THE SANDSTONES ARE SILURIAN. 163 



stacles to unity that he found, it would be extremely hazardous in us to state 

 that all the Blue ridge sandstones are upper Silurian. It is perfectly reason- 

 able that Cambrian sandstones exist in this topographic line. They exist in 

 Tennessee of various types, and with equal certainty Silurian sandstones are 

 almost side by side with them. In Tennessee they have been confounded ; 

 why not in Virginia ? To establish their existence here, no single section, 

 much less a single sequence of the beds, will suffice. Nothing in Appa- 

 lachian geology is less certain than a single sequence. A profound fault 

 may cut it without a trace ; the beds may be turned upside down ; an inter- 

 val of erosion or of non-deposition may intervene and no record be left. 

 Each of these is of frequent occurrence, and each may produce the same sec- 

 tion. In detailed work along the outcrop lies the only structural proof. 



In this region above most others nature has concentrated the means of 

 proof, and by frequently repeating the phenomena has clarified and empha- 

 sized them. The responsibility thrown on a single section has been dimin- 

 ished until error is practically eliminated. 



Summary. 



To sum up, there is no a priori reason to call these sandstones either Pots- 

 dam or Silurian. 



The series lies in synclines above the Shenandoah limestone. 



The beds grade from one to another and are conformable by dip. 



The series is the same along the strike throughout the area. Apparently 

 this is their original position as sediment, and they are Silurian. It' this po- 

 sition was acquired after deposition, it could only be by means of a remark- 

 able fault with none pf a fault's characters and nothing to suggest its 

 existence. 



In Tennessee there are both Cambrian and Silurian sandstones in prox- 

 imity. In Virginia the facts are discordant in different areas and probably 

 there are two sandstone horizons. 



Here the net of facts is close and all point to one conclusion : our conten- 

 tion simply is — here there are upper Silurian sandstones. 



Washington, D. C, December 28, 1890. 



DISCUSSION. 



Mr. C. D. Walcott : In the Adirondack region of northwestern New York 

 the Potsdam sandstone rests on pre-Cambrian rocks and is succeeded by the 

 Calciferous, Trenton and Hudson strata. On the southwest the Trenton 

 limestone is in contact, by overlap, with the pre-Cambrian rocks, not having 

 been removed by erosion ; and above it we find to the westward the Lorraiu§ 



XXIV— Bull. Geol. Soc, Aivj., Vol. 2, 1890, 



