676 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



Another of these intramarginal channels is indicated by the occurrence 

 of Levis graptolites in the Ouachita trough in central Arkansas and east- 

 ern Oklahoma. This channel opened into the Mississippi embayment on 

 the east, and probably extended southwestwardly from Oklahoma through 

 Texas. Its course in the latter State is, of course, unknown, being cov- 

 ered by the late Mesozoic and Tertiary formations which overlap the 

 close-folded and subsequently peneplaned basement of older rocks. 



Similar channels are indicated by graptolite shales in western Nevada, 

 but there is some doubt as to the exact age of these. The graptolite 

 fauna at Summit, Nevada, for instance, contains Phyllograptus, but 

 with it are many Diplograptidce whose alliances are with Ordovician 

 faunas rather than Canadian. Compared with graptolite zones else- 

 where, this Nevada fauna correlates best with the Diplograptiis dentatus 

 zone of Euedemann^s Deepkill section. Both may be late Canadian, but 

 on account of the preponderance of the Axonophora I am not ready to 

 say positively that they are older than basal Ordovician. However, the 

 Ashhill quarry zone in New York, likewise the upper Tetragraptus zone 

 in Arkansas, shows conclusively that the Axonophora had been developed- 

 before the close of the Canadian. 



The Ouachita shale in Arkansas. — Tlie Levis shale and fauna, as devel- 

 oped at Quebec and in the Deepkill and Schaghticoke outcrops in New 

 York, is now so well known that it is unnecessary to repeat the informa- 

 tion here. A few remarks concerning the equivalent Ouachita shale in 

 central Arkansas, however, seems desirable. The section of which it 

 forms a part has been well described by Purdue.^^^ At the base of the 

 section in the Ouachita Mountains is a dark, occasionally graphitic shale, 

 of undetermined thickness (200 feet or more), for which Purdue has sug- 

 gested the name Collier shale. This is succeeded by the massive Crystal 

 Mountain sandstone, with a maximum thickness of possibly 800 feet. 

 Although fossils have not been found in these two basal formations, I am 

 yet strongly inclined to refer them to the lower Cambrian. Next above 

 is a heavy bed of shale, about 1,000 feet in thickness, to which the name 

 Ouachita shale has been given by Purdue. At its base there is a con- 

 siderable thickness of sandstone interbedded with black and gray shale, 

 which I am inclined to regard as initial deposits of the Ouachita shale 

 rather than as the upper part of the Crystal Mountain sandstone. The 

 latter interpretation has been adopted by Purdue. 



The Ouachita shale consists for the most part of dark clay shale, occa- 

 sionally alternating with green bands. Thin layers of limestone occur 



«oi A. H. Purdue : The slates of Arkansas :. Geol. Survey of Arkansas, 1909, pp. 29-52. 



