230 J. C. Branner — Paleozoic Sediments in Arkansas. 



novaculites are about 1260 feet thick, making a total thickness 

 of 2560 feet of Silurian sediment exposed in the Ouachita 

 uplift.^ 



The lower Carboniferous beds are not represented, so far as 

 we know, in the Ouachita uplift, but the Lower Coal Measures 

 (Pennsylvanian) rocks seem to rest directly upon the Silurian 

 novaculites. 



In the Silurian area north of the Boston Mountains the 

 rocks are sandstones, cherts, magnesian limestones and a few 

 beds of shale. The lowest rocks in the Boston Mountain region 

 of which we have any knowledge are those penetrated by the 

 deep well bored near Cushman, Independence County. This 

 well gives a thickness of 1750 feet below the Izard limestone.f 



Overlying this unclassified (in Arkansas) group of sediments 

 is a bed I have called the Izard limestone, from its development 

 in the southeastern part of Izard county ; according to Pen- 

 rose this limestone has a maximum thickness of 285 feet:j: at 

 Penter's Bluff on the White River. From the manganese 

 region in Independence and Izard counties it thins out east, 

 west, north and south. 



Next above the Izard limestone is the St. Clair marble with 

 a maximum thickness of 155 feet at Penter's Bluff ; it also 

 thins out on all sides and is not known in the western part of 

 the state. 



According to Dr. H. S. Williams, who has carefully studied 

 the paleontology of these rocks, the lower part of the St. Clair 

 bed of Penrose is " about equivalent to that of the Nashville 

 group of Tennessee or the Cincinnati group of Ohio,"§ while 

 the upper part of it contains fossils " equivalent to the Waldron 

 fauna of Indiana or to the Clinton-Niagara fauna of New 

 York," and the middle is of " early Niagara." The last two 

 divisions are Upper Silurian. 



Next above the St. Clair marble is what I have called the 

 Eureka shale with a maximum thickness in Washington County 

 of 50 feet. II The age of this shale has not been definitely 

 settled. In Tennessee, Safford calls it the " Black shale " and 

 refers it provisionally to the Devonian ; Hershey thinks it is 

 the equivalent of shales of Devonian age in Iowa and Illinois.^f 



* Ana. Rep. Geol. Survey of Ark., for 1890, p. 206, and plate ITI, p. 209. 



\ Op. cit., vol. i, Manganese, by R. A. F. Penrose, Jr., pp. 1 17-118. 



t Annual report of the Geol Survey of Ark. for 1890, vol. i, Manganese, by R. 

 A. F. Penrose, Jr., Little Rock. 1891, pp. 121-122. Also marbles and other 

 limestones, vol. iv, by T. C. Hopkins, p. 108. 



§ On the age of the Manganese beds of the Batesville region of Arkansas, by 

 Henry S. Williams, this Journal, IK, 1894, xlvih, 326. ' 



II Ann. Rep. Geol. Survey of Ark. for 1888, vol. iv, Washington County, p. xiii. 



•ffThe Devonian series in southwestern Missouri, hj Oscar H. Hershey, Amer. 

 Geologist, 1895, xvi, 294-300. 



