46 UNIVERSITY OF MISSOURI STUDIES 



the Elvins formation suggests a broad area submerged by a shal- 

 low sea. This water was probably so shallow that a slight dis- 

 turbance of the sea-level was sufficient to shift the bottom above 

 or below the water. Such exposure would permit the muds to 

 become hardened and mud-cracks to develop. These small 

 blocks resulting from the mud-cracks would then be easily broken 

 up and slightly rounded by the waves before they were com- 

 pletely buried in more calcareous muds. The conglomerate beds 

 were developed in situ from the materials of the previously exist- 

 ing beds. The bottom of the sea was undulating as is shown by 

 the lenticular beds. 



The dolomite in places may be a chemical precipitate. Fos- 

 sils are found in the lower part of the formation and there they 

 may have played an important part in furnishing a source for 

 materials. The upper part, however, with its thick beds, is more 

 suggestive of a chemical precipitate. The calcium carbonate was 

 probably first precipitated by bacterial and chemical processes, 

 and then changed to dolomite while still a soft mud. The very 

 shallow waters, as shown by ripple-marks, mud-cracks, etc., were 

 also favorable to the development of dolomite and limestone by 

 aiding bacterial and chemical changes. 



No evidence of an unconformity between the Elvins and the 

 Potosi formations was found in the area, for not a single ex- 

 posure of the contact was seen. Both formations are parallel at 

 all points where their position was determined. Others have 

 concluded that there was an unconformity at the top of the El- 

 vins formation, but this is based on rather meager evidence. 



The Potosi seas were relatively quiet and free from terri- 

 genous muds, as the formation consists of a very pure dolomite 

 thruout. Its origin was the same as that of the carbonate rocks 

 described above. The writer believes that the large amount of 

 silica, in the form of drusy quartz and chert, w^as largely deposit- 

 ed as silica at the same time the dolomite was deposited and later 

 took its present form. A very small part was probably intro- 

 duced by the solutions which deposited the barite. 



In this region the Potosi dolomite seems to have graded up 

 into the Eminence chert, if it is present, without a break. As 

 far as the evidence goes the Eminence might be considered a part 



