174 S. W. McCALLIE 



abundant Cambrian fauna which has been studied by Professor 

 C. D. Walcott. 



The Knox dolomite is divided by an unconformity into a lower 

 and upper division. The upper division on paleontological evi- 

 dence has been referred to the Ordovician system and the lower to 

 the Cambrian, The formation as a whole consists of a great thick- 

 ness of magnesian limestones with much chert in places. It occurs 

 in a large number of broad and narrow belts traversing the Appa- 

 lachian Valley. 



ORDOVICIAN SYSTEM 



The Chickamauga formation represents both the Middle and 

 the Upper group of the Ordovician system. It consists of sedi- 

 ments laid down in two separate basins. The rocks in the western 

 part of the valley consist of interbedded limestones and shales 

 outcropping in a number of long, narrow belts, while in the eastern 

 part of the valley the formation is divided into the Chickamauga 

 limestone and the Rockmart slate. The latter consists chiefly of 

 dark-colored slates, with a few thin beds of sandstone and some 

 limestone usually high in calcium. 



SILURIAN SYSTEM 



The Rockwood formation, which unconformably overlies the 

 Chickamauga formation, is the only representative of the Silurian 

 system. It consists of olive-green shales and thin-bedded sand- 

 stone, with one or more beds of fossiliferous iron ores. It occurs 

 only in the western part of the Appalachian Valley, where it forms 

 narrow belts at the base of the higher ridges and mountains capped 

 with Carboniferous rock. 



DEVONIAN SYSTEM 



The Chattanooga black shale and the Armuchee chert are the 

 only representatives of the Devonian system in Georgia. These 

 two formations are separated from each other by an unconformity 

 and are supposed to represent the Upper and Lower Devonian 

 respectively, the middle part of the system being absent. The 

 black shale, which attains a thickness of not more than twenty 

 feet, is confined to the western part of the valley, where it occurs 



