92 UNIVERSITY OF MISSOURI STUDIES 



the barite and the marcasite. The former according to Lindgren 

 and W. H. Emmons is characteristic of modern and shallow 

 deposits. Descriptions of hundreds of deposits give proof of this 

 conclusion. Marcasite, according to Allen, Crenshaw, and John- 

 son,'' is found only in shallow deposits and is formed at relatively 

 low temperatures only. 



Therefore, deposition must have occurred relatively near the 

 surface and may have been influenced by the mingling of the de- 

 posited solutions with oxygen-bearing meteoric waters. The 

 shallow depth is further evidenced by the occurrence of the de- 

 posits in the solution cavities. The materials added during the 

 process of replacement must have aided in changing the character 

 of the solutions. That the last stages of deposition were charact- 

 erized by an abundance of sulfuric acid or sulfates is proved by 

 the large amount of barite then deposited. 



Time of mineralization 



The time of mineralization cannot be fixed because there are 

 no formations younger than the Pennsylvanian system in asso- 

 ciation with the rocks which contain the ore bodies. Even these 

 beds are so high up and far from the border of the mineralized 

 area that they do not add much to the determination of the time 

 of mineralization. In the Washingtin Coimty district the miner- 

 alization rarely extends above the Proctor dolomite. In the Cen- 

 tral district, however, deposits extend up into the Jefferson City 

 formation, and possibly certain occurrences of lead and zinc in 

 the Coal Measures can be included in this group. Those in the 

 top of the Jefferson City formation are younger than Ordovi- 

 cian. If the deposits in the Coal Measures belong to this group, 

 the mineralization is later than the Paleozoic period. This con- 

 clusion is doubtful. 



The mineralization occurred, then, in the interval following 

 the Ordovician, as the Jefferson City formation is of Ordovician 

 age. It is known that there were intrusions of igneous rocks 

 later than these rocks in Missouri ; for example, the pegmatite 



'Allen, P. T., Crenshaw, T. L., and Johnson, J., "The Mineral Sul- 

 fides of Iron, with a Crystallographic Study by P. S. Larson," Am. Jour. 

 Sci., 4th ser., vol. 33, pp. 169-239. 1912. 



