24 UNIVERSITY OF MISSOURI STUDIES 



dolomite and soft finely porous dolomite." It is about 50 to 60 

 feet thick and possibly thicker. 



It was found to be impossible to separate satisfactorily these 

 two members from the underlying formation and from each 

 other in the Washington County area, hence they are grouped to- 

 gether and called by the name first given to them by Ulrich, the 

 "Elvins." He states (ibid, p. 23) : 



"This name is proposed for the shales, shaly limestones and 

 more or less earthy dolomites that in St. Francois County inter- 

 vene between the shaly top of the underlying Bonneterre lime- 

 stone and the cherty limestones of the Potosi group above. 



"The bed of pink dolomite assigned to the base of the Potosi 

 in the foregoing section contains no chert nor drusy quartz, and 

 in that respect differs decidedly from the overlying beds of the 

 formation. It may therefore be regarded as more properly re- 

 ferable to the Elvins formation. This is not here, for the reason 

 that it or a similar bed inaugurates the Potosi sedimentation in 

 areas where the Elvins . formation is missing." 



This pink bed is here included with the Elvins for it is more 

 distinctly related lithologically to the formations below than to 

 those above. No evidence was found to indicate that the rela- 

 tionship between the Elvins formation and the Potosi dolomite 

 was other than one of conformity. Both Ulrich and Buckley 

 thot they might be unconformable locally, but did not cite the 

 evidence. Ulrich* states that the "Elvins" is in unconformable 

 contact with the base of the Ozarkian system (the Potosi). In 

 the paper cited, Ulrich defines his Ozarkian system and places 

 its base between the Elvins formation and the Potosi formation. 

 The structural, faunal, and petrological differences of these two 

 formations are certainly great. Whether the break between them 

 is sufficiently great to call it the line between two systems future 

 studies will determine. 



Potosi formation 



The Potosi formation covers about one-half the area map- 

 ped. It is nearly continuous from the southern end to the north- 

 em in the eastern half of the area, except where the streams have 



*Ulrich, E. O., Bull. Geol. Soc. Am., vol. 22, p. 623. 1911. Also pp 

 628-633. 



