28 ' MISSISSIPPIAN BRACHIOPODA 



greater number of species which have been described by the various 

 writers from the ''Chester" of Illinois, and a full faunal list of the forma- 

 tion would include nearly all of the Chester or Kaskaskia species in our 

 literature. 



The various members of the Okaw formation are well exposed in the 

 Mississippi River bluffs above and below the tributary Okaw or Kaskaskia 

 valley, this valley being entirely excavated from the rocks of this forma- 

 tion at its point of juncture with the valley of the Mississippi, hence the 

 name here adopted for the formation. 



Menard formation. The Menard is a conspicuous formation of lime- 

 stone with interbedded shales, well exposed in the middle portion 

 of the bluffs at Chester. One of its best exposures is immediately south- 

 east of the hospital for criminal insane at Menard. In its typical expres- 

 sion, this formation is a thin and moderately thick-bedded limestone, the 

 bedding planes being undulating and hummocky in character, with thin, 

 shaly partings. In places these shaly partings become thicker, and shale 

 beds of as much as five feet or more in thickness are present. The basal 

 portion of the formation, where it is exposed, is seen to be shale, as much 

 as 35 feet of fine, blue, clay shale being present in some localities between 

 the top of the Okaw and the typical limestone beds of the Menard. The 

 lithologic character of the limestone of the formation differentiates the 

 Menard rather sharply from most of the limestone strata of the 

 Okaw. The limestones of the lower formation are commonly more or less 

 crystalline or granular, often crinoidal, sometimes oolitic, and usually 

 free from chert. In the Menard the limestones are nearly always 

 close-textured, fine-grained rocks, and there may be a small amount of 

 chert ; they are brittle, and often exhibit a conchoidal fracture. Because 

 of the difference in texture, the weathered surfaces of the Menard are 

 commonly smooth, those of the Okaw usually being more uneven. The 

 color of freshly broken surfaces of the Menard is usually a bluish gray, 

 while that of the Okaw limestone is commonly lighter, some beds being 

 nearly white. Locally, there are crystalline strata in the Menard which 

 closely resemble certain of the Okaw beds, but such strata are always of 

 limited thickness and usually occur in the higher portion of the forma- 

 tion. The thickness of the Menard is about 80 feet. It is well exposed 

 in the Mississippi River bluffs from Chester to Rockwood, and the valley 

 of Mary's River, at its mouth, is excavated entirely through this forma- 

 tion into the higher beds of the Okaw. 



The fauna of the Menard is very different from that of the Okaw. 

 One of the most conspicuous species is a large pelecypod of the genus 

 Sidcatopinna. This is associated with several other species of pelecypods, 

 and with the large, typical forms of Composita subquadrata and Spirifer in- 

 crebescens. Pentremites and Archimedes are not common in the formation 



