136 THE CAMBRIAN. [bull. 8L 



In the sheet of sections accompanying "The Virginias " by Prof. Wil- 

 liam B. Rogers, printed in 1884, section 9 crosses the Primal series of 

 Rockbridge County, about 30 miles north of Balcony Falls. On the 

 eastern slope of the Blue Ridge a small synclinal basin of the Primal 

 series is represented as resting in a hollow in the gneiss; on the western 

 slope, the Primary series is shown as it occurs in numerous sections 

 crossing the Great Valley and the Blue Ridge. The editor of the sec- 

 tions, Mr. Jed. Hotchkiss, states in the note accompanying them that the 

 sections are "exact reproductions, in so far as the geology is concerned, 

 of the sections as Prof. Rogers left them." From this it is evident that 

 Prof. Rogers became aware of the presence of the Primal series on the 

 upper eastern slope of the Blue Ridge In Rockbridge County after the 

 publication of the third annual report in 1839, as he stated then that 

 the Primal rocks were confined to the western side of the ridge. 



Messrs. J. L. and H. D. Campbell, while studying the geological re- 

 lations of the Suowdon slate quarries in Amherst County, on the borders 

 of the counties of Rockbridge and Bedford, discovered the presence of 

 the Balcony Falls series of Cambrian rocks and found that the quartzite 

 passed beneath the slate of the quarries. 1 On the map accompanying 

 the paper (p. 170), an ideal section shows the shales and conglomerates 

 resting on the gneiss beneath the sandstone, which they have referred 

 to the Potsdam owing to its carrying Scolithus borings, and above the 

 latter is the slate belt, in which the quarries are located. Under the 

 title of "Geology of the Blue Ridge near Balcony Falls, Virginia; a 

 modified view," Prof. Campbell corrects the statement made in a former 

 paper, in the American Journal of Science in 1879, that the rocks of the 

 southeast slope are of Archean age. He adds a sentence as follows : 



We may conclude, therefore, that this portion of the Blue Ridge has heen formerly 

 spanned by a grand arch, or series of arches, of Cambrian age, upturned perhaps at 

 the time of their upheaval — the broken fragments of which have been carried away, 

 and only the abutments left to tell the story of a great catastrophe. 2 



In a letter on relation of Archean and associated formations in Vir- 

 ginia, Prof. W. M. Fontaine states that he finds in Virginia a valuable 

 guide to the true base of the Potsdam in the conglomerates. 



The lowest conglomerate is not always seen. It is often very coarse, with pebbles 

 sometimes 4 to 5 inches in diameter, and composed of Lanreutian or Huroniau ma- 

 terial, according to the nature of the underlying rock. The matrix is often shaly or 

 slaty. 



It is the conglomerates overlying this stratum that afford the best guide, for they 

 may always be seen. 



They are simply pebble beds in the slate or shale. The pebbles are from the size 

 of a musket bullet down, and usually of quartz, often pink in color. The pebbles 

 look as if they had been scattered over a muddy bottom, forming a very peculiar 

 conglomerate in which all the material except the pebbles is a fine slate or shale. 

 Sometimes some partially decayed feldspathic matter occurs with the pebbles. 



1 The Snowdon slate quarries. The Virginias, vol. 5, 1884, pp. 162, 163. 



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

 1884, vol. 28, pp. 222, 223. 



