THE BARITE DEPOSITS OF MISSOURI 25 



cut down to the Elvins formation. The other extensive area Hes 

 along Mineral Fork, Mine a Breton, and Bates creeks. 



The eastern area constitutes the Summit Platform as de- 

 scribed above. It also includes the more important farming areas 

 in the district. This is due largely to the fact that the Potosi 

 weathers to hills with less relief than any other formation having 

 much areal extent in the district. 



The exposures of the Potosi in the western part of the dis- 

 trict are due to the fact that the larger streams have cut thru the 

 overlying formations. About eight miles to the southwest of the 

 area mapped Hazel Creek has cut down to and exposed the Po- 

 tosi along its bottom. 



The bluffs along Mineral Fork, Old Mines Creek, and Mine 

 a Breton Creek have many excellent exposures of the Potosi for- 

 mation. Many of these exposures are 75 to 100 feet or more 

 high, and where the stream is cutting at the foot of the bluff, as 

 is common, there is no talus to obstruct the exposure of the lower 

 portion. These bluffs afford opportunity to study the forma- 

 tion. Good exposures are also found along Big River where it 

 flows across the Potosi formation. 



The mantle rock is thicker over the Potosi dolomite than 

 over any of the other formations unless it be the Proctor dolo- 

 mite which is very similar to the Potosi dolomite in many re- 

 spects. As a rule, the soil and subsoil derived from the Potosi 

 dolomite are free from quartz and chert, but where the slopes 

 are great enough to favor rapid erosion, these materials become 

 abundant, and on the sides of the sharper valleys the Potosi for- 

 mation itself outcrops. The soil is fertile and covered with a 

 heavy growth of oaks of various kinds. 



Petrography. — The Potosi formation is dolomite from top 

 to bottom and contains a large amount of drusy quartz, chalce- 

 dony, and chert. The dolomite is distinctly granular and the 

 greater part is medium-grained, with a few fine-grained beds. 

 The remarkably uniform granularity is striking, and the similar- 

 ity of this formation to the Proctor dolomite would make it dif- 

 ficult to differentiate between the two formations, if it were not 

 for the ever present drusy quartz of the Potosi. There are many 

 small cavities in the rock lined with quartz, dolomite, or more 

 rarely calcite crystals. 



