48 UNIVERSITY OF MISSOURI STUDIES 



line. Likewise, the extreme local character of the sandstone beds 

 demands changes which are rather abrupt. Other evidence of 

 shallow water are mud-cracks, ripple-marks, and cross-bedding. 

 As the materials of the sandstone beds are angular and have 

 crystal faces, their source must have been near at hand. The 

 presence of the quartz with crystal faces is interesting. It may 

 have been derived from the igneous rocks of the land to the south 

 which presumably included the present area of igneous rocks in 

 southeastern Missouri. The most probable source would have 

 been the rhyolite of the latter area which contains a considerable 

 amount or more or less idiomorphic quartz as phenocrysts. The 

 Potosi may have been another possible source, the drusy quartz 

 furnishing the grains. In this case it would have been the erosion 

 of this member from previously existing areas that furnished the 

 material. An examination of the gravels and sands in small 

 streams and along the roads in areas underlain by the Potosi 

 formation showed materials very similar to those in the Gascon- 

 ade sandstone, except that the Gasconade material was better 

 sorted. The limestone and chert have an origin similar to that 

 of the dolomite described above. The chert breccias, however, 

 suggest conditions different from those indicated by the other 

 members of the formation. To the writer, their origin appears 

 very similar to that of the conglomerates of the Elvins forma- 

 tion. The steps in their production are similar, thus involving 

 conditions of shallow water. 



There is unquestionably a break between the Gasconade and 

 Roubidoux formations in this region, because wherever the lat- 

 ter is found, its base is a conglomerate, in places coarse, but, for 

 the most part, consisting of materials less than an inch in size. 

 These materials indicate considerable transportation as they are 

 well rounded. The ripple-marks and cross-bedding tell of shallow 

 water. Sufficient silica has been introduced into the rocks to 

 convert many beds, or parts of beds, into quartzite. 



The upper part of the Roubidoux formation has been re- 

 moved from this area. That the Jefferson City formation was 

 once in this region, but has been removed, is very probable. It 

 would also seem very likely that other younger formations may 

 once have covered the district. 



