iSTRATlGRAPHlC TAXONOMY bt I 



rather commonly in the lower half. The next formation is a sandstone, 

 bnt recently discovered — in a letter from Professor Purdue the name 

 Blakely sandstone is proposed for it — that is absent to the west of Wom- 

 ble, but is locally developed to a thickness of about 500 feet east of that 

 town. The discovery of this intercalated sandstone tends to confirm the 

 high value of the stratigraphic hiatus between the Ouachita and String- 

 town shales which hitherto was inferred chiefly on paleontological evi- 

 dence. 



The Stringtown shale consists mainly of soft black shale, but varies 

 greatly in thickness from place to place. It attains its maximum devel- 

 opment of 800 or 900 feet where the Blakely sandstone underlies it, but 

 at other places it may not exceed 100 or 200 feet. A limestone, some- 

 times conglomeratic and 25 to 75 feet in thickness, often occurs at the 

 base of the formation in the latter localities, its place in the formation, 

 according to the section at Crystal Springs, being about 200 feet beneath 

 the top. Beneath this limestone only shale has been observed. Another 

 calcareous zone is locally found at the top, but this even does not seem 

 °to be the last deposit of the formation, a still higher graptolite shale 

 being indicated at several localities beneath the overlying Big Fork chert. 

 Fine collections of Normanskill graptolites have been procured from the 

 StriQgtown formation. They occur in three zones, clearly distinguish- 

 able by characteristic species. The most prolific is the Nemagraptus 

 gracilis zone, which lies between the two limestones. Out of 33 species 

 found in one exposure of this zone, 23 species have been listed from the 

 same zone in the Normanskill shale of New York by Euedemann, and 

 15 from the Glenkiln shale in England by Lapworth, Elles, and Wood. 



The Stringtown is succeeded by the cherty Big Fork limestone, about 

 700 feet thick, this by the Polk Creek shale, ? to 400 feet, and next by 

 the Blaylock sandstone, to 1,000 feet or more. All of these contain 

 graptolite faunas, comparable with British associations rather than any 

 described from America. 



Two graptolite zones have been found in the Ouachita shale. The 

 first occurs in the lower half of the formation near Womble, Arkansas. 

 The species recognized are as follows: Didymograptus nitidus Hall, D. 

 extensus Hall, D. similis Hall, D. fiUformis Tullberg, Tetragraptus amii 

 Lapworth, T. approximatus Nich., T. darkei Eued., T. fruticosus Hall, 

 T. cf. pendens E. and W., T. quadribrachiatus Hall, T. serra Brong., and 

 T. similis Hall. Of these 12 species, 8 occur in the Tetragraptus zone 

 and 4 in the Didymograptus bifidus zone in New York and Quebec, while 

 10 are listed by British authorities from the "Lower Arenig'^ and "Mid- 

 dle Skiddaw" slates in Great Britain. 



