288 STUART WELLER 



and were clearly derived from the underlying Ste. Genevieve or 

 St. Louis limestones. 



The presence of these conglomerate beds establishes the fact 

 that subsequent to the deposition of the Ste. Genevieve limestone, 

 the calcareous sediments hardened into limestones, the cherts 

 which are clearly secondary in origin were formed, and were in 

 essentially the same condition in which they are found today. 

 Then an erosion period set in and in places the entire thickness 

 of the Ste. Genevieve limestone was removed, along with a part of- 

 the St. Louis limestone. The subsequent sedimentation laid down 

 the sands of the Aux Vases formation. This interruption in the 

 deposition of the sediments of the Mississippi Valley section must 

 have represented a considerable length of time, and it must be 

 reckoned as an important break in Mississippian history. Other 

 phenomena connected with this sedimentary break will be dis- 

 cussed later, in connection with the geological history. 



Beyond Monroe and Randolph counties, to the south, the Aux 

 Vases sandstone has not been certainly identified. There is, how- 

 ever, a flaggy sandstone, about 20 feet thick, in the base of the 

 Chester section east of Anna, in Union County, Illinois, which 

 may be an extension of the Aux Vases, but in view of the fact that 

 this sandstone contains numerous fossils in some beds, while the 

 Aux Vases is quite barren of fossils, and further that sandstone 

 beds are commonly present in the Renault formation of Monroe 

 and Randolph counties, it is possible that this Union county sand- 

 stone may be younger than any part of the Aux Vases, and is per- 

 haps referable to the Renault. 



The maximum thickness of the Aux Vases sandstone is about 

 75 or 80 feet, and it varies from this amount to nothing at all, for 

 in places the overlying Renault formation overlaps the Aux Vases 

 and rests upon the underlying Ste. Genevieve limestone. In the 

 more southern counties of Illinois, east from Union County, the 

 position of the Aux Vases sandstone in the section is represented by 

 an unconformity in the midst of the so-called "Ohara Kmestone 

 member" of the Ste. Genevieve limestone, as described by Ulrich. 



Renault limestone. — It would perhaps be better to call this unit 

 the Renault formation, for in addition to its Hmestone content it 



