276 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



ieckei, Strophonella punctulifera, and Orthoihetes woolworthanus, common; Dalmanites micrurus, 

 not rare; Uncinulus schueherii, very scarce); fossiliferous clay, 15 feet; overlain by thin lime- 

 stone layers, 1 J feet, full of bryozoans. Upon these lies„a sandy bed, which may be the Hardin 

 sandstone member, the Camden chert being absent. 



In other local sections noted by Foerste the Linden varies in thickness, being 101 

 feet at Pyburn Landing, on the Tennessee, and thinning rapidly eastward. It is 

 succeeded in some places by the Camden chert, in others by the Hardin sandstone 

 member. 



The Camden chert was first distinguished by Safford, who recognized the 

 Oriskany facies of its fauna. It is 60 feet thick at Camden, Benton County ; is strongly 

 developed west of Tennessee River in Henry, Benton, and Decastur counties; and 

 thins toward the easi:. The Camden fauna was collected and studied by Schuchert,'"^ 

 who states: 



The fossils of this formation are, as a rule, natural casts both of the interior and exterior of 

 the organism, and preserve in detail the finest markings. This fauna is closely related to that 

 described by Meek and Worthen" from the "Clear Creek hmestone" of southern Illinois, in 

 Alexander, Jackson, and Union counties. From this region are known but 11 species, and 8 of 

 these are also found in Tennessee. They are Anoplia nucleata, AnoplotJieca flabeUites, Eatonia 

 pecularis, Spirifer worthenanus, S. Jiemicyclus, Megalanteris condoni, AmpMgenia curia, and 

 StropTiostylus cancellatus. 



The "Clear Creek limestone" of Illinois is intimately connected with the Helderbergian 

 below and is not less than 200 feet thick, being followed by a "quartzose sandstone" from 40 to 

 60 feet in depth. The latter is probably equivalent to the Upper or typical Oriskany of New 

 York and does not appear to be present in western Tennessee. From Prof. Safford's description 

 of the Camden chert, it is evident that the Lower Oriskany thins rapidly southward. In Tennessee 

 it is about 60 feet in thickness, while it is not less than 200 feet thick in Illinois, exclusive of the 

 Upper Oriskany, which is entirely absent in the former State. 



The Camden chert fauna contains 32 species, and 6 of these are restricted to southern 

 Illinois and western Tennessee. Of the entire fauna, 24 species are found either in the Helder- 

 bergian or in the Lower Oriskany of other regions, and 20 occur in the Upper Oriskany, or 

 Onondaga. After removing the 13 species common to both the Lower and Upper Oriskany 

 and the 2 restricted forms, 17 remain. Of these 10 occur either in Helderbergian or Lower 

 Oriskany rocks of other regions, while 6 are found in higher beds. This evidence, therefore, indi- 

 cates clearly a Lower Oriskany age for the Camden chert of Tennessee and the "Clear Creek 

 hmestone" of Ilhnois, which indication is the more marked because of the absence of such 

 characteristic Upper Oriskany species as Hipparionyx prorlmus, ChonostropJiia complanata, 

 Spirifer arenosus, Rensselaeria ovoides, MeristeUa lata, CamarotoecJiia pleiopleura, G. harrandd, 

 or 0. speciosa. 



Concerning the representative of the Onondaga in western Tennessee, Foerste ^*°° 

 says in part : 



On the western flank of the Cincinnati geanticline, along Harpeth River, between Newsom 

 and the bridge west of Pegram, the Silurian is directly overlain by a thin bed of Devonian lime- 

 stone, varying in thickness from 12 feet at the west to 3 feet at the most eastern point of outcrop. 

 * * * Nucleocrinus (Olivanites) vemeuili is a characteristic species. StropJieodonta demissa, 

 StropJieodonta perplana, RTiipodomella penelope, and Nudeospira condnna are also found. This 

 white and comparatively pure limestome is overlain by a darker and more sandy limestone. 



" Geol. Survey Illinois, vols. 1, 2, and 3. 



