Williams — Age of Manganese beds of Arkansas. 329 



Hopkins from immediately above the manganese deposit and 

 from about one hundred feet above. The fauna of this lime- 

 stone is now being studied. Over seventy species have already 

 been detected; over twenty species are identical with Waldron 

 species. Among the rare forms may be mentioned Strejotis 

 grayi, which has not, so far as I can learn, been hitherto re- 

 ported from American rocks. 



6. From this description, it will be noticed that a line of un- 

 conformity, or at least an interval of erosion and cessation of 

 deposits, separates the top of the St. Clair limestone from the 

 base of the Cason shale. 



This interpretation of the facts involves the following revis- 

 ion of the view advanced by Dr. Penrose in the Manganese 

 Report already mentioned. 



The breaking up of the St. Clair limestone of the Report 

 into three members. (A) a lower limestone of neoordovician 

 (Trenton) age for which the name St. Clair limestone in restricted 

 sense may be retained ; (B) an overlying irregular shale, con- 

 taining the deposit of manganese, separated from the former 

 by a line of erosion and unconformity, and of eosilurian (early 

 Niagara) age and here named the Cason shale ; and (C) an 

 upper member, a limestone of at least one hundred feet thick- 

 ness in the Cason tract containing a pure eosilurian fauna, 

 called the Cason limestone. The settlement of the age of the 

 original manganese deposits seems further to require the 

 abandonment of the theory that the present masses of ore are 

 derived from decay of the St. Clair limestone, because the ore, 

 when found, undisturbed in place as originally deposited, is not 

 only of later age than the St. Clair limestone, but was evidently 

 formed after that terrane had been elevated above the surface, 

 eroded and again depressed, thus separating the two by a line 

 of unconformity. The general principle set forth in the quo- 

 tation at the opening of this paper regarding the mode of 

 original accumulation of the magnanese is not questioned, 

 but the facts above cited give a simpler means of explaining 

 the accumulation in the position where it is, i. e. : the sink- 

 ing of the terrane from a position of elevation above sea level, 

 the accumulations taking place during the stage of shallow 

 waters and swamps. When the depression was greater the 

 manganese deposit ceased to appear in the deposits. 



The close resemblance between Orthis elegantula, which 

 is abundant in the Waldron fauna, and Orthis testudinaria of 

 the Trenton and Cincinnati faunas, suggests the importance of 

 stating the reasons for the belief in the ordovician age of the 

 lower limestone as seen at Polk Bayou, the Trent mine and 

 the O'Flinn mines, where it is a pink or purple marble often 

 containing manganese. 



