662 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



uniting the Cambrian on the one side with the Ordovician on the other. 

 But with advancing knowledge the difficulties have largely disappeared, 

 and it is now not only possible but incumbent on us to subdivide some 

 of the two comprehensive old formational units. In the case of the 

 Arbuckle, as partly stated on page 641, it is divisible into four well 

 defined parts, the lowest of which may be Ozarkian, but is more likely 

 to prove late upper Cambrian. All of these four parts are further 

 divisible into lithologic members and faunal zones. 



The second of these major divisions of the Arbuckle limestone con- 

 sists of about 4,300 feet of massive pure and more or less magnesian 

 limestone. The highly magnesian beds occur chiefly in the lower 2,000 

 feet of this great mass of sediments. Moreover, fossils seem to be entirely 

 absent in this lower member in the typical section of the Arbuckle, 

 though to the west, in the limestone areas flanking the Wichita Moun- 

 tains, a considerable fauna has been collected from beds apparently cor- 

 responding to this member. However, it is possible that this member 

 is absent there and that the fossils come from some bed of the overlying 

 member. 



The remaining 2,300 feet of the second division consist mainly of 

 fairly pure, fine-grained limestone. Fossils occur very sparingly, having 

 been observed at only three horizons, the first at the base, the second 

 some 1,200 feet above the first, the third about 600 feet above the second. 

 The first and second fossil-bearing ledges contain shells of Maclurea- 

 like gastropods, which show in cross-section in the face of the rock. 

 Though specifically undeterminable, they are evidently related to the 

 widely umbilicated, slender-whorled types, which seem to be confined 

 to Canadian rocks. Another horizon, apparently between the first and 

 second, afforded numerous specimens of Ophileta complanataf; and 

 another, apparently near the first horizon, but probably near the second, 

 contained Ceratopea keithi. The third horizon contains a species of 

 Eccyliopterus related, to E. volutatus. 



The third division is 700 feet thick and characterized by relatively 

 thin-bedded limestones and frequency of fossiliferous ledges. The upper 

 200 feet are all thin-bedded and in part even shaly. Occasional thin 

 layers of chert and maybe a few inches of sandstone are seen in the 

 lower two-thirds. Chert occurs also in the underlying heavy bedded 

 division, but is never a conspicuous feature in the Arbuckle limestones, 

 except where this third division forms the top of the series and is fol- 

 fowed by massive sandstone of the Ordovician Simpson formation. 



The commonest fossils of the third division are the articulate brachio- 

 pods, of which eight species have been distinguished. Two of these 



