wAucm.l SUMMARY — VIRGINIA. 293 



Feet, 

 The two last series of beds, 5 and 6, appear to be the equivalent of No. 

 (9) of the Balcony Falls section. Their combined thickness is about 1,450 

 feet, that is, provided no reduplication from folding occurs. This is 

 possible, though not probable, I think. 

 (7) "The strata now to be described are the equivalent of the Potsdam sand- 

 stone, and resemble No. (10) at Balcony Falls in their almost total free- 

 dom from iron and other coloring matters. * * * The system here 

 consists of a vast number of thin layers, in which we may distinguish 

 three classes of rock: (a) A shale pale gray to bluish gray when fresh, 

 composed almost entirely of kaolin, and occurring in thin plates. (&) 

 This is associated with a very fine grained kaolin sandstone, the quartz 

 grains being now perceptible to the naked eye. * * * (c) A kaolin 

 sandstone of moderately fine grain, often with a thickness of several 

 feet in the individual layers. The quartz grains are mixed with an equal 

 amount of kaolin , 300 



"The beds last described have a thickness of 300 feet, and are suc- 

 ceeded by a partially concealed interval, in which 50 feet of a similar 

 rock are shown." The strata between this point and the Auroral lime- 

 stone are largely concealed. 



The section as studied at Balcony Falls by Prof. Fontaine is as fol- 

 lows: 1 % 



Feet. 



(1) Unbedded quartzite, with crystals of feldspar 120 



(2) Brown, crumbling, argillaceous rocks or sandy shale, conglomerate in its 



upper portion 40 



(3) Brown, decomposing, thinly laminated and contorted shale 10 



(4) Conglomerate, like the upper portion of (2) 20 



(5) Shales, like (3; 12 



(6) Conglomerate, like (4) 15 



(7) Crumbling, brownish shales, passing in the upper portion into argillaceous 



sandstones of the same hue and texture 200 



(8) Massively bedded, coarse white quartzite (apparently Rogers's typical 



No. 1) 500 



Up to this point the rocks are well exposed in the cliffs which closely 

 border the canal. Proceeding west across the strike, we next encoun- 

 ter a series that forms No. (9). 



(9) Thinly laminated, gray and reddish shales. Probable thickness 600 



(10) Alternating beds of quartzite and kaolin shales. Probable thickness.... 700 



(11) The first rock seen next to the mountain is thinly laminated, fragile shale, 



of yellowish and reddish hues not fully exposed. Next, to the west, we 

 have bluish calcareous slate, and slaty limestone, 50 feet. Then very 

 thinly laminated, firm, deep red slates, 60 feet. Next a similar slate of 

 blue color, 15 feet. Then a bed of dark blue, hard, and dense limestone, 

 used for cement, 13 feet. Then thinly laminated red slates, 20 feet. 

 Then coarser and thicker bedded blue slates, 40 feet. Lastly, a coarse, 

 rough, massive, siliceous limestone, 40 feet 238 



2,455 



He compares the Balcony Falls section with that of Rockfish Gap, 

 stating that the thickness of Nos. 5 and 6 in the latter section is much 

 greater than that estimated for their equivalent, No. 9 in the Balcony 



l Am. Jour. Sci., 3d ser., vol. 9, 1875, pp. 363-365. 



