326 Williams — Age of Manganese beds of A rkansas. 



The age of the St. Clair limestone is described as interme- 

 diate between the Trenton and Niagara, based upon the deter- 

 mination of fossils made by me. Referring to these identifi- 

 cations Dr. Penrose says : 



Fossils collected on Polk Bayou four miles north of Batesville 

 have been determined by him as undoubted Trenton forms, while 

 fossils from St. Clair Springs, eight miles northeast of Batesville, 

 and from elsewhere in the country to the west, are considered by 

 him to be intermediate between those of Trenton and Niagara. 

 (I.e., p. 114). 



This quotation indicates the part I had contributed, at the 

 time the Report was written, toward determining the age of 

 the deposits in question. 



I differed with Dr. Penrose and the geologists of the Arkan- 

 sas Survey in holding the opinion that two distinct limestones 

 were confused under the name St. Clair limestone. 



Since the year 1890 I have had considerable correspondence 

 with the director of the survey Dr. Branner, and with Dr. Pen- 

 rose regarding this perplexing question ; and about a year ago 

 in order to make certain that the collections were from typical 

 localities Mr. T. C. Hopkins, who was stil! in Arkansas, col- 

 lected for us a set of test samples from some of the mines par- 

 ticularly described in the Report. These I examined, and was 

 able as the result of a preliminary study to report about a year 

 ago that all of the limestone samples from below the manga- 

 nese beds were of Trenton age or older ; and that all the lime- 

 stone described as St. Clair, but lying above the Manganese 

 beds, was of the age of the Clinton-Niagara formation ; and 

 that in neither limestone was there confusion of the two 

 faunas. Mr. Hopkins referred to this decision in his Report 

 on Marbles,* and other references have been made to my inter- 

 pretation, but I have not, hitherto, published the evidence 

 upon which it was based. 



I have recently re-examined the whole evidence with the 

 hope of making a more exact determination of the case, and 

 have succeeded to the following extent. 



1. The highest formation underlying the Batesville manga- 

 nese beds is a limestone containing a fauna of neoordovician 

 age, about equivalent to that of the Nashville group of Ten- 

 nessee or the Cincinnati group of Ohio. This is the lower part 

 of the St. Clair limestone of Penrose and may well retain the 

 name. 



2. The formation immediately overlying the manganese bed, 

 where the rocks are in their original position, contains a fauna 



* Ann. Eept. Geol. Surv. Ark. for 1890, vol. iv, Marbles and other Limestones. 

 By T. C. Hopkins. 1S93, pp. 213 and 225. 



