24 MISSISSIPPIAN BRACHIOPODA 



Brewerville formation. The lower portion of the Chester group, as 

 recognized by Worthen, and by Engelmann, is for the most part arenace- 

 ous, and the name Cypress sandstone, as used by Engelmann, and later by 

 Ulrich, was originally applied to this lower sandstone member of the 

 gioup. 1 Recent field studies have brought out the fact that two distinct 

 formations are present in the Cypress, as it was originally defined, sep- 

 arated by a distinct unconformity and overlap. The lower of these two 

 formations is the Brewerville, and the upper, the Renault formation. 

 The Brewerville sandstone is well-exposed in the Mississippi River bluffs 

 in Brewerville and Prairie du Rocher townships, between Modoc and 

 Prairie du Rocher, in Randolph County, where the thickness of the for- 

 mation is about 80 feet. 



The formation consists of massive beds of fine-grained sandstone, 

 which as a rule exhibit much cross-bedding. In its unweathered condition 

 the sandstone is a light yellowish-brown, locally nearly white, but upon 

 the exposed ledges it is more or less reddish-brown. It is the so-called 

 "ferruginous sandstone" of some of the earlier writers upon the geology 

 of the Mississippi Valley. 



Wherever the Brewerville has been critically studied by the writer, it is 

 separated from the subjacent formations by a distinct erosion uncon- 

 formity, and rests locally upon the Ste. Genevieve and elsewhere directly 

 upon the St. Louis limestone. In much of this area a conspicuous basal 

 breccia is present at the contact of the Brewerville with the St. Louis. No 

 fossils have ever been recorded from the formation and none have ever 

 been found by the writer. 



Renault formation. The Renault formation has its typical development 

 in the valley of Horse Creek and its tributaries in the eastern portion 

 of Renault Township in Monroe County. It continues northward, where 

 it is well developed east and northeast of Waterloo. To the south it is 

 exposed in the Mississippi River bluffs below Modoc. The formation is 

 exceedingly variable in its lithologic characters and includes sandstone, 

 shale, and limestone members. Some of the sandstone members at or near 

 the top of the Renault closely resemble the Brewerville, but they are com- 

 monly thinner bedded and are associated with arenaceous shales and with 

 limestone lenses or more or less continuous strata of limestone. In many 

 localities fossil tree trunks, lepidodendroids, are present in the sand- 

 stones of this formation, while no fossils of any sort have been observed 

 in the Brewerville. Northeast of Waterloo, a conspicuous bed of varie- 

 gated red and blue or gray shale is a notable member of the formation, 

 and other less important beds of similar variegated shales occur elsewhere 



l Engelmann also recognized certain beds of limestone and sandstone in Johnson 

 County which he believed to be sub-Cypress in position, but no sub-Cypress Chester 

 was ever recognized by Worthen in the typical Randolph County area. 



