OF ARKANSAS. 



91 



The black shale has a slight dip to the north-east, which will reduce 

 the thickness from 70 feet— the height from its base to its highest observed 

 out-crop — to 40 or 50 feet of actual thickness. It has concretions of sep- 

 taria, both calcareous and ferruginous, disseminated especially through 

 its upper part, some of which contain a considerable per centage of iron. 

 There are also saline exudations of sulphate of alumina and sulphate of 

 iron, with, perhaps, some chloride of sodium or common salt; these attract 

 the game and stock of the country, which resort to it as a " lick." 



The space between the shale and the millstone grit, that forms the top 

 of the mountain, is chiefly occupied by alternations of sandstones and 

 limestone belonging to the upper division of the subcarboniferous group. 



The following section exhibits the succession, and relative approximaet 

 thickness, of the principal rock formation of which the Boat mountain is 

 composed. 



SECTION OF BOAT MOUNTAIN. 



I c 



I o I 



I o-| 



4) 



40 



200 



Conglomerate, forming the summit of Boat 

 mountain, 40 to 50 feet visible. 



Sandstone of the millstone grit series, two 

 hundred feet in thickness. 



The upper member of 

 the millstone grit series 

 is here pebbly, and un- 

 derlaid by grits without 

 pebbles ; all the members 

 together occupying a 

 thickness of nearly two 

 hundred and fifty feet. 



These repose upon the 

 yellow upper strata of 

 the subcarboniferous 

 group, including the Ar- 

 chimedes and pentrimital 

 beds, which are, however, 

 mostly concealed by for- 

 est and vegetation ; in all, 

 about one hundred and 

 sixty feet in thickness. 



Beneath these, are 

 coarser - textured, a n d 

 subcrystalline members 

 of the same group, occu- 

 pying a space of three 

 hundred and ten feet. 



These coarse-textured, 

 subcarboniferous lime- 



