26 GEOLOGICAL SURVEY OF ALABAMA. 



* 



dolomites and limestones, all more or less charged with 

 cherty matter, which occurs sometimes in great concretion- 

 ary masses, sometimes approaching sandstone. The residual 

 soils from the disintegration of these upper beds of the Dolo- 

 mite, are usually of gray to yellowish color, and the surface 

 is generally covered with angular fragments of the chert. 

 Where the chert predominates, it often forms the ridges of 

 gray flinty gravel which are so characteristic of this forma- 

 tion. The chert usually exhibits concretionary structure, 

 and has small rhombohedral cavities interspersed through it, 

 the empty moulds of crystals of dolomite which once filled 

 them. This peculiarity was first pointed out by Dr. Safford, 

 and seems to be fairly characteristic of the chert of this form- 

 ation, though we have occasionally noticed it also in the 

 Fort Payne chert. Other details will be given below in con- 

 nection with the account of the brown iron ore deposits of 

 this valley. 



CAMBRIAN. 



Coosa Shale,* Sparry Limestone. The next member be- 

 neath the chert, or Knox Dolomite chert, as it is sometimes 



*In the classification and description of the Cambrian rocks of Alabama, 

 we are confronted with many difficulties. Dr. Safford has described the 

 rocks of this period occurring in East Tennessee under the names Knox 

 Shale, Knox Sandstone and Chilhowee, to which may perhaps be added 

 his Ocoee group. In the eastern part of our Coosa Valley region we find 

 the precise lithological equivalents of all these Tennessee divisions, and in 

 the Report of this Survey for 1875, which relates to that section, we have 

 identified and described the three types of Dr. Safford the Knox Shale, 

 the Knox Sandstone and the Chilhowee (Potsdam). Later, when we came 

 to study the Cambrian of the valley regions along the Coosa river and 

 westward, we found these divisionsno longer applicable, for the sandy shales 

 and their bedded sandstones, representing the East Tennessee, Knox Shale 

 and Knox Sandstone, are replaced here by their bedded limestones with 

 clay seams. So also we here find none of the great sandstone and con- 

 glomerate masses, which Safford named Chilhowee or Potsdam. 



In many places east of the Coosa river these thin bedded limestones, to 

 which we gave the name of Coosa Shales, appear to pass below the 

 variegated siliceous shales of the East Tennessee type, to which we had 

 given the name Montevallo or Choccolocco Shales. 



We were therefore led to assign the Coosa shale to the base of this division ; 

 yet in Murphree's Valley and its south-western prolongation, we find sim- 



