750 C. 0. Dunbar — Stratigraphy and Correlation 



interbedding of the upper layers of the Clear Creek chert 

 with the basal layers of the Grand Tower. He also 

 clearly showed the intimate relation of the faunas of 

 these two formations, and therefore assigned the Clear 

 Creek to the highest Oriskany, assuming that deposition 

 was continuous in southern Illinois from the Lower into 

 the Middle Devonian. At that time, however, the typical 

 upper Oriskany was entirely unknown in the Mississippi 

 basin and the Camden chert seemed best to occupy this 

 interval, the uniqueness of its fauna being ascribed to 

 the fact that it belonged to a different basin from that of 

 the Appalachian trough. The finding by Weller of typi- 

 cal Oriskany in Missouri and especially the discovery of 

 its good development in western Tennessee, where it is 

 unconformably succeeded by the Camden chert, give a 

 new vista to the problem of correlation. 



The Camden chert is a white to yellow brittle novacu- 

 lite disposed in thin hard layers, usually from 1 to 3 

 inches thick — rarely as much as 8 or 10 inches — which are 

 commonly separated by gritty clay along the bedding 

 planes where weathering has begun. Locally there are 

 irregular, more or less vertical pockets of white silica, 

 apparently the result of leaching along ground-water 

 passages. The rock breaks with an irregular fracture 

 into angular, sharp-edged fragments, and it is always so 

 thoroughly fractured that even the fresh quarry faces 

 quickly break down into a talus slope, while natural out- 

 crops appear only as a loose rubble of angular pieces of 

 buff " chert' ' or novaculite, mostly smaller than one's fist. 

 So characteristic is this broken-up condition of the rock 

 that the quarries where it is extensively worked for bal- 

 last or road metal are generally known as "gravel pits." 

 It has proved to be one of the finest of road metals, as it is 

 very slightly soluble and has the important quality of 

 "bonding' ' well so as to form a hard surface under traf- 

 fic. The formation displays these physical characters 

 in all the known exposures except the one at the "whirl" 

 in Buffalo River, 4 miles north of Bakerville. Only the 

 upper layers are here exposed and these are directly 

 followed* by the Pegram limestone. They consist of 

 an alternation of layers of yellowish chert, 2 to 9 inches 

 thick, and layers of dense bluish gray limestone. The 

 fauna of these layers indicates that they are stratigraph- 

 ically higher than those elsewhere exposed where the 



