298 THE CAMBRIAN. [bull. 81. 



Prof. J. L. Campbell concludes, from the structure, that this portion 

 of the Blue Ridge has been formerly spanned by a grand arch or series 

 of arches of Cambrian strata, upturned, perhaps, at the time of their 

 upheaval. ! 



In 1885 Mr. H. I). Campbell published a more detailed section of the 

 Blue Ridge at Balcony Falls, to illustrate the view that the Blue Ridge 

 at this point is a portion of an anticlinal fold, over which the Cambrian 

 strata formerly extended from the west to the east, forming a synclinal 

 basin on the western side of the Blue Ridge. He described the Cam- 

 brian section at Balcony Falls as follows : 



Feet. 



1. Conglomerate 120 



2. Sandstone 360 



3. Slates 500 



4. Hard sandstone 150 



5. Slates 700 



6. Sandstone 90 



7. Slates v 120 



8. Scolithus sandstone ,.. 350 



Total 2,390 



He further states that, away from the river some distance, there is 

 about 600 feet more of sandstone and friable slate before reaching the 

 limit of the Potsdam formation. 2 This gives a greater thickness to the 

 Cambrian section of Balcony Falls than that measured by either Prof. 

 J. L. Campbell or Prof. Fontaine. 



The next complete description of the Cambrian in the southwestern 

 portion of the State is that of Prof. J. J. Stevenson, in his account of 

 a reconuaisance of Bland, Gyles, Wythe, and portions of Pulaski and 

 Montgomery Counties. On the map accompanying the paper, the geo- 

 graphic distribution of the Cambrian rocks is shown for Montgomery, 

 Pulaski, and Wythe Counties. He includes in the Cambrian the lower 

 Knox shales and the Potsdam, stating that the former are probably 

 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. 3 The Knox shales are described as being 

 reddish, sometimes streaked with white color, usually more or less 

 greasy, often talcose-looking on the slipped surfaces. The shales are 

 hard enough to form bluffs. The thickness could not well be ascer- 

 tained, but it can not be less than 600 feet. 



The Potsdam forms the great mass of Lick Mountain, is the sandstone of Draper 

 Mountain, and is found along the southern border of Wythe, Pulaski and Montgom- 

 ery Counties. The upper beds are alternations of saudstones and shales. 



1 Geology of the Blue Ridge near Balcony Falls, Virginia ; a modified view. Am. Jour. Sci., 3d ser., 

 1884, vol. 28, p. 223. 



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

 vol 29, 1885, p. 472. 



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

 Counties of Virginia. Am. PhiL Soc. Proc, vol. 24, 1887, pp. 86, 87. 



