SILURIAN. 



229 



A preliminary study of the fossils occurring in the dark basal shale shows the following 

 forms : 



DiplograptUB ep. undetermined. 

 Climacograptus ap. near typicalis. 

 LeptograptuB sp. undetermined. 

 Lingula; short, obtuse form. 

 Lingulops? sp. nov. (platform obsolete). 

 Leptobolus sp. near insignis. 



Leptobolua? sp. nov. (baa six strong radiating plications). 

 Conularia sp. nov., with surface sculpture very similar to 



that of the Trenton C. papillata Hall. 

 Conodonts of forms resembling those referred by Hinde to 



Prioniodus and Polygnathus. 



The fauna of the Sylvan shale as far as known is generally remarkably similar to that of the 

 Utica. The specific differences, however, are such as to distinguish them; yet a study of the 

 fossil nvinces one that the fauna of the Sylvan is a direct development from the much older 

 fauna of the Utica shale and not of the intervening faunas of the Cincinnati group. From this 

 evidence it would seem that the position of the Sylvan shale in the time scale is problematical. 

 The highest known fauna of the Ordovician, that of the Richmond group, occurs at the top 

 of the Viola limestone, beneath the Sylvan shale, and the Clinton occurs immediately above the 

 Sylvan, in the base of the Hunton limestone. According to the present classification, therefore, ' 

 the Sylvan shale should be correlated with the Medina, and the line, separating the Ordovician 

 and Silurian in the Arbuckle Mountains should be placed between the Viola limestone and the 

 Sylvan shale. 



The green clay shales of the Sylvan [Silurian] are succeeded abruptly by hard white to light- 

 blue limestone of the Hunton formation. Usually the contact rock at the base of the Hunton is 

 a massive bed of oolitic limestone. Locally hard bluish limestone beds are found at the base, 

 with rarely a thin layer of brecciated limestone or limestone conglomerate in contact with 

 Sylvan shale. * * * 



The Hunton formation is composed of hard, thick crystalline limestones, thin earthy lime- 

 stone, and marls. It may be separated into three fairly distinct members according to litho- 

 logic characteristics, and to a less extent according to color. Except where the formation is 

 very thin * * * the three members can always be distinguished. An average section 

 of the formation on the south side of the Arbuckle Mountains west of Washita River is approxi- 

 mately as follows: 



Section of Hunton limestone. 



Feet. 

 Upper member: Semicrystalline limestone, in places cherty, interstratified with occasional 



thin marly layers 30 



Middle member: Marly limestones and calcareous clays, with some hard limestone layers, 



more abundant in the lower part 170-190 



Lower member: Thick-bedded crystalline limestone succeeded by hard thin limestone with 



occasional marly layers. At the base of this limestone is an oolite, 4 to 5 feet thick, which 



locally is silicified 35-40 



The lithologic differences by which the members are distinguished are accompanied . by 

 faunal changes which afford a basis for age distinctions. The more massive crystalline beds in 

 the lower part of the basal member, ranging in thickness from a thin bed to 25 feet or more, 

 contain forms which are distinctly Clinton. Four Bryozoa and Tripleda ortoni are character- 

 istic of the Clinton limestone of Ohio, and their horizon occurs in the St. Clair limestone of 

 northern Arkansas. * * * 



The hard thin limestone beds in the upper part of the basal member contain fossils which 

 indicate Niagara age. * * * 



The middle member of the Hunton limestone as a whole is abundantly fossiliferous. * * * 

 There can be no question as to the Helderbergian age of this member of the Hunton formation. 



The uppermost member of the Hunton formation is probably lower Oriskany in age and 

 equivalent to the Camden chert of Tennessee. * * * 



According to the classification of the Paleozoic rocks at present in vogue, this member should 

 be called Devonian, but it is so intimately united, both faunally and structurally, with the under- 



