630 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



Potosi dolomite. — Tliis is tlie oldest of the Ozarkian foniiations in Mis- 

 souri. It is a light gray to dark bluish gray, rather massive dolomite, 

 the surface of the rock weathering hackly and many of the beds with 

 drusy quartz masses, seams, and incrustations. Surfaces underlain by 

 this formation exhibit abundant masses of cavernous banded calcedony 

 and drusy quartz inclosed in a stiff deep red residual clay. This drusy 

 quartz is very characteristic of the formation. Maximum thickness, 300 

 feet or more; well exposed in Washington and Saint Francois counties, 

 especially in the vicinity of Potosi. Fossils exceedingly rare and poorly 

 preserved. So far but two specimens have been seen, one an undeter- 

 minable gastropod about one inch in height, the other a fragment of 

 Cryptozoon. 



This is not the Potosi group of Bain and Ulrich, who used the term 

 under a misapprehension of the wishes of the then State geologist of 

 Missouri, but the Potosi formation of Buckley, which is entirely satis- 

 factory. 



Eminence chert. — This is the proposed name of a very cherty dolomite 

 that rests, apparently unconformably, on the Potosi or overlaps that for- 

 mation and then usually comes into contact with the pre-Cambrian por- 

 phyry. Above it is limited by the base of the Proctor dolomite, the in- 

 terval between the top and bottom being not less than 200 feet in Shannon 

 County. The Eminence is widely distributed in Missouri, being espe- 

 cially well displayed in the valleys of Carter and Eeynolds counties, which 

 adjoin Shannon on the east. It comes to the surface also in some of the 

 deep valleys near the Osage, in the northern part of Camden and the 

 southern part of Morgan counties. Though of varying kinds, a large 

 proportion of the chert of this formation is white and dense and much of 

 it is fossiliferous. Many species have been collected and most of them 

 are well marked and characteristic of the formation. They comprise sev- 

 eral large species of umbilicated Holopea-like gastropods, which, having 

 a deeply notched outer lip, I am referring to a new genus — Sinuopea. 

 Holopea sweeti Whitfield is a congeneric species. Another of the more 

 striking gastropods is a large trochoid shell which also belongs to an un- 

 described genus. Among the cephalopods are three species reminding 

 of Piloceras and one or two of Cyrtocerina. Associated with the mol- 

 lusks are a few trilobites — Dikelloce'phalus and lUwnurus—ihe latter ap- 

 parently being confined to the Eminence in Missouri. 



Characteristic species of the Eminence fauna have been found at a 

 number of localities in Alabama and Tennessee and near Lexington, Vir- 

 ginia, in the lower part of the Copper Eidge division of the Knox ; also 

 in the lower part of the Oneota dolomite in the upper Mississippi Valley 



