Geology of the Virginias. 195 



belongs to the Salina (6), additional explorations are required 

 to settle the question definitely. 



Lower Helderberg, VI, R. (7), and Oriskany, VII, R. (8). — In 

 the recent revision of Professor Dana's "Table of Geological 

 Formations, 1885," we find that he makes the Helderberg the 

 topmost period of the Silurian age, and makes the Oriskany 

 the base of the Devonian. Whatever may be the chronological 

 relations of these two formations, we find them in Virginia to 

 be entirely conformable, to be coextensive with the Appa- 

 lachian range, to blend considerably into each other in some 

 places — the limestones of the former becoming very siliceous 

 in its upper beds, while the sandstones of the latter are some- 

 times, though less frequently, calcareous where they come in 

 contact with the underlying limestone beds. While the organic 

 remains of the Helderberg bear close relations to those of the 

 preceding Salina and Niagara periods, they seem to be just as 

 nearly related to those found in the Oriskany, though much 

 more abundant. In Virginia the relations of the Oriskany 

 fossils to any remains found in the Devonian seem to be rather 

 remote. Therefore we are still inclined to hold that "the Oris- 

 kany sandstone strata are the passage-beds between the Silurian 

 and Devonian," but truly Silurian. This formation is charac- 

 terized in Virginia by extensive beds of limonite ores. 



We are here treating these groups in conjunction because in 

 our mountain ranges they are most intimately associated. The 

 one rarely, if ever, appears without the other; and where their 

 beds have not been ruptured by contortions and foldings — that 

 is, where the undulations have been such as to give the strata 

 moderate dips — the durable sandstone of the Oriskany is left 

 undecayed in many localities to form the crowning beds of 

 mountain ridges of considerable height, and thus protects 

 against erosion the less durable beds of Helderberg limestone 

 beneath.* As a rule, however, where the sandstones of the 

 Niagara period have been thrust up to a considerable height, 

 the Helderberg and Oriskany strata have been so much rup- 

 tured that their fragments have been removed from the higher 

 parts of the ridges by denuding agencies, and their outcropping 

 edges alone are now to be found low down on the slopes and 

 spurs. The Helderberg limestones being more easily disinte- 

 grated than the Oriskany sandstones, are often hidden from 

 vfew, but their position is still indicated by a depression 

 between the sandstone and the main ridge, forming a well- 

 defined "bench" on each spur, while in the adjacent ravines, 



* "We have a striking and very interesting illustration of this arched structure at 

 the celebrated Blowing Cave in Bath county, three miles west of Millboro Station 

 on the C. & 0. Railroad. By a singular oversight Professor Rogers locates the 

 cave in the Oriskany instead of the Helderberg. See this Journal, vol. xviii, pp. 

 121 and 125 (1879). 



