26 UNIVERSITY OF MISSOURI STUDIES 



The predominating color of the formation is light bluish- 

 gray, but dark grays, buffs, and in some places red and pink are 

 seen. It weathers to a rather dark gray, with occasional reddish 

 and buff members. 



The Potosi formation consists of beds ranging from a foot 

 or so in thickness up to 9 or 10 feet. While they appear mas- 

 sive, some of them weather into thinner beds, but in no instance 

 thinner than 6 or 8 inches. The weathered surface of some of 

 the fine-grained beds is very smooth and appears to be due to 

 temperature changes rather than to solution. Where solution is 

 important the resulting outcrop is very rough and hackly, the 

 surface being marked by many irregular depressions. In a few 

 places stylolites, up to an inch or more in lengtli, appear in the 

 lower part of the formation. 



The microscope shows that the crystals of dolomite are an- 

 hedral, with very irregular outlines, and that they are strongly 

 interlocked. The grains all show considerable kaolin. They 

 average .3 to .5 mm. in diameter. Steidtmann^ has recently call- 

 ed attention to the fact that dolomite grains are anhedral where 

 they are in contact with each other and rhombohedral where in 

 contact with calcite. Using this as a criterion, no calcite would 

 be present in the dolomite, a fact borne out by the analyses. 



The composition of the Potosi dolomite is shown by the fol- 

 lowing analysis : 



SiO^ 2.03 



AI2O3 14 



Fe.03 ) 



FeO [ ^^ 



MgO 20.98 



CaO 30.30 



CO, 45.35 



H2O 30 



99.64 

 The Potosi dolomite contains an abundance of silica, not only 

 in the form of drusy quartz, but as chalcedony, and to a less ex- 



'Steidtmann, E., "Results of a Study of Dolomitization," Science vol 

 44, pp. 56-57. 1916. 



