420 R. M. Field— Middle Ordovician of Central 



angusticollis, I. americanus, Thaleops ovatus, Encrinurus 

 tuberculosus, Calymene senaria. 



Salona Formation. — The black argillaceous limestones 

 which follow the Rodman are decidedly different from it 

 lithologically as well as from all of the lower Stones 

 River formations. They appear to attain their maxi- 

 mum thickness at Salona but there is evidence that they 

 also thicken slightly to the east. Although this forma- 

 tion contains a few characteristic fossils such as Conula- 

 ria trentonensis, Endoceras proteiforme elongatum, etc.. 

 it is principally characterized by Brongiartiella trenton- 

 ensis, another species of the same genus and several 

 new species of Isotelus. The lowest beds of the Salona 

 contain Cryptolithus tessellatus, and its upper beds bear 

 large symmetrical ripple -marks with an amplitude of 



Fig. 3. 



A 



Center Hatt 



"Hodman 



VcUenV>ne lagoon 



Carl! 



from one and one-half to two feet. It is closed in all 

 sections by several feet of finely cross-bedded limestone, 

 exceedingly poor in fossils. The Salona undoubtedly 

 belongs to the Trenton group, but since it is decidedly 

 different from the superjacent limestone both faunally 

 and lithologically, and is in fact a phase peculiar to the 

 central Pennsylvanian province alone, the writer has 

 decided to give it a distinct formational name. Salona 

 is a village in the Mill Hall gap and opposite the best 

 exposure of the Trenton in the whole province. 



Coburn Formation. — The upper Trenton limestone is 

 both faunally and lithologically different from the lower 

 Trenton or Salona. It has thus been recognized as a 

 distinct formation and named after the town of Coburn 

 whose location has already been described. This forma- 

 tion begins in all sections, where exposed, with a well- 

 defined zone of Parastrophia hemiplicata. The lower and 



