306 THE CAMBRIAN. 



| HULL. 81. 



quite massive, and made up of lumps of pebbles of opalescent quartz 

 and opaque white feldspar, held in a matrix of greenish talcoid matter. 

 Of the second division, or the Potsdam, it is said : 



Next in ascending order, to the slates and conglomerates of the preceding group, 

 comes a series of sandstones which form the most conspicuous mouutaius outside the 

 Metamorphic and Acadian areas. The direct superposition of this sandstone upon 

 the slates of the Acadian age, I have not seen in Alabama, unless on the eastern edge 

 of that belt, the prominent sandstone or quartzite ridge be, as seems quite probable, 

 metamorphosed Potsdam sandstone. In the disturbed region of the Kahatchee Hills, 

 also, half metamorphosed slates are seen, some of which may belong to the Acadian 

 epoch, whilst others are evidently altered Knox shales. 



Fine-grained conglomerates, heavy-bedded sandstones, and sandy shales make up 

 the great mass of the rocks of this formation. I have noticed, also, occasionally, 

 masses of a brownish porous chert, which, from its association with the sandstones, 

 seems to be of the same age. 



In general, the rocks of this formation are heavy bedded, almost massive, and the 

 higher crests of the Potsdam Mountains are usually covered with huge blocks of 

 sandstone and fine-grained conglomerate. In the lower parts of these ranges, sandy 

 shales are the prevailing rocks. * * * The most characteristic markings of the 

 rocks of this formatiou are the sandy rods, caused by the filling in with sand of the 

 burrows of a marine worm, Scolithus linearis. Upon the bedding planes of the sand- 

 stones, small rounded depressions or dots mark the cross-sections of these Scolithus 

 burrows. As yet I have found no other marks of organic origin in strata of this 

 horizon. * * * 



The nature of the rocks of this group, conglomerates, sandstones, and sandy shales, 

 the fossil markings, principally the burrows of marine worms, etc., all point to a sea- 

 shore origin. 1 



From the preceding descriptions, the equivalency of the Chilhowee 

 sandstone and quartzite with that of Alabama is extremely probable, 

 if not certain. 



Of the succeeding formation, the Knox sandstone, the following de- 

 scription is given : 



The rocks of this group succeed next, in ascending order, the Potsdam sandstone, 

 and they are often, no doubt, found resting directly upon that rock. I have not, 

 however, often seen them in this position ; but much oitener just on the southeast side 

 of a fault by which they have been raised to the level of a much higher formation. 

 This sandstone is more generally associated with the shales of the next higher group, 

 and these with dolomite, so that three groups are usually closely associated geograph- 

 ically as well as liihologically. 



A calcareous saudstone, sometimes thick, sometimes thin bedded, is the character- 

 istic Kuox sandstone. 



It is associated with hard calcareous shales much like the shales of the next higher 

 division. The bedding planes of the sandstone commonly show ripple marks and 

 irregular raised markings, which are commonly supposed to be fucoidal impressions. 

 The bedding planes are also frequently smooth and shining as if polished. Green 

 grains of glauconitic mineral are usually to be seen upon a fresh fracture of the sand- 

 stone ; upon weathered surfaces the brown color of hydrated ferric oxide is often 

 the result <ff the decomposition of this mineral. The colors of the Knox sandstone 

 are pleasing to the eye, and are gray, greenish brown, buff, chestnut colored, etc. 

 Beds of dolomite, impure and cherty, are found in the upper part of this formation; 

 two such calcareous beds were noticed in the exposure of Knox sandstone at the 

 Jackson shoals and also at Helena. * * * 



1 Op. cit., p. 13-15. 



