wakjott.] SUMMARY — VIRGINIA. 297 



assume the form of lenticular masses, usually built up of concrotiouary foruis. Theso 

 ores are unlike those found in and under the Potsdam sandstone, and not rarely are 

 mingled with oxide of manganese or associated with it, forming ferro-manganese. In 

 the same shales, occurring under conditions similar to the iron and formed in like 

 manner, we find, not rarely, deposits of quite pure manganese oxide. This is always 

 in the form of Psilomelane or hard manganese. 



The lowest of the strata of this group have the character of kaolin shales and 

 flags. They graduate insensibly into the Potsdam quartzite member. In ascending 

 they become purely argillaceous shales, generally of lead, or bluish gray color, when 

 fresh. With these, moreover, occur strata yellowish-reddish in color, the red often 

 assuming a pinkish shade. Towards the top, ledges of pretty firm pure purplish 

 shales and flags are found that are sometimes of the nature of fine-grained sandstone. 

 The shales towards the top are often calcareous, and graduate into the impure lime- 

 stoue with which the Calciferous group may be assumed to begin. 



We thus have in the Primordial formation, in the belt now being described, at 

 least three horizons of iron ore. The lower strata contain specular iron ore usually 

 of low grade, the middle portions, extending as far as the Potsdam quartzite, con- 

 taining limonites, either impregnating definite strata, or filling what were once fis- 

 sured and crushed bands. Hence all of these usually appear in massive or rocky out- 

 crops. Finally, in the Ferriferous shales, we have interrupted deposits of limonite, 

 inclosed in clay and often mixed with manganese. (No. 12885, U. S. G. S. Library.) 1 



The Balcony Falls section has also been studied by J. L. and H. D. 

 Campbell with a slightly varying result. As measured by Prof. J. L. 

 Campbell the section is as follows : 



Feet. 



1. Conglomerates 50 



2. Sandstones 350 



3. Slates 600 



4. Sandstones (thickness not given). 



5. Slates 180 



6. Scolithus sandstone 340 



7. Shales GOO 



Total 2,120 



A comparison of this with the sections measured by Prof. Fontaine, 

 in 1879, shows essentially the same succession of strata, although dif- 

 fering materially in the thickness of the various divisions based on the 

 character of the rocks. Subsequently, Prof. Campbell added to the 

 preexi stent information of the section at Balcony Falls, by showing 

 that the primal series is present upon the southeastern slope of the 

 Blue Ridge, a short distance north of the James river. This was fol- 

 lowed by a paper on the Snowdon slate quarries, by himself and his 

 son, in which it is said that the sandstones are penetrated with Sco- 

 lithus and dip beneath the Snowdon roofing-slate quarries. 2 An ac- 

 companying ideal section, crossing the Blue Eidge 3 miles north of 

 James River Gap, represents the Primal series at the summit dipping 

 to the westward and on the eastern slope dipping at a high angle to 

 the eastward and resting unconformably upon the Archean gneiss. 3 



1 Op. cit, p. 43. 



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



» Op. cit., p. 170. 



