328 Williams — Age of Manganese beds of Arkansas. 



The name St. Clair limestone can be retained for only this 

 lower part lying below the manganese deposit. When 1 made 

 the identification referred to in the Report it was not clear to 

 any of us what different relations the manganese deposits bore 

 to the two limestones which I interpreted by these fossils to be 

 of different age. The reference made to the identification of 

 the upper limestone as " intermediate between Trenton and 

 Niagara" was based upon the judgment as to the age of a single 

 species, — Spirifer radiatus Sow., var. near that described by 

 Hall from the Clinton group near Louisville, Ivy.* Compari- 

 son of the specimens with it and with typical forms from the 

 Niagara suggested that the Arkansas form was of a still earlier 

 horizon than that of the specimens from Kentucky. 



3. Later studies have conclusively shown that, in all the 

 cases described of which I can get any information, the manga- 

 nese beds lie either upon this restricted St. Clair limestone, in 

 hollows in it, or in hollows in the underlying Izard limestone. 

 I have seen no specimens of this limestone containing manga- 

 nese in which there is not indication of the manganese having 

 been deposited in cracks or cavities or on the surface of fossils 

 after the limestone was formed. 



4. The second division (C), which I have named the Cason 

 shale, lies immediately upon this St. Clair limestone or some 

 lower rock, and at the Cason mine it is a stratified shaly calcare- 

 ous deposit with concretionary masses of the manganese in the 

 rock, flattened in the plane of the stratification as above stated, 

 and undoubtedly in place as originally deposited. There are 

 also, according to Dr. Wolff, rounded and angular fragments 

 of quartz, feldspar and mica in this shale. (1. c, p. 170.) In 

 the more calcareous layers a few minute fossils have been de- 

 tected, indicating the beginning of the fauna of the overlying 

 limestone. The same fauna is found in samples from the 

 O'Flinn mine. These two mines are the exceptional cases in 

 which Dr. Penrose finds the ore in place. The position, con- 

 dition and fossils of this Cason shale are evidence that the 

 original accumulation of the manganese-bearing ore was made at 

 an age later than that of the formation of the St. Clair limestone. 



5. Above the Cason shale, at the typical locality on the 

 Cason tract is seen in place a hundred or a hundred and ten 

 feet of limestone capped above by the chert ; and " the ore-bear- 

 ing stratum runs under the limestone" as described by Dr. 

 Penrose.f This limestone, which I have called the Cason 

 limestone, contains the pure eosilurian fauna, equivalent to 

 the Waldron fauna of Indiana, or to the Clinton-Niagara 

 fauna of New York, as shown by specimens collected by Mr. 



* 27th Regent's Report on State Museum, Albany, p. 196. 

 f Manganese Report, p. 220. 



