24 GEOLOGICAL SURVEY OF ALABAMA. 



several places in the other anticlinal valleys. It exists in 

 abundance in many regions, and particularly in the regions 

 of the Mississippi river, where it has been most closely 

 studied. It is known as the Keokuek Limestone. 



The irregularity in the exposures above mentioned made 

 it impossible to define the extent of these calcareous deposits, 

 but they are evidently local. One of them in sec. 27, T. 13,. 

 R. 1, east, was estimated to be at least 80 feet thick. It is 

 bedded on the Black Shale. This limestone is generally of 

 excellent quality, much of it crystalline. It is apparently 

 the best body of limestone seen anywhere in the valley, and 

 will doubtless be utilized in the future. 



The other deposit of this rock is in sect. 24, T. 14, R. l y 

 west. Its apparent thickness about the same, say 80 feet. 

 Its base could not be seen, but as it was overlain by about 

 the same amount of siliceous strata as the other, it was in- 

 ferred -that it had also the same base. The quality of the 

 rock, however, is generally different; some of it is good, but 

 most of it has too much silica and other impurities to be of 

 economic value. A large cave opening down into the heart 

 of this limestone, into the lower chambers of which a small 

 stream sinks and disappears, is a noticeable feature. 



DEVONIAN. 



The Black Shale. This is a thin but very persistent mem- 

 ber, which marks the boundary line between the rocks of 

 the Carboniferous era and those of the Silurian. Itsaverage 

 thickness is about ten feet, in some places less, and occasion- 

 ally twenty, thirty or forty feet. It is highly bituminous, 

 indicating an organic origin, and its included fossils are 

 wholly marine. It is doubtless of Devonian age, and seems 

 to have been deposited about the middle of the Devonian 

 period. It probably belongs to the bituminous shales of the 

 Marcellus period, or the overlying shales of the Hamilton 

 period, as suggested by Prof. Tuomey. 



