THE BARITE DEPOSITS OF MISSOURI 57 



the cavities in which it was deposited and its inner side is 

 covered with crystal faces. Crystals, usually cubes modified by 

 octahedrons an inch or more in diameter, are quite common. An 

 interesting mode in which pyrite occurs is as stalactites. These 

 may be four or five inches in length and up to one-quarter of an 

 inch in diameter, although the majority are two or three inches 

 long and about one-sixteenth of an inch in diameter. They are 

 distinctly cylindrical and are usually hollow. The larger ones 

 show distinct growth-rings. They are always pendant and are 

 so close together as to resemble very closely the organ-pipe coral. 

 A similar type of limonite occurs among the secondary iron ores 

 of Missouri and is called "pipe-ore" because of this resemblance. 

 In one or two places these stalactites were completely cemented 

 along a plane thru which the stalactite passed. This plane of 

 cementation was at right angles to the stalactites and suggests 

 that the water level in the cavity stood at that point for a time. 



The pyrite is found to be later than the quartz when both 

 minerals are present. It is always older than the barite. Its re- 

 lationship to the marcasite has been discussed above. 



Sphalerite. — Sphalerite is found only in vein deposits. Here 

 some large masses are found, but more commonly it occurs as 

 smaller masses in a gangue of barite. Galena may accompany it 

 but not in large quantities. At the Eye mine the sphalerite and 

 pyrite are intergrown, in part, but more commonly the sphalerite 

 is later than the pyrite. Rarely, the sphalerite occurs as small 

 ruby crystals. Where sphalerite is abundant pyrite is a minor 

 associate. In some localities the sphalerite is disseminated thru 

 the dolomite, replacing it. In all cases it is older than the barite. 



Oxides 



The following oxides were observed: quartz, chalcedony, 

 chert, limonite, and hematite. 



Qiiarts. — Quartz is a common associate of barite. As a rule, 

 only the barite found in the formations above the Potosi exists 

 without it, but there are considerable areas in the Potosi forma- 

 tion in which there is little quartz in immediate association with 

 the barite. This may be either where the barite occurs in veins 

 or where it is disseminated. The quartz, usually drusy and 



