wAi-coTT.j VIRGINIA. 137 



These shaly conglomerates, as we may call them, might sometimes he mistaken for 

 certain amygdaloids of the Uuronian when the latter are weathered, hnt careful ex- 

 amination will always detect the difference. I have never seen any shaly conglom- 

 erate in the Huron ian. 



I am now of the opinion that the conglomerates at the railroad hridge at Harper's 

 Ferry are these lower Potsdam strata, hut would require a reexamination of them 

 before I make up my mind positively. I am also inclined to think that Frazer's 

 Mountain Creek conglomerate is the same. 1 



During the year 1885 Prof. H. D. Campbell published a more detailed 

 account of the discovery of the Potsdam group east of the Blue Ridge^ 

 A resume of the section is given as it occurs on the northwestern slope 

 of the Blue Ridge at Balcony Falls for the purpose of comparing the 

 rocks as found upon the southeastern slope. On the eastern slope the 

 strata lie in a basin of Archean rocks forming a synclinal, a section of 

 which practically reduplicates the lower portion of a section of the 

 " Potsdam group " of the western slope. A map and figure of the sec- 

 tion accompany the paper. 2 In a second article in connection with his 

 father, the Cambrian-Primordial (No. 1 of Rogers) is described in con- 

 siderable detail, 3 and mention is made of the discovery of another area 

 referred to the " Potsdam group " on the eastern slope of the Blue 

 Ridge, at Tye River gap, in Nelson County. 



Prof. J. J. Stevenson identifies the Cambrian rocks, crossing Mont- 

 gomery, Pulaski, and Wythe Counties, as the Lower Knox shales and 

 the Potsdam of the Tennessee section. Under the title u Cambrian" 

 he says : 



Here are placed the Lower Knox shales and the Potsdam. The former are proh- 

 ahly equivalent to the Hydromica schists of Pennsylvania and the lower part of the 

 Calciferous of New York; the latter is the Potsdam of New York, vastly increased in 

 thickness. 



No fossils were observed in the Knox or the Potsdam shale with the 

 exception of Scolithus linearis. The geographic distribution of the for- 

 mations referred to the Cambrian is shown on a map accompanying 

 the paper. Further reference will be made to Prof. Stevenson's paper 

 in describing the Cambrian rocks of Virginia. 4 



In a paper read before the Geological Society of America December 

 31, 1890, by Messrs. H. R. Geiger and Arthur Keith, it is stated that 

 the shales and quartzite at Harper's Ferry represent the Hudson and 

 Medina series and not the Primal series as stated by Profs. Rogers and 

 Fontaine. They present a number of sections showing the stratigraphic 

 position of the beds. In these the normal succession is limestone, 

 shales, and sandstone. The shales succeed the limestones, and overlap 



'(Letter on relations of Archean and associated formations in Virginia.) G-eol. Chester Co ,Pa., 

 2d Geol. Surv., C4, 1883, p. xv. 



2 The Potsdam group east of the Blue Ridge at Balcony Falls, Virginia. Am Jour. Sci., 3d ser., 

 v ol.29, 1885, pp. 470-474. 



3 Review of William B. Rogers's Geology of the Virginias. Am. Jour. Sci., 3d ser., vol. 30, 1885, pp. 

 364-368. 



4 A geological reconnaissance of Bland, Giles, Wythe, and portions of Pulaski and Montgomery 

 Counties of Virginia. Am. Phil. Soc. Proc, vol. 24, 1887, pp. 8C-87. 



