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



In comparing the Lower Carboniferous of southern Pennsyl- 

 vania with that of southwestern Virginia, the great difference 

 in conditions must not be overlooked. Owing to absence of 

 faults within the Cumberland Valley of Pennsylvania (the 

 same with the Great or Shenandoah Valley of Virginia) and to 

 the presence of great folds on the westerly side of that valley, 

 the first outlier of Lower Carboniferous rocks is on Licking 

 Creek Mountain in Fulton County 33 miles from the Ar- 

 chaean of South Mountain ; the next is on the west side of the 

 same county 8 or 9 miles farther west, and is separated by 

 an interval of 30 miles from the next exposure on the border 

 of Somerset county. 



But such is not the condition in southwest Virginia. The 

 Draper Mountain fault Hn Pulaski County shows Umbral and 

 Vespertine on the downthrow side at not more than 12 miles ' 

 from the Blue Ridge Archasan ; the Price Mountain faults in 

 Montgomery County show the same groups at not more than 

 15 miles from the Archaean ; the Walker Mountain fault 

 shows Umbral and Vespertine on its downthrow side from 

 Smyth County to many miles be} 7 oncl New River; while the 

 Saltville fault* shows the Lower Carboniferous from the Ten- 

 nessee line to the edge of Giles County. So there are good and 

 extensive exposures, giving satisfactory sections at 12, 15, 21 

 and 23 miles from the Archaean, while the outcrop on the side 

 of the great coal-field is reached at barely 15 miles farther 

 N.N.W. The line of the Saltville fault is very nearly equiv- 

 alent to that of the most easterly outcrops in southern Penn- 

 sylvania ; and for purposes of comparison, it will be taken as 

 such. 



The conditions observed in the easterty outliers within Penn- 

 sylvania do not change rapidly toward the south. Prof. W. 

 B. Rogers's assistants obtained the following measurements 

 near Westernport on the Potomac :f 



Umbral. 



Shale and sandstones 650' 



Limestone 4' 6" 



Shale and sandstone 186' 



Vespertine. 

 Siliceous limestone 79' 



Sandstone and conglomerate 200' 



No very noteworthy change in the character of the rocks 

 occurs until within 75 miles of the Tennessee line ; for Brushy 



"For descriptions of these faults, see the writer's paper in this Journal for 

 April, J 887. 



f Report on Geology of Virginia for 1839, p. 89 and 91. I have rearranged the 

 section so as to throw the Siliceous limestone into the Vespertine, the Formation 

 X of Prof. Rogers, instead of leaving it in his Formation XI, tbe Umbral. 



