472 C. W. HAYES COOSA VALLEY IN GEORGIA AND ALABAMA. 



by a belt of deep red soil, with which beds of limonite are usually as- 

 sociated. It seems probable that this ferruginous clay represents the 

 deep residual mantle of an old land surface. This is at present, how- 

 ever, only a working hypothesis and requires further field-work to de- 

 termine its value. 



West of the region mapped the evidence of this overlap is more con- 

 clusive. It consists in a heav}^ bed of conglomerate or breccia which 

 occurs at the base of the Chickamauga limestone, and is composed of 

 more or less angular pebbles of Knox dolomite chert imbedded in a 

 calcareous matrix. 



The second erosion overlap occurs about the top of the Silurian. It 

 is marked by much more pronounced folding than the earlier one, but 

 is confined to a smaller area. It produces the greatest unconformity in 

 the vicinity of Frog mountain. As show^n upon the map, the Frog 

 Mountain sandstone and shale come in contact at different points with 

 the Connasauga shale, Knox dolomite, Chickamauga limestone and 

 Rockmart slate. To bring about this relation by faulting would require 

 a period of folding and erosion followed by the horizontal transfer of 

 younger rocks across the eroded surface of the older. There is no reason 

 why such .a process might not take place under proper conditions, but 

 the evidence is against the hypothesis of faulting in this particular case 

 and in favor of the hypothesis of overlap. No rocks are found in this 

 region belonging to the Rockwood (Upper Silurian), while toward the 

 northeast and southwest this formation carries heavy ■ conglomerates, 

 which represent shore conditions of deposition adjacent to a Silurian 

 land area. 



The entire absence of the Chattanooga (Devonian) black shale south 

 of the Coosa river indicates a third overlap. The corresponding period 

 of erosion must have covered nearly the whole of the Devonian, but was 

 probably marked by slight elevation, and consequently little degrada- 

 tion, so that the Carboniferous rocks appear to rest conformably upon 

 the Frog Mountain (Lower Devonian) or Rockwood (Upper Silurian) 

 sandstones. 



Geologic Structure of the Region. 



The structure of the region between Rome and Weisner mountain is 

 probably as complicated as that of any portion of the Appalachians. 

 Three types of structure, more or less closely related genetically, are here 

 found associated, and the complication is further increased by extensive 

 deposition overlaps and by abrupt lithologic changes. Only the main 

 features of this structure could be presented here, even if all the details 

 had been or could be worked out. For reasons given above all attempts 



