40 J. J. Stevenson — Lower Carboniferous groups of the 



TJmbral. 



Shales and sandstones with thin limestones S00' 



Limestones and calcareous. shales 1470' 



Limestones, cherty, with calcareous shales 655' 



Vespertine. 

 Concealed 40' 



In coming northward from Tennessee, one finds much diffi- 

 culty in attempting to draw a line between TJmbral and Ves- 

 pertine ; the writer in his memoirs on southwestern Virginia 

 used the classification adopted for Tennessee 'by Prof. Safford. 

 But there is no room for doubt to one coming southward ; for 

 the Pennsylvania Vespertine is easily recognized in the coal- 

 bearing series of Montgomery, Pulaski, Wythe, Bland and 

 Smyth Counties of Virginia. One can follow this group along 

 the side of the Saltville fault, as the thickness diminishes, until 

 the whole mass of not less than 700 feet has its representative 

 in the concealed interval of the Mendota section, in which a 

 thin bed of coal is said to exist. Whether or not any trace 

 of this group remains at the Tennessee border could not be de- 

 termined from the exposures. 



Northwestward, on lines of exposure bounding or within the 

 great coal field, equally notable variations exist. Near the 

 Pennsylvania border, the limestone of the TJmbral increases in 

 importance westward until under Chestnut ridge it is certainly 

 one third of the whole mass, which is not far from 200 feet 

 thick. The Vespertine, however, shows little change other 

 than in thickness. Southward from the Pennsylvania line, the 

 first measurement is that by Prof. I. C. White* in Monongalia 

 county of West Virginia : the section in the gap of Cheat River 

 through Chestnut ridge showing: 



TJmbral. 



Shales , 295' to 300' 



Limestone 95' to 120' 



Vespertine. 

 Sandstones with siliceous limestone at the top.. 125' 



As the fold of Chestnut ridge decreases rapidly southward, the 

 next measurement, also by Prof. White,t was obtained in the 

 gap made by the same river through Briery Mountain in Pres- 

 ton County of West Virginia, at about 30 miles southeast from 

 the other. This shows : 



* Proc. Amer. Phil. Soc, vol. xx. p. 484 to 438. 



f The Virginias, July, 1882. I have re-arranged the sections so as to place the 

 siliceous limestone, 105 feet, at the top of the Vespertine. 



