1887.] ^^ [Stevenson. 



acteristic of the rock along an outcrop of fully 400 miles. Some brown 

 or reddish-brown layers occur in this division, but their thickness was not 

 ascertained. Aside from ArthropJiycus liarlani and a 8colithus-\\\<.Q form, 

 no fossils occur in the sandstone. 



The Lower Medina, composed of reddish sandstones and shales, is the 

 imperfect terrace on the northerly side of the mountains, and, as was 

 stated in a previous memoir, this is clearly the same with the Terrace 

 group of South-central Pennsylvania, which has been identified by Lesley, 

 and therefore by the writer in his Pennsylvania report, as the Red Medina 

 of New York. The thickness cannot be given with any degree of cer- 

 tainty, as the rock passes downward without break into the Hudson ; or- 

 dinarily it is much disturbed. The best exposure in detail is on the Taze- 

 well and "Wythe pike as that road descends the northerly side of Big 

 Walker mountain toward Sharon Springs. There, within 100 feet verti- 

 cally below the White Medina, is a fossiliferous bed, six to eight feet thick, 

 containing Bhynchonella, OrtJiis, Modiolopsis, Avieula emacerata, Ambo- 

 nyeliia rciMnta and fragments of Orthoceras, which were collected during 

 an examination lasting but a few minutes. 



The relation to the Lower Silurian, therefore, is intimate, so intimate 

 that the writer is more tlian ever convinced that there is room for doubt- 

 ing the accuracy of any identification which makes the Terrace group of 

 Southern Pennsylvania the equivalent of Medina in New York. 



The Lower Silurian. 



The Lower Silurian is represented by Hudson, Trenton and Knox lime- 

 stone. Hudson beds occur only on the steeper slopes of the Medina 

 mountains ; Trenton is at the foot of such mountains, but within the 

 counties described in this memoir, is rarely seen in the broader limestone 

 valleys. The Knox limestone is the surface group in the "Rich valley" 

 of Bland and Giles ; on both sides of New river in Giles* county ; and is 

 the great limestone group cropping out over so macli of the " Great val- 

 ley " in Wythe, Pulaski and Montgomery counties. 



The Hudson consists of red to yellow sandy shales, which pass with 

 equal indefiniteness into the Red Medina (?) above and into the calcareous 

 shales of the Trenton below. The Trenton beds are calcareous shales 

 with streaks of impure limestone passing downwards into pure massive 

 limestones, some of wliich would be valuable as marbles. The lower beds 

 become more or less silicious and show embedded nodules of chert. 



These cherty beds afford transition to the Knox limestones, which are 

 very cherty in the upper portions. The lower portions contain much shale 

 with silicious limestones, and near the base is a curious limestone, some- 

 what silicious, which is veined with white spar and contains great blotches 

 of the same material. This is a notable bed, which is recognized in many 

 places where it promptly affords a key to the puzzles. Its presence on 

 New river and at several other localities in Pulaski county shows the rela- 

 tions of the rocks on both sides of the Walker and Pulaski faults, and 



