330 Williams — Age of Manganese heels of Arkansas. 



At the Polk Bayou locality four collections are in my hands : 

 one made by Dr. Branner marked (74/13), second, specimens 

 collected by myself from the same cliff, with Dr. Branner and 

 Dr. Penrose (1235A),* third, a collection made by Mr. Weller 

 (1235A') from the light colored limestone below A, and B' 

 from a ledge of the same color up the creek a quarter mile, 

 and the fourth made by Mr. T. C. Hopkins. 



The purple marble (A) contains great numbers of the 

 Orthis (Dalmanella) testudinaria, which I so identify ; it 

 also resembles 0. elegantula of the ]STiagara. Davidson states 

 in his description of 0. elegantula that he has found well char- 

 acterized examples of O. elegantula in rocks of both lower 

 and upper Silurian age.f 



It also contains the following species : 



Rhynchonella capax. 



Leptwna {Plectombonites) sericea, of the typical lower Silurian 

 type. 



Orthis (Dinorthis) pectinella. 

 Strophomena (Raphinesquina) alternata. 



This is the typical manganese-stained limestone called St. 

 Clair limestone by Dr. Penrose. 



In the lower beds not so stained, but yellowish white or 

 occasionally pinkish, A' of Weller's section, are seen the same 

 Orthis, called 0. testudinaria, the same type of Zeptcena, 

 called Z. sericea, Rhynchonella capax, Strophomena alternata 

 and also unmistakable specimens of Orthis (Platystrophia) 

 biforata, of the laticosta type, and other species which taken 

 together leave no doubt as to the Ordovician (Trenton-Hudson 

 River) type of the fauna. 



In the other limestone (B of "Weller's section), are the speci- 

 mens of Rhynchonella capax and what appears to be the eye 

 and part of the cheek of a large Asaphas gigas. 



This seems to leave no question as to the age of the lower 

 limestone. 



The Rhynchonella of the " Rhynchonella beds" of the 

 O'Flinn mine is the R. capax in a similar limestone as 1235 A' 

 of the Polk Bayou section. 



The proof of the age of the upper limestone is found in the 

 following species : 



Spirifer radiatus. The type seen in the Clinton group near 

 Louisville. PI. 9, figs. 17, 18, and 19 of the 24th Regent's Re- 

 port, Albany, published first with the 27th Report. 



L. (Plectambonites) transversalis var. elegantula Hall (see 

 Foerste's fig. 6, pi. 17, vi, B. S. N. H. xxiv.) 



* Locality number of collections of the United States Geological Survey, 

 f Brit. Foss. Brae, III, p. 212. 



