r.AiN.] CARBONIFEROUS ROCKS. 25 



usually be discriminated from the sandstones al)ove and below by the 

 interbedded limestones and calcareous shales, which are easily recog- 

 nized by (he connuon presence of various species of Arclthnedci^. 



Thickness. — Not enough detailed observations were made in Illi- 

 nois to allow an estimate of its thickness to be made. Mr. IJlrich 

 estimates it at 400 to 680 feet, and it can only be said that so far as 

 general impressions go this estimate seems sufficiently conservative. 



Distribution. — The Birdsville is the most widespread of the Ches- 

 ter formations in southern Illinois. It outcrops over much of Har- 

 din, Pope, and various counties to the west. On the maps accom- 

 panying this report the greater portion of the area assigned to the 

 Birdsville, Tribune, and Cypress formations, which for convenience 

 are mapped together, is really occupied by the Birdsville. Outside 

 of these areas the larger portion of the region shown on the general 

 map (PL I) as underlain by the Mississippian has the Birdsville 

 formation at the surface. The particular areas underlain by the 

 earlier formation have already been described. 



Since the Tribune limestone is not generally recognized, its absence 

 is presumably due to faulting, by which the Birdsville and Cypress 

 are. brought together. Such faults are not easily detected, since 

 rocks of similar lithologic character are brought into juxtaposition. 

 In the Rosiclare area there are probably a number of such faults 

 which are not indicated on the special map (PI. II). The sand- 

 stone in the western part of section 28, for example, is presumably 

 Cj^press, since it rests in apparent conformity upon the Ste. Genevieve, 

 and shows a normal erosion edge. In section 30 the sandstone at the 

 surface is interbedded with thin limestones, characteristic of the 

 Birdsville, and yet between the two there is neither any sign of the 

 Tribune limestone nor of a fault such as the stratigraphy calls for. 

 This raises a question as to the integrity of the Tribune formation, a 

 question which can not be settled on the basis of the evidence locally 

 available. The determination must accordingly rest upon Mr. Ulrich's 

 wider studies, and on this basis faulting would have to be inferred as 

 indicated. 



Name. — ^The name Birdsville was given to the formation by Mr. 

 Ulrich from the typical exposures near Birdsville, Ky. 



MANSFIELD SANDSTONE. 



Character. — The topmost member of the Carboniferous within the 

 district is a massive to thick-bedded sandstone, which is especially 

 characterized by the common presence of small well-rounded peb- 

 bles. These are mainl}^ of clear white quartz, but some chert is also 

 present. In the absence of the pebbles it is difficult to distinguish 

 the Mansfield from the Cypress and certain beds in the Birdsville. 



