154 GEOLOGICAL SURVEY OF ALABAMA. 
ore is some twenty feet in thickness, just across the Cahaba 
Coal Field in the Cahaba Valley about six miles distant, the 
Red Mountain, or rather its representative, contains no ore 
at all in the greater part of its length, nor does it seem to 
contain any of the Clinton rocks. As is well known this 
formation furnishes the greater part of the material used in 
our furnaces. In places, the ferruginous limestone of this 
formation would make a fine building stone, and the same 
is true of the sandstones. It would be difficult to give the 
average thickness of the Red Mountain rocks proper, in the 
region of the present map; 100 feet might perhaps be a fair 
average, for the Red Mountain as a topographic feature, is 
made up of the rocks of different ages, Trenton, Clinton and 
Sub-Carboniferous, together with the usually very thin black 
shale of the Devonian. 
The thickness of the whole Silurian in this part of the 
State given above as about 5,000 feet, is only an estimate. 
The true thickness it will be very difficult to determine, 
especially in the case of the most important member, the 
Knox Dolomite, since it is in great part made up, so far as 
surface materials are concerned, of loose fragments of chert 
in which the bedding planes are seldom to be seen. A 
greater part of the area of our valleys is held by this forma- 
tion than by any other, and its importance is still further 
enhanced by the fact that it is the chief source of the brown 
iron ores of the State. Many of the noted big springs issue 
from this formation. 
THE DEVONIAN.—The only representative in Alabama 
of this system of rocks, which in the States further north is 
of great thickness and importance, is a thin bed of Black 
Shale, averaging perhaps ten or fifteen feet, but being ap- 
parently absent altogether in some places. A few fossils 
have been found in it in the Valley of the Tennessee in North 
Alabama, which serve to fix its position as a member of the 
Devonian. The shale being soft and somewhat easily 
eroded, is usually covered and concealed by the debris of 
the adjacent rocks, so that it does not commonly come under 
notice even where it is present. Itis of importance chiefly, 
perhaps, as being the source of some of our best known 
sulphur springs. The shale usually contains a large amount 
