156 GEOLOGICAL SURVEY OF ALABAMA. 
This character of the Sub-Carboniferous chert, and the 
presence of the rhombohedral cavities in the chert of Knox 
Dolomite enable us in almost every case to distinguish be- 
tween the two. 
Now, in tke anticlinal valleys south of the Tennessee 
river we find itimpossible to carry out this two-fold division 
of the lower.or Siliceous member of the Sub-Carboniferous, 
for the entire member shows, upon the surface at least, little 
else than chert, which appears in a mantle of angular frag- 
ments, covering usually one side of all our Red Mountain 
ridges. a 
We have called this the Fort Payne Chert, and it is prob- 
ably the representative of both the subdivisions of the lower 
Sub-Carboniferous or Siliceous group, of North Alabama 
and Tennessee, as long ago conjectured by Prof. Safford. 
Its thickness is not very great as compared with that of the 
upper member. 
The Upper Calcareous member is variable in composition. 
In North Alabama it is chiefly a limestone called Mountain 
Limesione, from the fact that it forms the flanks of most of 
the mountains in that section that are capped with the Coal 
Measures. b 
Within this limestone there is interbedded a layer of 
sandstone of variable thickness, perhaps 100 feet at a maxi- 
mum in the Tennessee Valley, while the over and underly- 
ing limestones are many times that. As we come southward, 
the sandstone becomes more important, and the lower sec- 
tion of the limestone appears to give way to, or to be re- 
placed by, a series of black shales closely resembling those 
of the Devonian but many times more massive. In many 
places in the anticlinal valleys, and especially the further 
south we go, the upper limestone also appears to be want- 
ing or to be replaced by the shales and sandstones above 
named. The limestone which comes next below the Coal 
a We have already adverted to the fact that these Red Mountain 
ridges are formed of the Clinton, the Black Shale and the Sub-Carboni- 
ferous chert, and the same structure has been mentioned by Safford as 
characterizing the Dye Stone ridges of Tennessee. 
b The name, however, comes from Europe, where it appears,in similar 
relations to the Coal Measures. 
