STRATIGRAPHIC TAXONOMY 635 



Basal division of the Knox dolomite (s. st.). — Rather generally in east 

 Tennessee and southwest Virginia grayish dolomite and limestone, prac- 

 tically free of chert, constitute the lower part of the Knox proper. So 

 far as known, this lower division, which I formerly thought to represent 

 the Ketona, but now believe to be a younger and quite distinct formation, 

 does not occur on the west side of Murphrees Valley, Alabama, nor has it 

 been recognized in Cahaba Valley. Even in the Tennessee basin it 

 varies considerably in thickness, possibly being absent altogether locally, 

 as in Eiver Eidge north of Morristown, while in other places it seems to 

 exceed 600 or 700 feet. The latter figure is attained between Clinchport 

 and Speers Ferry, Virginia. The formation, which for the present re- 

 mains unnamed, was recognized at Knoxville; hence it is a part of the 

 tjrpical Knox dolomite of Safford. In Tennessee and southwestern Vir- 

 ginia, where the Briarfield, Ketona, and Potosi formations have not been 

 detected, the Knox rests on upper Cambrian formations, either the Noli- 

 chucky or the Conasauga shale. When present the lower member of the 

 Ejnox is readily distinguished by its more calcareous and much thicker 

 beds. Most of its beds are magnesian, but few, if any, are dolomite. 

 Many, on the other hand, contain so little magnesia that they may justly 

 be called limestone. The latter are fine-grained and not infrequently 

 contain more or less shaly layers. Chert is very sparingly developed, not 

 only in the unweathered rock, but also in the residual clays of the surface. 

 The practical absence or scarcity of chert and the presence of nearly pure 

 and often shaly limestone distinguishes this lower member of the Knox 

 from the profusely cherty main mass of the formation which overlies it 

 and for which the name Copper Ridge chert is proposed. 



Copper Ridge chert (new). — Of the three divisions commonly recog- 

 nizable in the Knox, the highly cherty ridge-making middle division is 

 the most persistent and by far the greatest. This middle division, for 

 which the term Copper Ridge chert is here proposed, is readily distin- 

 guished from the lower and upper divisions by the hard white or gray 

 chert which is developed by segregation and liberated under the slow 

 process of subaerial decomposition of the dolomitic matrix. The resist- 

 ant character and finally great abundance of this chert almost invariably 

 gives rise to broad and long ridges, among which that known as Copper 

 Ridge, in northeast Tennessee, is the excellent example chosen to supply 

 the name and type of the member or formation. 



As a formation the Copper Ridge chert is best displayed, and probably 

 also best developed, in the middle and western parts of the Appalachian 

 Valley in Tennessee and Alabama. Here its average thickness is nearly 



