358 CHARLES R. KEYES 



deposits of Arkansas, and especially those of the western part 

 of the state, is the prevalence of sandstones and shales above 

 the strata of the Augusta stage (Boone chert) of the Mississip- 

 pian. There are, however, in the region occupied by the Boston 

 mountains, two limestones of unusual thickness which lie at 

 levels considerably above the Augusta beds. These have been 

 called 1 the Archimedes and Pentremital limestones. They are 

 separated from each other by 40 to 75 feet of shale. The inter- 

 val between the first mentioned and the limestones beneath is 

 about 200 feet and is occupied by shales and sandstones. The 

 Pentremital limestone may be taken, with but little doubt, 

 as corresponding to the Kaskaskia limestone, of western Illinois. 

 Its stratigraphical, lithological and faunal characters all agree 

 in this respect. 2 There do not appear to be any indications of 

 a plane of unconformity at the top of this heavy Pentremital for- 

 mation, as there is at the summit of an equivalent stratum alongthe 

 Mississippi River. The Arkansas sequence of the Carboniferous 

 seems unbroken and undisturbed from base to top. The Pentre- 

 mital limestone may be therefore regarded as the uppermost 

 member of the Mississippian of the region, and the overlying 

 coal-bearing shales as the basal portion of the Coal Measures, or 

 Des Moines series, of the more northern localities. 



In regard to the upper limits of the Arkansas valley Coal 

 Measures, or of any definitely determinable horizon in the great 

 succession, there are not yet at hand any very critical data. The 

 lately issued notes of Smith 3 on the fossils collected in the region 

 by the members of the Arkansas Geological Survey give the 

 'first tangible facts that have been obtained, and by which any 

 comparison whatever can be made with the better known region 

 north of the Boston mountains. 



All who have worked in the Arkansas valley agree in 

 assigning a very great thickness to the Coal Measures of that 

 region. This is readily inferred from the writings of Chance, 4 



t Simonds, Arkansas Geol. Surv., Ann. Rept. 1888, Vol. II, p. 26, 1891. 



2 American Geologist, Vol. XVI, pp. 86-91, 1895. 3 Loc. cit. 



"Trans. American Inst. Min. Eng., Vol. XVIII, pp. 653-661, 1890. 



