286 STUART WELLER ' 



cases, locally at least, more shale than limestone, and some of them 

 do include minor arenaceous layers. They are called Hmestones, 

 however, because they are primarily calcareous as distinguished 

 from the alternating sandstone formations. Each of these forma- 

 tions will be considered briefly, their leading Kthologic and faunal 

 characteristics will be pointed out, as well as their geographic dis- 

 tribution in the state, and in some cases their distribution beyond 

 the limits of Illinois, in part at least. This will be followed by 

 some statements concerning the geological history of the Illinois 

 basin in Chester time, and its relations to the history of the pre- 

 ceding Iowa time. 



LOWER CHESTER GROUP 



Aux Vases sandstone. — The Aux Vases sandstone is typically 

 exposed in the Mississippi River bluffs of Randolph County, 

 Illinois, and Ste. Genevieve County, Missouri. It is the formation 

 that was called "Ferruginous sandstone" by the early Mississippi 

 valley geologists, the name Aux Vases being first used by Keyes in 

 1892,^ from the exposures in Ste. Genevieve County, Missouri, 

 near the mouth of River Aux Vases. It was the belief of Engel- 

 mann and also of Worthen that this basal sandstone in the Missis- 

 sippi River section was the exact equivalent of sandstone No. 8, 

 or Cypress sandstone of Engelmann's Johnson County section. 

 With such a correlation accepted, Keyes name would be synony- 

 mous with the earlier Cypress. In the assumption that the Aux 

 Vases-Cypress correlation was correct, the name Aux Vases was 

 abandoned in our earlier work in IlKnois. It was early recognized, 

 however, that there was a stratigraphic break within the arena- 

 ceous beds of the basal portion of the Chester series in Monroe and 

 Randolph counties, and with the belief that the name Cypress 

 covered all of these beds and that the Aux Vases was the exact 

 equivalent of the Cypress, the name Brewerville^ was used by the 

 writer for that portion of the sandstone which lies beneath the 

 break. Later, when studies in the more southern counties of 

 Illinois established the fact that the Cypress and the old "Fer- 

 ruginous sandstone" were not equivalent, and when studies 



' Bull. Geol. Soc. Amer., Vol. Ill, p. 296. 



== Trans. III. Acad. ScL, Vol. VI (1913), p. 121. 



