296 THE CAMBRIAN. [bull. 81. 



flaggy and gray in color; some reddish and brownish beds, however, are found. This 

 change to a more siliceous nature is accompanied by a great increase in the accom- 

 panying quartzites. These strata everywhere, owing to their comparatively greater 

 capacity for yielding to strains, have suffered much from contortions and minor 

 faults. 



(4) Upper Gray Shales and Flags. — This group, as the name implies, is somewhat 

 like No. 2, but in it the conglomerates and specular ledges are wanting and the 

 amount of quartzite is usually much less. Owing to the brittleness of the material 

 of the beds and the ease with which many of them yield to crushing, portions of the 

 strata are often found crushed to loose fragments. In the fractured and crushed por- 

 tions important deposits of limoniteare sometimes found that are evidently produced 

 by the concentrating action of percolating waters. The reddish and brownish rocks 

 found in No. 2 are also wanting here. The predominant rocks are shales and flags, 

 usually all highly argillaceous, and when fresh of a gray or greenish gray color, but 

 on weathering apt to assume a grayish white color. Towards the top quartzite ledges 

 are interstratified with these and cause a gradual passage into the Potsdam quartz- 

 ite. In some places the amount of quartzite here is very large and should properly 

 he counted as a portion of the Potsdam member of the Primordial. Perhaps the pre- 

 dominant rock is a gray shale that resembles a hardened mudstone and which grad- 

 uates into a sandy shale. This rock often decays to a pure white clay, and no doubt 

 furnishes the material for the pure potter's clay sometimes found immediately west 

 of the Blue Ridge. These strata for the sake of distinction may be called kaolin 

 shales or flags. 



(5) Potsdam Quartzite. — The members of the Primordial below this, so far as ob- 

 served by us, show no indications of the former existence of life. This portion of the 

 formation, however, shows in some beds numerous casts of Scolithus borings, some 

 of which are remarkably long, being visible for at least 3 feet. The brothers Rogers 

 considered this member as identical with the Potsdam sandstone of New York. This 

 portion of the Primordial is more constant in lithological character than the subdi- 

 visions that underlie it. Quartzite is always the predominant rock in it, the remain- 

 ing portions being mostly composed of the kaolin shales and flags. To the south, 

 and especially in the interval from Mount Torrey to Buena Vista furnace, the quartz- 

 ite is very massive and siliceous, composing nearly all of the rock at this horizon. 

 The material is more properly called a quartzite than a sandstone. The upper and 

 lower portions are often flaggy, and cause a passage into the underlying and overly- 

 ing shales and flags. So far as observed, these quartzitea are never conglomeratic, 

 and they are free from the infiltrations of silica, the diffused chlorite, and other char- 

 acters that mark the lower quartzites. Near the junction of this member with No. 

 4, the strata are often crushed, the crushed band holding important deposits of a 

 dark limonite. The Quartzite members of this subdivision are sometimes broken up 

 and crushed, forming a curious band of breccia, cemented by iron or manganese. 

 Sometimes workable deposits of limonite are found in these disturbed portions. 



(6) Ferriferous sliales. — It might be a question whether these shales ought not rather 

 to be counted with the Calciferous group. It is certain that there seems to be a 

 gradual passage from the shales into the pure Magnesian and Siliceous limestones, 

 that correspond to the Calciferous sandstones of New York. No fossils are found to 

 settle the matter, and it seems best to draw the dividing line at the first marked 

 change in the lithological character of the strata. This change occurs with the low- 

 est limestone beds, that usually occur interstratified with reddish and yellowish 

 shales. 



The Ferriferous shale group is noteworthy for containing throughout it deposits of 

 limonite which are often suited for the manufacture of a neutral iron. These depos- 

 its seem to be of concretionary origin, formed by the decay of the shales which con- 

 tain the iron in a diffused form, probably, for the most part, as a carbonate, but 

 sometimes as pyrite. These ores are consequently usually imbedded in clay. They 



