86 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



and in others as massive beds, frequently several hundred feet in thickness. In some places 

 the limestone, instead of forming continuous beds, occurs as flat lenticular masses, an inch or 

 less in thickness, rather closely crowded in the fine shales. The lower portion of the formation 

 consists whoUy of fine clay or slightly sandy shales, which appear yellow or brown at the surface 

 and dark bluish gray or black below drainage. 



These three subdivisions merge into one another without sharp boundaries. The formation 

 contains no stagle stratum sufficiently characteristic to be identified at different outcrops and 

 thereby used as a datum for the subdivision of the formation, so that the sihceous beds are 

 represented only as a lentil in the formation. The rocks of the Coosa VaUey are everywhere 

 highly contorted and are probably also intersected by numerous faults. For these reasons, and 

 also because large areas in the valley are deeply covered by recent gravels, it is impossible to 

 estimate even approximately the thickness of the formation as a whole or of any of its subdi- 

 visions. The fact that the formation occupies such a broad area suggests, however, that its 

 thickness must be very considerable, probably several thousand feet. In addition to the areas 

 already described, the Conasauga forms a number of narrow belts in the northwestern portion 

 of the quadrangle. These occupy the axes of narrow anticlinal rolls or are brought to the surface 

 by faults. The formation here has much the same character as in the type locahty; that is, 

 it consists of fine clay shales with occasional beds of blue limestone. The Conasauga formation 

 differs so widely in character in the closely adjoining regions that, except for the evidence 

 derived from the fossils, the two phases would scarcely be correlated. Nothing resembling 

 the upper sihceous division occurs in the region east of the Rome sandstone belt, and, on the 

 other hand, the characteristic oohtic limestone of this eastern region is wholly wanting in the 

 Coosa Valley. 



Two explanations of these differences are suggested. The first is that there may have been 

 a barrier of land between the two areas of deposition, so that the rocks of the Coosa Valley 

 and those south of the Coosa fault were laid down in separate, though contiguous, basins. 

 Deriving their sediments from different sources, they would differ in lithologic character, while 

 the faunas might be essentially the same. No trace of such a land barrier, however, has yet 

 been found, and the rocks in question contaia none of the characteristic marks of littoral 

 deposition. The second explanation is that the rocks now occupying adjacent areas at the 

 surface were originally deposited in comparatively remote parts of the same sea, but have been 

 brought side by side by folding and faulting, and that the observed lithologic differences are 

 due to the gradual change which is always found upon tracing a bed for a considerable distance. 

 The contrast in character is greatly heightened by the elimination of the intermediate varieties 

 and by the most widely different types being brought into immediate contact, where comparison 

 reveals differences which in the normal relation of the beds might escape notice. The two 

 regions are separated by the Coosa fault, which will be more fuUy described later. It is quite 

 possible that rocks now in contact on opposite sides of this fault were originally separated by 

 an interval of 10 or 15 miles, and that the intervening rocks, now entirely concealed, would, 

 if restored, present all the intermediate varieties between the two sharply contrasted phases 

 of the formation. 



The Knox dolomite consists of from 4,000 to 5,000 feet of massively bedded and somewh9,t 

 crystalline gray magnesian limestone. From the few fossils which have been found it appears 

 probable that a transition from Cambrian to Silurian occurs in the lower third of the formation, 

 but it is generally impossible to determine this fine of division, so the whole formation is classed 

 as Silurian on the historical-geology sheet. This limestone, or, more properly, dolomite, 

 contains a large amount of sihca in the form of nodules and layers of chert, an impure variety 

 of flint. Upon weathering, that part of the rock which consists of the carbonates of lime and 

 magnesia is dissolved, leaving behind the chert, usually embedded in red clay. This residual 

 material covers the surface to a great depth, and the dolomite is seldom seen except in the 

 stream channels. Unlike the underlying Conasauga formation, the Knox dolomite affords 

 some indication of having been deposited in proximity to land toward the east. In a few 

 places the chert beds are replaced by coarse sand disseminated through the dolomite, in some 



