THE BARITE DEPOSITS OF MISSOURI 53 



are common. As in the other formations it is found consistently 

 along certain horizons and in any of its numerous modes of oc- 

 currence. The sandstone is in beds up to two feet or more in 

 thickness. The texture is fine to medium-grained, with occa- 

 sional conglomeratic phases. The quartz grains are well- 

 rounded, as a rule. The conglomeratic phases contains many well- 

 rounded fragments of chert. The sandstones are cemeted with 

 silica, dolomite, and rarely with iron oxides. The quartz grains are 

 covered with a coat of chalcedony, giving the beds an oolitic tex- 

 ture. Ripple-marks, cross-bedding, and sun-cracks are common 

 features. Sun-cracks are found also in the dolomite. The for- 

 mation is evidently a shallow-water deposit. 



The structure of these beds is the same as that of the Gas- 

 conade below. All the beds are essentially horizontal with a gen- 

 eral northward dip, and are broken occasionally by small faults. 



The Gasconade grades upward into the Roubidoux without 

 a break, so far as known. 



The Jefferson City formation. — The Jefferson City forma- 

 tion is from 200 to 250 feet thick. It consists dominantly of 

 dolomite; contains chert nodules; and in places is interstratified 

 with thin beds of chert, sandstone, and shale. The dolomite is 

 (1) very hard, dense, and fine-grained; or (2) soft, argillaceous, 

 and arenaceous, as in the Cotton Rock; or (3) coarse, vesicular, 

 and hackly. The Cotton Rock is very fine-grained and dense but 

 relatively soft. It has an earthy texture and usually breaks with 

 a conchoidal fracture. The color is white, gray, yellow, or buff. 

 Most of the beds are only a few inches in thickness, and the 

 weathered surfaces are very thin. The hackly, pitted dolomite is 

 very uniform and persistent over most of the district. It is in 

 beds five or six feet thick. The dense phase is represented by a 

 few beds. 



The chert is much less abundant than in the formation be- 

 low. It is in nodules, irregular masses, and beds. The beds are 

 rarely as much as two feet in thickness and most of them are 

 only three to six inches thick. The colors are the same as in the 

 other formations. Oolitic and brecciated chert are occasionally 

 found. As a rule, the nodular variety which is abundant in this 

 formation is associated with the Cotton Rock. The nodules are 



