412 



INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



where they are weathered, appear sandy. Where the Moorefield shale is absent this formation 

 rests directly on the Boone formation and is usually found in this position between Marshall 

 and Fayetteville. The thickness of the formation in the Yellville quadrangle is about 75 feet. 

 The conspicuous portion of it is an even-bedded yellowish-brown sandstone. 



The conspicuous portion of the Fayetteville shale is a bed of black carbonaceous shale, 

 which is usually thinly laminated. It frequently exhibits a jointed structure and upon disin- 

 tegration falls into small fragments. Its thickness varies from about 50 feet up to 200 feet. 

 It is named from exposures around Fayetteville, where it is conspicuous in the valley of the 

 West Fork of White River. This formation rests upon the Batesville sandstone or, where 

 that formation is absent, upon the Boone formation. The lower portion of the black shale in 

 the Fayetteville formation contains lenses of dark-blue siliceous limestones at some places. 



Southeast of Fayetteville and westward to the State line the Fayetteville shale is succeeded 

 by a sandstone formation varying in importance but usually having a thickness of between 50 

 and 150 feet. It is thin bedded and heavy bedded and carries some interstratified shale. 



Succeeding the Fayetteville formation in the lead and zinc region there is nearly every- 

 where a bed of limestone. It is not always conspicuous, since it frequently occurs on slopes 

 and is covered with debris of sandstones and shales from the higher formation. Its thickness 

 where well developed is usually from 10 to 40 feet. In exceptional cases it is thicker. It 

 sometimes has associated with it sandstone beds which contain the same fossils found in the 

 limestone. The Pitkin limestone has been called the "Archimedes limestone" by the Arkansas 

 Survey, because of the presence of this easily recognized bryozoan, the screwlike stems of 

 which are seen in the weathered surface of the rock. In the northwestern part of the State, 

 around Fayetteville, the limestone thins out to a pebbly stratum and disappears from the 

 section. Along the north front of the Boston Mountains, however, it is a very persistent ledge, 

 and the beds are frequently massive and give rise to an escarpment, a bench being developed 

 on its upper surface. This formation is regarded as the highest one of the Mississippian series. 

 It is probable that the thinning to the north and west is due to shore conditions, and its absence 

 in some cases may be due to erosion previous to the deposition of the Pennsylvanian series 



which succeeds it. 



J-K 16. KENTUCKY AND INDIANA. 



Along the west side of the Cincinnati axis the Mississippian presents a section 

 similar to that of the series along Mississippi River. At the base the strata He 

 conformably upon the Devonian and are somewhat sandy and shaly. The bulk 

 of the deposit is limestone. The top part, which includes sandstone,- is limited 

 above by an unconformity. The local section for Indiana, as given by Blatchley 



and Ashley 

 follows : 



and substantially quoted by Chamberlin and Salisbury, ^^*'' is as 

 Generalized section for Indiana. 



