GEOLOGICAL INTRODUCTION 27 



Okaw formation. Succeeding the Ruma formation is an alternating 

 series of limestones and shales attaining a thickness of about 200 feet. 

 The shales of this Okaw formation are commonly blue or gray, rarely with 

 a slight admixture of red and blue, which is such a conspicuous feature of 

 some of the shale horizons lower in the section. The limestones vary 

 greatly in texture, some being highly crystalline, some oolitic, and others 

 rarely cherty. Their color is also variable from gray or blue to nearly 

 white. At least four conspicuous limestone horizons are present in the 

 Okaw, the highest of which is the quarry ledge at Menard in the South- 

 ern Illinois Penitentiary prison yard. 



Both the shales and the limestones of the Okaw formation are com- 

 monly fossiliferous, and in many localities abundantly so. Within the 

 formation are several more or less distinct faunal zones, although the de- 

 tails of the faunal distribution of the fossils has not yet been wholly 

 worked out. At the base of the lowermost limestone member there is 

 commonly present a zone, several feet in thickness, which is especially 

 marked by the brachiopod genus Martinia, this being the only horizon 

 from which members of this genus have been recognized. Associated 

 with Martinia, in most localities, are great numbers of bases of the crinoid 

 Agassizocrinus, along with other characteristic Chester forms. A few feet 

 higher, still in the basal limestone member of the formation, wing-like 

 plates from the ventral disk of Pterotocrinus depressus occur so abund- 

 antly in many localities as to almost entirely cover some surfaces of the 

 limestone, in fact these plates of several species of Pterotocrinus consti- 

 tute a very notable element in the faunas of the lower members of the 

 Okaw formation. Archimedes is especially abundant in the faunas of the 

 lower members of the formation, as well as a great variety of other 

 bryozoans. 



Some 60 feet above the base of the formation occurs a very constant 

 oolitic limestone member, 10 or more feet in thickness, with a fauna in 

 which many small pelecypods and gastropods are present. 



The summit beds of the formation are usually calcareous shales with 

 interbedded thin limestones, and locally a sandstone ledge 10 to 12 

 feet in thickness is present a few feet above the highest heavy ledge of 

 limestone. These shaly, upper Okaw, beds are commonly abundantly 

 fossiliferous, the large blastoid, Pentremites sulcatus, so far as it has been 

 observed by the writer, being restricted to this horizon. Another species 

 which has been found in most localities where fossils have been col- 

 lected from these higher beds, and which has been seen at no other 

 horizon, is Archimedes laxus. 



The brachiopod species Camarophoria expla.nata is present in most of the 

 Okaw faunas, and has been observed by the writer only rarely in any 

 other Chester horizon. This formation has been the source of by far the 



