26 FLUORSPAR DEPOSITS OF SOUTHERN ILLINOIS. I bdll. 255. 



It is (][iiite possible that outliers of it have been overlooked. It very 

 coninioiily carries small flalces of inica, and these are rare, at least 

 in the other sandstones. This fact is of material assistance in recog- 

 nizing it. The formation seems to rest nnconformably on the Birds- 

 ville. 



Distril>utio7i. — The Mansfield sandstone outcrops around the north- 

 ern and eastern sides of the district, and its edge practically marks 

 the limit of the productive territory. It rises in sharp bluffs, form- 

 ing Karbers Eidge, as already discussed. South of this escarpment 

 there are a number of outliers formed of fault blocks let down into 

 the older formations. A few of them are indicated on the general 

 map. No attempt has been made to map all of them, and the Mans- 

 field has not been especially studied in the course of this work, and 

 no estimate of its thickness was made. 



Ncmie and correlation. — The name used here for these beds is that 

 adopted by the Indiana geological survey. Formerlv the beds were 

 commonly referred to as the Conglomerate measures or the Millstone 

 grit. The Mansfield forms the base of the Pennsylvanian series of 

 coal measures and is regarded as of late Potts ville age. 



TERTIARY DEPOSITS. 



In the southern part of Pope County, including the highland south 

 of Bay Bottoms, scattered pebbles and thin beds of gravel make their 

 appearance on top of the Carboniferous formations. The pebbles are 

 well i-ounded and seemingly waterworn. They vary in size from a 

 half inch to two inches in diameter and exhibit consideral)le hetero- 

 geneity as to both composition and material. Pieces of quartz, qiiartz- 

 ite, sandstone, and chert are common. The chert frequently contains 

 fragments of fossils, so far as observed, of common Carboniferous 

 species. The pebbles were not observed elsewhere in the district, 

 but near the old shaft immediately east of the main working shaft 

 of the llosiclare mine there are numerous similar pebbles scattered 

 over the sui-face. Among them was one rounded and apparently 

 waterworn pebble of galena showing quartz crystals in a small druse. 

 It is possible that these represent remnants of a former extension of 

 the formation. In general all the pebbles observed are such as might 

 have been derived from the destruction of local formations. 



No special stud}' of these gravels was made, nor were the green 

 sands, said by Worthen to accompany them in Pulaski Comity." 

 examined. Worthen determined these beds to be of Tertiary age. 

 Mr. McGee ^ has referred them more specifically to the Lafayette 

 formation. 



° Geol. Survey Illinois, vol. 1, 1866, pp. 44-47. 



6 Twelfth Ann. Rept. U. S. Geol. Survey, pt. 1, 1891, p. 469. 



