22 MISSISSIPPIAN BRACHIOPODA 



In Illinois the Ste. Genevieve limestone has a notable development in 

 the valley of Fountain Creek and its tributaries in Monroe County, and 

 extends northward as far as Alton, where it caps the Mississippi River 

 bluffs. Worthen. included all of these beds in the upper portion of the 

 St. Louis limestone. 



The name Ste. Genevieve has been revived in recent years by Ulrich, 1 

 in his study of the Mississippian section in western Kentucky, where he 

 has recognized three members of the formation, the Fredonia oolite, the 

 Rosiclare sandstone, and the Ohara limestone. This author's criterion 

 for the recognition of the formation seems rather to be a lithologic suc- 

 cession, limestone below and above with an intermediate sandstone, than 

 any faunal characteristics. In some localities the beds referred by him 

 to the Ohara carry a characteristically Chester faunal assemblage, while 

 at other points the Ohara fauna lacks these Chester characteristics and 

 is distinctly older in general complexion. Because of this uncertainty 

 as to the correctness of Ulrich 's interpretation of the Ste. Genevieve, 

 his subordinate member or formation names, Fredonia, Rosiclare and 

 Ohara are not adopted in this place, although in certain sections in Mon- 

 roe County, Illinois, there is a sandstone stratum with limestone both above 

 and below it in the Ste. Genevieve. According to Ulrich the maximum 

 thickness of his Ste. Genevieve in Kentucky is at least 245 feet, but the 

 greatest measured section in Monroe County, Illinois, is about 75 feet, and 

 it probably nowhere exceeds 100 feet. The thickness of the formation in its 

 typical localities near Ste. Genevieve, Missouri, probably does not exceed 

 that in Monroe County, Illinois. It is believed by the writer that some 

 room for doubt exists as to the correctness of Ulrich 's correlation of some 

 of the beds in the extreme southern counties of Illinois which have been 

 referred to the Ste. Geneveive limestone by him, and the description of 

 the formation herein given applies only to its typical Mississippi Valley 

 expression. 



The Ste. Genevieve includes certain remarkably cross-bedded lime- 

 stones which are commonly more or less arenaceous; some of the most 

 conspicuously oolitic limestones in our entire Mississippian section; and 

 also some beds which are wholly arenaceous. In the bluffs at Alton about 

 48 feet of limestone and more or less arenaceous strata are referable to' 

 this formation. The thickness increases to the south, for about 75 feet 

 was actually observed in Monroe County, and a maximum thickness of 

 100 feet may be present. In Monroe County, where the formation is best 

 known by the writer, the basal portion comprises the remarkably cross- 

 bedded, more or less arenaceous limestones above mentioned. On Andy's 

 Creek, in this county, a conspicuous 12-foot sandstone stratum may be 

 seen lying about 20 feet above the lowermost observed beds of Ste. Gene- 



ITJ. S. Geol. Survey, Prof. Paper, No. 36, p. 39 (1905). 



