634 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



The Briarfield consists chiefly of medium thick-bedded blue and gray 

 siliceous dolomite. On weathering the basal part is marked by abundant 

 dense residual chert. Except for this the lower half of the formation is 

 without chert. Through some 400 to 500 feet of thickness above the 

 middle the weathered rock is streaked with convoluted plates of silica 

 and weathers cavernous with drusy incrustations. This is followed by 

 about 200 feet of blue dolomite, which forms the top of the Briarfield as 

 provisionally drawn. The last is followed by 275 feet of typical Ketona 

 dolomite. The only fossils observed are of Cryptozoon cf. proUferum, 

 of which a well developed reef, consisting of coalescing individuals 3 to 24 

 inches in diameter, occurs about 400 feet above the base of the formation. 



Ketona dolomite. — It is well developed in Birmingham and Cahaba val- 

 leys in Alabama, but elsewhere unknown. Eecently described by Butts 

 as a basal member of the Knox dolomite. Thickness variable, commonly 

 300 to 400 feet, with a maximum development of 800 feet or more, as 

 observed by Butts in. the Cahaba Yalley about 5 miles north of Monte- 

 vallo. As noted above, this thickness decreases to 275 feet at Six-Mile, 

 some 12 miles south in the same valley. The Ketona consists almost en- 

 tirely of gray dolomite which is nearly pure and especially noteworthy 

 because of the small percentage of silica contained in it. Apparently un- 

 fossiliferous. Succeeded by the Potosi, the lower Knox, or the Copper 

 Eidge. 



Potosi (?) dolomite. — A formation that forms the basal division of the 

 Ozarkian in Missouri, and is so closely simulated by deposits found in the 

 Cahaba Valley of Alabama that a distinct name seems unjustifiable. So 

 far as known, the Potosi is entirely absent in the Appalachian Valley to 

 the west and north of a point in the Cahaba Valley about 5 miles north 

 of Montevallo, where, according to unpublished observations by Charles 

 Butts, it wedges in between the typical Knox and the Ketona. The max- 

 imum development of the Potosi in the Cahaba Valley is estimated by 

 Mr. Butts at something like 500 feet. Near the bridge over Six-Mile 

 Creek, at Six-Mile, it seems to be only about 275 feet. The rock is a 

 blue or bluish gray fine-grained dolomite, notably siliceous and cavernous. 

 On weathering the surface of the ground is covered with large and small 

 masses of cavernous drusy-surfaced quartz, which also incrusts the weath- 

 ered surface of the outcropping beds. Except that it is more abundant, 

 this residual material recalls the middle to upper part of the Briarfield 

 dolomite. In the Cahaba Valley the Potosi is succeeded directly by the 

 Copper Eidge cherts of the Knox proper. No fossils of any kind have 

 been seen here in the Potosi. They are exceedingly rare also in the sup- 

 posed equivalent beds in Missouri. 



