walcott.] SUMMARY ALABAMA. 307 



It [Knox sandstone] would naturally be looked for overlying directly the Potsdam 

 sandstone of the mountains enumerated in a previous section ; but it is not always 

 easy to distinguish it ; since, however, these ridges of Potsdam saudstone have a well 

 defined belt of Knox shales on their eastern and southeastern flanks, the Knox sand- 

 stone, probably in most cases, intervenes between the two. 1 



It will be noticed in the description given of the Knox sandstone 

 that its direct superposition upon the massive "Potsdam" sandstone 

 has not been observed. I think that as in Tennessee the Knox saud- 

 stone may be, in part, the equivalent of the sandstone reterred to the 

 Potsdam or the Chilhowee sandstone. It is probably the accumula- 

 tion of sand and shale deposited while the sediments of the more 

 massive sandstones were accumulating nearer the coast line. 



The Knox shales rest conformably upon the Knox sandstone series 

 and correspond in their description closely with the Knox shales of 

 Tennessee. Of these beds Prof. Smith says: 



The characteristic rocks of this subdivision of the Knox group are calcareous shales 

 of bright and agreeable colors, usually gray, buff, greenish, brown, chestnut colored, 

 and red. The shales are tolerably soft, aud in some portions in weathering, break up 

 into small angular pieces resembling shoe pegs. Strata of dark blue limestoue, some- 

 times banded with argillaceous layers, are found, especially in the upper part of this 

 division. The weathering of such limestones brings into relief the bands or stripes 

 of argillaceous matter, and the limestone appears very distinctly banded. Where 

 these impurities are not so regularly disposed in layers, but in patches, the promi- 

 nence given to them by the weathering away of the limestone gives them a striking 

 resemblance to half-exposed fossils. 



In some places layers of dark colored oolitic limestone have been observed, one of 

 the best localities of this peculiar rock being at the foot of the mountain at Alpine 

 Station in Talladega County. 



In the upper part of the division blue limestone layers become more frequent, and 

 the transition into the overlying Dolomite is so gradual that a line between them 

 lithologically is hard to draw. 2 



The only instance mentioned to indicate the relations of the so-called 

 Potsdam sandstone to the Knox shales is in the following paragraph : 



Between the Ladiga Mountain and the hills of Acadian slate in Calhoun County, 

 there are numerous small ridges of Potsdam sandstone, on the flanks of which the 

 Knox shales seem never to be wanting. 3 



No fossiliferous strata were observed in this division. 



The distribution of the Cambrian rocks of Alabama is shown on a 

 map accompanying Prof. E. A. Smith's outline of the geology, published 

 in 1878. On this map the geographic distribution of both the Cambrian 

 and Lower Ordovician rocks is delineated under one color called the 

 Silurian. It includes all the formations from the Clinton down to the 

 Acadian. Of these formations the Cambrian includes, at the summit, 

 the Quebec or Knox shale and Knox dolomite, or at least the lower 

 portion of it, and subjacent to this the Calciferous or Kuox saudstone, 

 beneath which the Potsdam and Acadian are placed, the latter forming 

 the base of the Paleozoic series. 4 



1 Op. cit., pp. 17, 18. 2 Op. cit., p. 19. 3 Op. cit., p. 20. 



4 0ntline of the geology of Alabama. Berney's Handbook of Alabama, 1878, p. 139. 



