470 C. W. HAYES — COOSA VALLEY IN GEORGIA AND ALABAMA. 



apparent break by the Rockwood formation, and that by the Carbonifer- 

 ous, Fort Payne chert and Floyd shale. South of the Coosa fault the 

 succession above the Chickamauga is quite different. 



Devonian Rocks. — A few miles southeast of the region mapped the 

 Rockmart slate is overlain by a thin bed of white quartzose sandstone, 

 and this by fossiliferous chert supposed to correspond to the Fort Payne. 

 In the area mapped no rocks are found ^vhich can be shown to rest con- 

 formably on the Rockmart slate. As show^n upon the map, there aj;e 

 between Indian and Weisner mountains several small areas occupied by 

 a formation which comes in contact with all the older rocks thus far 

 described. It consists of coarse ferruginous sandstone, in some places 

 white, resembling quartzite, and in others yellow or gray and weathering 

 to incoherent beds of sand. Beneath this sandstone and usually deeply 

 covered by its debris are shales, also variable in composition and appear- 

 ance. No satisfactory measurement has been made of their thickness, 

 but this is probably as variable as their physical appearance. 



A number of fossils have been found i^ these sandstones of Frog 

 mountain. They include Zaphrentls sp. (?), Chsetetes complanata (?), Spi- 

 rifera arenosa (?), S. arrecta (?), Pterinea sp. (?), Platyceras sp. (?), Orthis 

 musculosa (?) and Peatomerus, cast like that of P. ohlongus conocardiiim. 

 Concerning these fossils Mr Walcott says '^ that " all the specific determi- 

 nations are uncertain, as the material is not in a satisfactory condition, 

 but the horizon of the Oriskariy sandstone is strongly suggested by the 

 general fades of the fauna." 



The only formation in the Appalachians south of Tennessee which 

 has hitherto been regarded as Devonian is the Chattanooga black shale, 

 and this is wanting south of the Coosa river. If the Frog mountain 

 sandstone proves to be Oriskany a part of the break between the Silurian 

 and Carboniferous will be filled. 



Unconsolidated Formations. — Reference has been made to beds of silt 

 and gravel in the Coosa valley which are worthy of some further descrip- 

 tion. These deposits cover a considerable proportion of the valle}^, en- 

 tirely concealing the underlying rocks over many square miles of its 

 surface. They "are found at two distinct levels, separated by a vertical 

 interval of about 140 feet. At both levels they rest directly upon a 

 smoothly cut surface of the highly contorted Cambrian shales. 



The low-level deposits are most extensive and occur from 30 to 40 feet 

 above the Coosa river. , At their base, resting on the erosion surface, are 

 from 1 to 5 feet of gravel and coarse sand showing some indications of 

 bedding. The gravel is coarsest at the bottom, sometimes containing 

 pebbles 6 to 8 inches in diameter. When fresh exposures are examined 



* Am. Jour. Sci., vol. xlvii, 1894, p. 237. 



