294 THE CAMBRIAN. [bult,.81. 



Falls section, showing that the proportion of shaly matter in the entire 



group at Rockfish Gap has greatly increased. 1 



In summing up the physical characters of the sections as described, 



he says : 



It will be seen from the above notes that in Virginia we have below the calciferous 

 limestones a great development of sandstones, shales, and conglomerates, which 

 attain in the middle portion of the State a thickness of over 2,000 feet, and increase 

 in the proportion of coarse materials to the southwest. They probably attain greater 

 thickness in that quarter, while to the northeast the amount of sediment diminishes, 

 and the proportion of tine matter increases. This change is plainly due to the 

 increasing development to the southwest of the syenitic rocks which formed the shores 

 of the ancient seas and to the greater violence in that direction of disturbing forces. 

 The Potsdam sandstone forms one of the upper members of this group. Much further 

 study of these strata is required to settle the question whether the eDtire series is a 

 great expansion of the Potsdam or whether divisions may be made corresponding to 

 other epochs. The fact that at Rockfish Gap, and to the southwest, a great body of 

 ferruginous slaty shales separates the lower highly siliceous and altered, sandstones 

 from the upper Kaolin sandstones of probable Potsdam age, seems to indicate a change 

 in the conditions of sedimentation sufficient to justify such a division, in which the 

 Acadian strata may be found. 3 



With the single exception of a Scolithus, no fossils have been found 

 in these rocks which would indicate their age. 



In 1883 Prof. Fontaine supplemented his detailed description of the 

 sections by a synoptic description, embracing the general character of 

 all the formations that he considered belonging to the Primordial series. 3 

 Beginning with the base of the Primordial he recognized six divisions 

 as follows : 



Feet. 



1. Lower Primordial conglomerate to 30 



2. Lower gray shales and flags 500 



3. Red shales and flags 400 



4. Upper gray shales and flags 600 



5. Potsdam quartzite 350 



6. Ferriferous shales 500 



2,380 



The description of the various formations is so clear and full that I 

 quote them : 



(1) Lower Primordial conglomerate. — I would confine this appellation to the lowest 

 conglomerate that forms the base of the Primordial where it is present, excluding the 

 conglomerate bands that follow higher up, but are separated from the basal conglom- 

 erate by shale and slate. * * » This member, thus limited, I have not seen at all 

 the places where the junction of the Primordial with the Azoic is exposed. * * 



The conglomerate now in question is of special importance, since, when it is found, 

 we may be sure that we have the true base of the Primordial. I have never seen it 

 without finding the Azoic rocks immediately under it. It consists of a finer matrix 

 that is shaly or slaty in texture, in which particles, angular or rounded in shape, 

 of the Azoic strata are imbedded. The lithological character of tbe pebbles varies 



1 Op. oit. f p. 421. 

 8 0p.cit.,p.424. 



■ Notes on the mineral deposits at certain localities on the western part of the Blue Ridge. The 

 Virginias, vol. 4, 1883, pp. 22, 42-44. 



