SILURIAN. 239 



Rondout formation: Shales and earthy limestones; sparsely fossiliferous; name restricted Feet. 



to the upper cement bed of the section at Rondout 39 



Maniius limestone: Thin bedded, knotty, dark blue to black; fauna limited, exhibits 

 from base to summit a passage from life conditions suitable only for Leperditia to normal 

 marine conditions; gradual transition from Rondout faunally and stratigraphically; 

 pronounced faunal break in passing to the overlying Coeymans (basal Helderbergian) 

 Devonian limestone 35 



4,424 



The Silurian rocks described by Weller extend into Pennsylvania, but with 

 changes which are described in great detail in the county reports of the Second 

 Geological Survey of Pennsylvania under the names Lewistown limestone, Blooms- 

 burg shale, and Bossardville limestone. The facts are assembled by Lesley.^^^ A 

 typical section, which shows the great thickness of strata and their intimate litho- 

 logic similarities or gradual changes, is that of Logan Gap, Miflflin County, in the 

 central part of the State. The descending section is summarized by Lesley ^^^* as 

 follows : 



Feet, 

 j. Salina gray shale 164 



k. Salina variegated shale 

 1. Logan limestone 

 m. liOgan shale 



423 feet. 



348 



^ 



72 



n. Salina upper red shale 432 



o. Salina upper lime shale 326 



p. Salina lower red shale 272 



q. Clinton lower lime shale.) 



r 251 



r. Clinton upper olive shalel 



s. Ore sandstone and ore beds * 38 



t. Clinton middle olive shale ." 178 



' u. Iron sandstone 7 



V. Clinton lower olive shale 571 



w. Medina white sandstone 820 



X. Medina red sandstone and shale 1,280 



y . Oneida red conglomerate 309 



z. Oneida gray sandstone 313 



a. a. Hudson River slates. 



5,384 



The Silurian rocks recognized in New Jersey have been traced southward 

 through Pennsylvania. The strata become thinner to the south. The unconformi- 

 ties observed in New York and New Jersey at the base of the Silurian are not noted 

 in the Pennsylvania sections, where the basal gray sandstone succeeds the Martins- 

 burg ("Hudson River") shale (Ordovician) in apparent conformity. 



The littoral and calcareous marine strata which constitute the Silurian in 

 Pennsylvania pass across Maryland into Virginia. They have been described by 

 Prosser **' and O'Harra."' The section, which measures 2,200 to 2,400 feet in thick- 

 ness, is fully described and correlated by Prosser, the following being a condensed 

 section of the subdivisions recognized by him, from the base up : 



Juniata formation: Alternating beds of shale and sandstone, deep red, unfossiliferous, prob- Feet, 

 ably equivalent to the older part of the Medina of New York 550+ 



Tuscarora formation:, Quartzose sandstone, white to light gray; Arlhrophyacs harlani 250-300 



Clinton formation: Shales, yellowish green to red, with thin fossiliferous limestones in upper 

 part, and two beds of iron ore 550-600 



Niagara formation: Thin-bedded blue limestone with thin shale partings, becoming shaly 

 and blackish above; represents the Decker Ferry formation of New Jersey and is equiva- 

 lent in time to Rochester shale and Lockport limestone or Niagara of New York 250-300 



Salina formation [Cayuga group]: Shales, impure limestones including four cement beds, 

 and sandstone, gray and blue, sparsely fossiliferous 700 



