GEOLOGICAL INTRODUCTION 25 



in it. The thickness of the Renault varies from 40 feet or less to a maxi- 

 mum of 100 feet or more. 



The Renault formation commonly overlaps the subjacent Brewerville to 

 the west and rests directly upon the St. Louis or Ste. G-enevieve lime- 

 stone in unconformable contact, and this unconformity is also present 

 between the Renault and the Brewerville itself. On Hickman Creek, north- 

 east of Columbia, the Renault rests upon the Ste. Genevieve with a con- 

 spicuous basal conglomerate, and elsewhere in the same general region it 

 rests upon the Brewerville with a basal conglomerate, mainly of limestone 

 pebbles, but with some pebbles of crystalline rock. Eastward from the 

 western margin of the Renault, away from the shore-line of the period, 

 the limestone members of the formation become more and more con- 

 spicuous. 



In some of the limestones and associated calcareous shales in the 

 Renault, fossils are exceedingly abundant, the facies of the fauna being 

 typically "Chester." The bryozoan genus Archimedes, however, is as 

 a rule sparsely represented in the faunas and is in many places wholly 

 absent, although in at least one locality it has been observed in abundance. 

 The genus Lyropora, on the other hand, is in many localities exceedingly 

 abundant. The crinoid fauna of these beds is the most varied and best 

 preserved of any observed in the Chester group, Talarocrinus being one 

 of the commonest genera of these organisms. 



Yankeetown formation. Overlying the Renault formation is a thin but 

 most persistent siliceous formation of peculiar lithologic character, locally 

 quartzitic. Although this bed is entirely unfossiliferous, so far as it has 

 been observed, it has served as a key formation in the interpretation of the 

 Chester succession to a greater extent than any other formational unit. 

 Some of its most typical exposures may be seen in the region adjacent 

 to the Yankeetown school, about 6 miles southwest of Red Bud, from which 

 locality the name of the formation has been derived, but it occurs as well 

 in thse extreme north of the area of the outcrop of the "Chester", on 

 Hickman Creek, northeast of Columbia, and near Millstadt. It is also 

 well exposed in the Mississippi River bluff section one and one-half miles 

 below Modoc, and at many intervening localities. 



The thickness of the Yankeetown is probably nowhere over 20 feet, 

 and commonly is less. Its color is light, commonly gray or yellowish, 

 or in many localities nearly white. It is very irregularly, and more 

 or less cross-bedded, having a decidedly knotty appearance, and locally is 

 distinctly banded. It is commonly more or less arenaceous and in some 

 localities certain beds are quartzitic. "Where the formation is encountered 

 in wells it appears to be a very hard, siliceous limestone, and in dug wells 

 it usually puts an end to any further excavation. The Yankeetown is so 

 hard and resistant that over considerable areas the superjacent beds were 



