632 E. O. ULRICTI REVISION OV TTIE PALEOZOIC SYSTEMS 



south of Roaring Spring, Pennsylvania. Apparently it occurs also in 

 Iowa. Finally, a very good representation of the Gasconade fauna has 

 been collected from the Chepultepec chert in the vicinity of Chepultepec, 

 Alabama. Evidently this sea was nearly as large as the preceding Emi- 

 nence submergence. 



Jioubidoux formation. — It is 70 to porliaps 225 feet in thickness, and 

 succeeds the Gasconade. Probably a considerable hiatus separates tlie 

 two. In some places at least evident unconformity exists ; in others 

 the break is not very clearly indicated. The formation consists of alter- 

 nating beds of sandstone, quartzite, conglomeratic and oolitic chert, mas- 

 sive chert, dolomite, and shale, all exceedingly variable in thickness and 

 areal extent. The thinner sandstones are often ripple-marked and sun- 

 cracked. The proportion of clastic matter in the Eoubidoux is greater in 

 the northwestern half of the Ozarkian area than in the southeastern, but 

 the siliceous components — in the form of sandstone and chert — are every- 

 where abundant. The cherts look much like those in the Gasconade, and 

 were it not for the beds of sandstone and conglomerate associated with 

 them it would often be practically impossible to distinguish the two for- 

 mations. Fossils are very rare in the Roubidoux, and none that could be 

 determined were collected. 



Jeiferson City dolomite. — The Roubidoux seems to be merely the intro- 

 ductory stage of this formation which is the last of the Ozarkian forma- 

 tions in Missouri. It consists mainly of two kinds of dolomite, tlie fine- 

 grained, argillaceous, earthy-textured, relatively soft, white to buif or 

 gray form known as "cotton rock," and the more massive, medium- 

 grained variety weathering hackly on the surface. The two are inter- 

 bedded with each other and with thinner beds of sandstone, shale, and 

 more or less chert. A large part of the chert is in the form of silicified 

 masses of Cryptozoon minnesotensis, which species is characteristic of 

 the formation in Missouri and usually very common. Other fossils are 

 rare, especially in the middle and lower portions. In thickness the for- 

 mation varies from less than 100 to 200 feet in the eastern and southern 

 parts. This variation may be largely due to Paleozoic erosion. At any 

 rate, the formation seems to be thickest in northern Arkansas, where it is 

 locally overlain by the Yellville formation of the Canadian system. The 

 contact with this Canadian formation is almost as clearly unconformable 

 as when some later Ordovician to Pennsylvanian deposit rests on it. 



At many places in Missouri and Arkansas, but so far as known only 

 on the southern and western sides of the uplift, very fossiliferous cherty 

 beds are found overlying the Cryptozoon-bearing typical Jefferson City. 

 At two or three localities evidence of unconformable relations between 





