THE BARITE DEPOSITS OF MISSOURI 39 



Fork of the Fourche a Renault. It is confined to the very crests 

 of the ridges in sections 29, 30, 31, 32, and 33, T. 37 N., R. 2 E. 

 Probably residual materials from the Roubidoux are on some of 

 the other ridges, but it is difficult to be sure when the only means 

 of identifying the formation are petrological ones, and since 

 there are members in the underlying formations which are some- 

 what similar. 



Petrography. — While the Roubidoux formation in other 

 parts of Missouri consists of dolomite, sandstone, quartzite, and 

 chert, the only member found in this area is a conglomeratic 

 sandstone. No doubt much of the chert on the crests of the di- 

 vides is also residual from the Roubidoux formation for some 

 of it answers to the descriptions of the chert from that formation. 



The sandstone is medium-grained with some pebbles of 

 quartz and chert which make it conglomeratic The color is 

 mostly white with a yellowish tint, weathering to a red. Much 

 of the rock is quartzitic. 



The chert fragments in the conglomerate phases are as much 

 as two inches across, and are angular. They are embedded in 

 rounded grains of limpid quartz. 



The sandstone is cross-bedded, ripple-marked (with ripple 

 marks up to three inches from crest to crest), and exhibits many 

 mud-cracks. These features are characteristic of it wherever 

 seen. The thickness is unknown since at no place was it seen 

 in section, but it is thin. 



The formation breaks down to a sandy soil. Many large 

 boulders show concentric rings due to oxidation. Angular bould- 

 ers and fragments result from temperature effects on the sand- 

 stones and quartzites. 



Age and correlation. — From its position, and from fossils 

 found elsewhere, the formation is known to be Lower Ordo- 

 vician. Swallow called it the Second Sandstone. Nason^ in 

 1892 used the name Roubidoux for it, but was under the impres- 

 sion that it was the First Sandstone of Swallow's classification. 



^Nason, F. L., "Report on Iron Ores," Mo. Geol. Sur., vol. 2, pp. 85- 

 115, 1895. 



