEROSA VAR. STEVENSON! VAK. NOV. 

 Tlate 12, Figures 1, 2, 3 



The most abundant, and in some respects the mosl striking, species 

 represented is a large, extremely rotund and short-hinged form of Platy- 

 strophia lynx (^ponderosa Foerste). The Larger specimens measure 

 28 millimeters in height and 33 millimeters in width, with a hinge area 

 16 millimeters long. A typical specimen of P. ponderosa from Cin- 

 cinnati measures 29x37 millimeters, will) a hinge line 88 millimeters 

 long. The Walker Mountain specimens are very rotund, the sinus ap- 

 pearing first as a gentle depression somewhal less abrupl than in the 

 Cincinnati specimens. Anteriorly, however, if is quite as marked, and 

 generally car ries three plications, though occasionally a fourth appears. 

 A striking feature of nearly all adult shells is the great thickness on 



