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



when the same beds are traced for a considerable distance. The contrast 

 in character is greatly heightened, since all the intermediate varieties are 

 wanting and the most different types are brought into immediate con- 

 tact, where comparison reveals differences which in their normal relation 

 would entirely escape notice. The visible displacement on the Rome 

 fault is about 4 J miles. Upon a very moderate estimate of subsequent 

 folding and erosion, the original displacement. must have been at least 

 twice, and was more probably three times, that amount. If an equal 

 displacement has occurred on the Coosa fault-plane, the original 10 or 

 15 miles of interval between rocks now adjacent would easily account 

 for these observed lithologic differences. 



Paleozoic History of the Region. 



From the facts detailed above a tolerably definite idea may be ob- 

 tained of the history through which this region has passed. It begins 

 in Cambrian time, when the land lay to the southeast and a broad ex- 

 panse of sea stretched far to the west and northwest. The rivers brought 

 down sediments from the crystalline rocks on the east. At first, while 

 the land was high, coarse sand was deposited in great lenses about the 

 river mouths, and as' the land was worn down the sediments became 

 finer, till at the close of the Connasauga epoch only the finest mud was 

 deposited. Then followed a long, period of calcareous precipitation, with 

 conditions, not clearly understood, favoring the deposition of amorphous 

 silica as chert or flint. This was the Knox dolomite epoch, covering 

 late Cambrian and early Silurian time. Following this was an uplift 

 with gentle folding which brought portions of the region above sealevel. 

 Probably most of the area covered by the map was dry land undergoing 

 subaerial degradation, with the formation of a deep residual mantle and 

 the deposition about its border of conglomerates composed of the in- 

 durated portions of the eroded rocks. FolloAving this comparatively 

 short period of uplift was a much longer one, during which the sea cov- 

 ered the region and the Chickamauga limestone was deposited. After a 

 few hundred feet of blue limestone had formed changes occurred in the 

 •altitude of the land toward the southeast, so that fine mechanical sedi- 

 ments replaced pure limestone, and a great thickness of calcareous clay 

 shales, now the Rockmart slate, was laid down. This long period of 

 quiet deposition was terminated by a period of disturbance similar to 

 the one which brought the Knox dolomite epoch to an end, but the fold- 

 ing was very much more profound in the second than in the first. 

 Within the area mapped, and extending some distance toward the south- 

 east, rather sharp folds were produced, trending nearly north and south. 



