302 STUART WELLER 



places is as much as lo feet thick, elsewhere being wanting alto- 

 gether. This layer is best exhibited in the vicinity of Chester, in 

 the outside prison quarry at Menard, between Menard and Ches- 

 ter, and just below Cole's mill in Chester. This sandy layer in the 

 Okaw is undoubtedly the attenuated margin of the Hardinsburg 

 sandstone which has its greatest thickness in the southeastern part 

 of the state, for the limestone beds above it possess many faunal 

 characters which unite them with the limestone formation overly- 

 ing the Hardinsburg in the southern counties. 



Glen Dean limestone. — The Glen Dean limestone is another 

 formation that has been named by Butts from exposures in Ken- 

 tucky.^ In the southern counties of Illinois the formation resembles 

 the Golconda in general character, being composed of interbedded 

 limestone and shale layers, but in most localities the proportional 

 amount of shale is much greater in the Glen Dean, in places nearly 

 the whole of the formation being shale. Many of the limestone 

 beds in the formation are similar lithologically to those of the 

 Golconda, being gray in color and crystalline in texture for the 

 most part, but locally certain of the layers are somewhat more 

 dense and compact. 



The Glen Dean has certain faunal characters that differentiate 

 it rather sharply from the Golconda. One of the best index fossils 

 is a species of bryozoan, Prismopora serrulata. Examples of this 

 species are triangular in cross-section, with three faces bearing 

 zooecia, these prismatic zoaria dividing at intervals. This bryo- 

 zoan is not entirely confined to the Glen Dean, for it has been 

 observed rarely in the Golconda, and is not uncommon in the 

 Vienna limestone, still higher than the Glen Dean, but it is far 

 more common in the Glen Dean than elsewhere, and in places 

 some of the limestone ledges of this formation are veritable Pris- 

 mop or a gardens. Pentremites spicatus is another characteristic 

 form, which has not been observed outside of this formation, but 

 it is far less common than the Prismopora. A number of other 

 bryozoans and some other fossil forms are more or less conspicuous 

 in this formation, which are nearly everywhere or entirely un- 

 known from other Chester horizons. 



' Miss. Form. W. Ky. (1917), p. 97. 



