134 THE CAMBRIAN. [bull. 81. 



The description of formation No. I in the third annual report is as 

 follows : 



■ The lowest of the Appalachian rocks consist for the most part of a close-grained 

 white or light gray sandstone, in some places containing beds of a rather coarse con- 

 glomerate of white silicious pebbles. Near the bottom in many instances a brown- 

 ish slaty sandstone occurs alternating with the former, while towards the top or 

 approaching the next formation, the sandstone passes into reddish and browuish 

 and olive-colored argillaceous slates. 



This formation in Virginia is exclusively confined to the western slope of the 

 Blue Ridge and the narrow belt of rugged hills and mountains extending thence to 

 the commencement of the valley limestone. 1 



In a paper on "The Lower Silurian brown hematite beds of America," 

 Mr. B. S. Lyman states that all these beds seem to lie within the Vir- 

 ginia and Pennsylvania geological formation No. I, wholly below the 

 Oalciferous Sandrock of No. II. 2 



On the map accompanying the reprint of the annual reports and 

 other papers on the geology of the Virginias, published in 1884, Prof. 

 Eogers has included under one color, formations I to III or the Primal, 

 Auroral and Matinal series. On the large series of sections, however, 

 the three are differentiated and No. I or the Primal series is shown in 

 sections 1 to 19, and also in 89. 



Prof. W. M. Fontaine has added many details to the section of the 

 Primal series described by Prof. Eogers, at Balcony Falls in Rockbridge 

 County. The sketch of the Balcony Falls section 3 is supplemented by 

 that of Rockfish Gap and Harper's Ferry. 



For a further notice of the results of Prof. Fontaine's work the reader 

 is referred to the description of the Cambrian rocks of Virginia. 



In a note upon the Potsdam or Primal group of Virginia, prepared for 

 Macfarlane's American Geological Railway Guide, Prof. W. B. Rogers 

 puts as equivalent formations No. I of the Pennsylvania and Virginia 

 surveys or Primal of the annual reports and Potsdam group. The 

 latter is used in the nomenclature of the rocks of Virginia and West 

 Virginia. In the note explaining the Potsdam group, he says: 



The Potsdam, or Primal group, includes in Virginia, where complete, besides the 

 Potsdam proper, the ferriferous shales next above, and the slates, shaly grits, and con- 

 glomerates, below this formation. It is exposed in varying mass and completeness 

 on the western slope and in the west flanking hills of the Blue Ridge throughout 

 much of its length, often, by inversion, dipping to the southeast, in seeming con- 

 formity beneath the older rocks of the Blue Ridge, but often, also resting unconform- 

 ably upon or against them. These older rocks, comprising masses referable probably 

 to Huronian and Laurentian age, include also a group of highly altered beds, corre- 

 sponding apparently to the copper-bearing or Keweenian series of northern Michigan, 

 and perhaps to the lately described Dimetian rocks of Wales. 4 



•Report of the progress of the geological survey of Virginia, for 1838 (Richmond, 1839), p 6. A re- 

 print of the annual reports and other papers, on the geology of the Virginias. New York, 1884, 

 pp. 197, 198. 



2 On the Lower Silurian brown hematite beds of America. Am. Assoc, Proa, vol. 16, 1867, p. 114. 



8 On the primordial strata of Virginia. Am. Jour. Sci., 3d ser., vol. 9, 1875, pp, 363-369. 



•Rogers, W. B. Virginia and West Virginia. [Geological formations.] Macfarlane's Am. GooL 

 R. R. Guide, 1879, p. 181. 



