THE BARITE DEPOSITS OF MISSOURI 89 



In the other types two dominant processes are evident ; simple 

 cavity filling and replacement. The former may appear in all 

 sorts of open spaces, but mainly in fissures, shrinkage cavities, 

 or solution cavities. Replacement is found to be associated with 

 all these types of openings. 



Cavity filling. — The cavities originated in several ways. The 

 numerous small openings in the dolomite are original sedimentary 

 features due to shrinkage in volume when the dolomite was 

 formed on the sea bottom, and to a less extent during recrystalli- 

 zation. Most of these small openings are lined with dolomite 

 or quartz crystals, tho the fissures due to shrinkage are only 

 rarely lined with these minerals. 



The larger fissures, joints, and fractures are also due in part 

 to this recrystallization, as it causes a decrease in volume and the 

 stresses so set up would find relief in movements which would 

 produce such openings. They are also in part due to deforma- 

 tion. It is the latter that has caused the major joints, fissures, 

 and faults, and the former process that produced the numerous 

 small irregular fractures. It is to be expected that these smaller 

 openings would be less persistent than those produced by defor- 

 mation because the forces involved are less intense. 



The solution cavities were formed after the region had been 

 uplifted above sea level and probably during the periods of ero- 

 sion now represented by unconformities. According to Ulrich 

 there is one after the Potosi period and another at the top of the 

 Proctor formation. While no evidence as to the upper uncon- 

 formity could be found in this district, some evidence suggestive 

 of one between the Proctor and the Potosi formations was ob- 

 tained and it is believed that both unconformities are present. 

 The solution cavities were probably developed while the forma- 

 tions were exposed. The caves filled with sand and chert were 

 probably formed at this time and also the various other solution 

 cavities. Some of these contained broken blocks of dolomite, 

 and less commonly, fragments of drusy quartz and chert, show- 

 ing that these minerals were in existence before the uplift and 

 erosion. The quartz crystals in the sand were derived from the 

 Potosi dolomite. This is further evidence that the quartz and 

 chert antedated the deposition of the barite. 



