THE CHESTER SERIES IN ILLINOIS 287 



across the Mississippi River, in Missouri, showed that the typical 

 section of the Aux Vases was the exact equivalent of the beds for 

 which the name Brewerville had been used, the latter name was 

 abandoned and the name Aux Vases adopted for the lowest sand- 

 stone formation of the Chester series in the Mississippi River 

 section. 



In its surface outcrop this formation is restricted to a belt 

 through Monroe and Randolph counties, IlHnois, continuing into 

 Ste. Genevieve County, Missouri. The formation is a very mas- 

 sive, fine- or medium-textured sandstone in thick beds, in most 

 places more or less conspicuously cross-bedded. Its color on 

 freshly broken surfaces is a soft brown tint, in some localities 

 becoming nearly white. Not infrequently it is mottled with small, 

 dark-brown specks. On long-exposed weathered surfaces, the color 

 in most localities is a darker brown than that of freshly broken sur- 

 faces. The massiveness of the formation is well shown in the 

 Mississippi River bluffs between Prairie du Rocher and Modoc and 

 in some of the picturesque gorges which have been eroded in the 

 formation where it is crossed by stream valleys. No fossils of any 

 sort have been found in the Aux Vases sandstone in Monroe or 

 Randolph counties. 



The unconformable relations of the Aux Vases sandstone upon 

 the underlying Ste. Genevieve limestone are well shown in a num- 

 ber of places in Illinois. The uneven fine separating the two for- 

 mations can be clearly seen in the Mississippi River bluffs above 

 Modoc. Elsewhere there is an important basal conglomerate in 

 the Aux Vases, such conglomerates being well exposed two miles 

 southeast of New Design, in S.E. J, S.W. |, Sec. 28, T. 3 S., 

 R. 9 W., and again five miles southeast of Waterloo in the bluffs 

 of Rock House Creek, in S.W. \, Sec. 4, T. 3 S., R. 9 W. Still 

 another excellent exposure of the basal conglomerate, apparently 

 resting upon the St. Louis limestone rather than the Ste. Genevieve, 

 is about 6 miles west of Red Bud, in S.E. \, N.E. \, Sec. 4, T. 4 S., 

 R. 9 W. A very excellent exposure of this same basal conglomerate 

 is exposed in the Mississippi River bluffs just below McBride, 

 Perry County, Missouri. The pebbles in these conglomerates are 

 practically all chert, they are more or less angular for the most part. 



