58 UNIVERSITY OF MISSOURI STUDIES 



nearly always containing some chalcedony, has been deposited on 

 the sides of the cavities and openings, which may later have been 

 filled with barite. Some of the quartz crystals are nearly one and 

 a half inches in length and they range down to minute sizes. 

 There is a great deal of quartz in beds in the Potosi, which upon 

 weathering becomes associated with the residual barite. The mass 

 of fragments of drusy quartz with the clay is called "moory" by 

 the miners. It is also known as gravel, but the latter may include 

 fragments of chert, etc. Where veins approach or cross these 

 beds the interstices in the drusy quartz are commonly filled with 

 barite. The numerous openings furnish a convenient place for 

 deposition. 



Chalcedony. — The cryptocrystalline, fibrous variety of 

 quartz known as chalcedony is a frequent and common associate 

 of the drusy quartz. Alternate layers of chalcedony and quartz 

 is the dominant mode of occurrence. These layers are very thin ; 

 50 or more occur in three-quarters of an inch. The chalcedony 

 is usually the first to be deposited, and, as shown in the weather- 

 ed specimens, preserves perfectly the rhombohedral imprints of 

 the dolomite crystals upon which it was deposited. In a few 

 cases there is a layer of chalcedony immediately under the sul- 

 fides or barite. The chalcedony is always white unless it has 

 been stained by iron oxides since weathering out of the dolo- 

 mite. There is an abundance of chalcedony only in the Potosi 

 formation ; none is seen in the formation above \^ 



Chert. — Chert is not found in immediate association with 

 the barite, except in the residual deposits, where it has been con- 

 centrated by the weathering of the dolomite. It is more abund- 

 ant in the Central district where the barite occurs in the cherty 

 Lower Ordovician formations. The barite diggings in the Jef- 

 ferson City formation contain a great deal of chert- 



Limonite. — Limonite, dark-brown or almost black, is very 

 common in the barite diggings of the Potosi and Proctor forma- 

 tions. It occurs as a layer under the barite and ranges from a 

 thin film to an inch or more in thickness. The larger part of it 

 is pseudomorphic after pyrite and marcasite, hence its distribu- 

 tion is similar. It is possible that some of the limonite has been 

 deposited from ground water. No specific instances of this were 



