THE BARITE DEPOSITS OF MISSOURI 27 



tent as chert. In two or three localities a small amount of flint 

 was observed and rarely some bright green chert was found. 

 Both the flint and the green chert are near the top of the forma- 

 tion. The chert is dense, gray to white in color, and breaks with 

 a conchoidal fracture. It is in masses up to two or three feet 

 thick and several feet long. It is not in continuous beds, and the 

 smaller masses are very irregular. There is little banding in the 

 chert. The drusy quartz and chert are either in distinct beds, or 

 parts of individual beds of dolomite. Many beds of the latter 

 are entirely free from either form of silica. 



It is the drusy quartz and chalcedony that distinguish the 

 Potosi formation from all other formations in this area. They 

 are found in very irregular masses of various shapes; in large 

 honey-combed masses, two to two and one-half feet thick ; and in 

 veins. 



The drusy forms are similar to geodes, tho differing from 

 them in some respects. The majority of the drusy cavities are of 

 such a shape as to have the inner surfaces dominantly convex in- 

 stead of concave as in geodes (PI. V, B). These convex sur- 

 faces are covered by the crystals of minerals which are in the 

 cavities. They have an infinite variety of shapes. As a rule the 

 larger cavities, those more than two inches long, do not contain 

 dolomite or calcite, tho these minerals are in the smaller open- 

 ings. The quartz crystals are of various sizes, from very minute 

 ones that cannot be distinguished with the naked eye, to those 

 nearly half an inch in diameter and an inch to an inch and a 

 quarter in length. The quartz and chalcedony are in alternating 

 layers from a fifth to one hundredth of an inch thick (PI. Ill, 

 E). There are as many as 40 to 50 in a single banded zone in 

 some cases. On a few specimens thin bands of limonite were 

 found coating the quartz layers and covered by chalcedony. No 

 limonite was seen on a freshly fractured surface, hence it is prob- 

 ably due to a slight infiltration along cracks or porous bands. 



The large honey-combed masses of drusy quartz have the 

 appearance of a series of pipes coalescing at intervals. The cen- 

 tral part of the pipes, in fresh specimens, contains dolomite, 

 around which the quartz is deposited (PI. Ill, D). Chalcedony 

 is usually lacking in these honey-combed masses. The first min- 



