190 C. R. KEYES — A DEPOSITIONAL MEASURE OF UNCONFORMITY 



ceptional conditions. The great development of the Coal Measures in 

 Arkansas is not widespread, but is confined to a comparatively limited 

 area. 



The noteworthy feature in the lithology of the Arkansas Coal Meas- 

 ures is their make-up of shales and sandstones, with an almost total 

 absence of marked limestones. While this characteristic is remarkable 

 through sucli an extensive succession, it points clearly to attendant 

 physical conditions that are unmistakable, and that are now known to 

 be in perfect harmony with the historical record of other parts of the 

 region. 



RELATIONS TO THE UNDERLYING MISSISSIPPIAN 



The Lower Carboniferous formations are well understood in Arkansas. 

 It is now known that the Boone cherts are essentially the Augusta forma- 

 tion of Missouri, and are continuous with that formation as developed 

 in the southwestern part of the last-mentioned state. The widely recog- 

 nized Batesville sandstone has been proved by Weller,* without much 

 doubt, to be the equivalent of the Aux Vases sandstone of the Missis- 

 sippi River region, the basal mem])er of the Kaskaskia formation. 



It is now generally agreed that the Boston group of northwestern 

 Arkansas is the equivalent of the Kaskaskia limestone and Chester shales 

 of the Mississip})i river. Typical Kaskaskia fossils have been found in 

 the shales of this group in the extreme northwestern corner of the state f 

 and in the adjoining parts of Missouri. 



The exact line of demarkation between the Lower Carboniferous and 

 the Coal Measures has not been drawn in Arkansas. In the northwest- 

 ern part of the state, Simmons, | without giving any reasons or data for 

 deducing his conclusions, has regarded a thin, shaly limestone as the 

 topmost number of the Mississippian. As the shales beneath the Kessler 

 limestone carr}'' thin coal seams, with an abundant flora, it may be that 

 these, as well as the Kessler, may eventually prove to belong more prop- 

 erly with Coal Measures. 



In this section it is at present uncertain just where the separating 

 line between the Mississippian and Coal Measures should be placed. In 

 the Boston mountains the stratigraphic succession is api)arently un- 

 broken from the Boone chert (Augusta) upward. Above the Batesville 

 sandstone the undoubted Kaskaskia beds upward assume more and 

 more the character of the Coal Measures, and into the latter the former 



♦Trans. New York Acad. Sci., vol. xvi, 1897, p. 251. 



t American Geologist, vol. xvi, 1895, pp. 86-91. 



X Arkansas Geol. Survey, Ann. Rept,, vol. iv, 1888, p. 104. 



