Stevenson.] ido [Nov. 21, 



contorted that the true thickness can be ascertained only after detailed 

 examination. This part of the Medina is very closely allied to the Cambro- 

 Silurian. It contains Ambonychia radiata and Rhynchonella capax in 

 Southern Pennsylvania ; it contains the same forms, with others equally 

 characteristic of the Hudson, in Southwest Virginia. These fossils can be 

 collected in Lyons gap on the northerly side of Big Walker mountain in 

 Smyth county and in Hayter'a gap on the north side of Clinch mountain 

 in Kussell county. In each case the exposure is conspicuous, being at the 

 roadside and well known to the people living in the neighborhood. 



The Cambro- Silurian or Lower Silurian. 



This is represented by Hudson, Trenton and Knox or Calciferous, the 

 Utica being absent or so changed that it cannot be separated from the 

 Hudson or Trenton. Rocks of the Trenton and Knox are the surface beds 

 in the Great valley as well as in the broad area between Clinch mountain 

 and the Clinch series of faults. Hudson beds occur at the northerly foot 

 of Big Walker and Clinch mountains and outcrop around the synclinal 

 mountains with Medina crest, which are seen in Tazewell county north- 

 ward from Clinch mountain. 



The Hudson consists of red to yellow sandy shales and the passage to 

 lower Medina is wholly imperceptible. The yellow shales below become 

 calcareous and the passage to the Trenton is equally gradual. The upper 

 beds of the latter group are very argillaceous, but the limestones become 

 much purer lower down in the column, until the marbles are reached near 

 the bottom of the group. These are thoroughly characteristic. Toward 

 the bottom of the marbles some massive limestones occur containing much 

 black chert ; those mark the passage to the silicious Knox group in which 

 are several beds of white chert and many thick beds of very hard and 

 slightly calcareous sandstones. This is merely the Calciferous of New 

 York, vastly increased in thickness. 



No detailed section of any portion of the Cambro Silurian was attempted, 

 but enough was observed to show that the writer's estimate of 3350 feet 

 for the thickness of the Knox group is materially below the truth. 



III. THE AREA DRAINED BY THE HOLSTON RIVER. 



In going from Bristol northward toward Mendota, one rides on Trenton 

 and Knox beds until the Saltville fault is reached at a little way beyond 

 the Rich Valley road. The Walker Mountain fault is crossed at about four 

 miles north from Bristol, not far from the Reedy Creek road. The; red, more 

 or less calcareous shales, forming the top of the Lower Carboniferous and 

 dipping sharply toward the south-south-east, are exposed in the railroad 

 cut at Wolf Run summit just beyond the Saltville fault. A well-marked 

 anticlinal is shown on Wolf run at a few rods above the school-house ; the 

 place of the synclinal was not determined, as exposures are somewhat in- 

 definite below Whetstone run, but long before the river has been reached 



