ABSTRACTS 763 



lies within the great A{)palachian Valley, here occupied by the Ten- 

 nessee River, which flows at an altitude of about 700 feet, and above 

 which rounded hills and ridge rise from 300 to 500 feet higher. The 

 valley ridges have a uniform northeast-southwest trend, parallel with 

 the Cumberland escarpment, their location depending on outcrop of 

 narrow belts of hard rocks. 



Geology. — West of the Cumberland escarpment the geologic struc- 

 ture is very simple. The strata remain nearly horizontal, as they were 

 originallv deposited, except in the Crab Orchard Mountains, where 

 they bend upward, forming a low arch. East of the escarpment the 

 strata have suffered intense compression, which has forced them into a 

 great number of narrow folds whose axes extend northeast and south- 

 west. The strata dip more steeply on one side of the_arch than on 

 the other ; and as a further effect of compression, the beds on the 

 steeper (generallv the northwestern) side have been fractured and the 

 rocks on one side thrust upward across the broken edges of those on 

 the other. In this manner the folds first formed have in most cases 

 been obliterated, and there remain narrow strips of strata separated by 

 faults, and all dipping to the southeast. 



The rocks appearing at the surface are entirely sedimentary — 

 limestones, shales, sandstones, and conglomerates — and include repre- 

 sentatives of all the Palaeozoic groups. The Cambrian formations con- 

 sist of the Apison shale, Rome sandstone, and Conasauga shale, a 

 series which is calcareous at top and bottom and siliceous in the mid- 

 dle. The Conasauga passes upward through blue shaly limestone into 

 the Knox dolomite, a formation about 4000 feet in thickness, composed 

 of siliceous or cherty magnesian limestone. Probably the lower por- 

 tion is of Cambrian age, while the upper is undoubtedly Silurian. 

 Above the dolomite is the Chickamauga limestone, whose upper por- 

 tion toward the eastern side of the district changes from blue flaggy 

 limestone to calcareous shale, and is called the Athens shale. The 

 next formation is the Rockwood, which also changes toward the east 

 from calcareous shale to hard, brown sandstone. These changes in 

 the character of the rocks indicate that, while they were forming, the 

 land from which their materials were derived lay to the southeast. 

 The Devonian is represented in this region by a single stratum of car- 

 bonaceous shale, the Chattanooga black shale, which rests, probably 

 with a slight unconformity, on the Rockwood. Above the Chat- 

 tanooga are the Fort Payne chert and Bangor limestone of the lower 



