THE SILURIAN SECTIONS 553 



Silurian. Cayugan series, about 1,350 feet (Rogers). The upper 700 feet are 

 made up of more or less of light bluish ribbon limestones (= water- 

 lime) that are often much mud-cracked, and the remainder consists 

 of greenish shales interbedded with an occasional limestone (300 

 feet), followed by variegated marls and red shales (350 feet). 



Great break. Most of Niagaran series absent. 



Silurian. Niagaran' series. 



Clinton formation, about 1,000 feet (Rogers). The upper 46 feet consists 

 of five thin zones of limestones in greenish shales, then 65 feet of red 

 and greenish shales, then 50 feet having the flaxseed iron-ore beds, 

 while most of the remainder of the section is concealed. 



Silurian. Medina series. 



Tuscarora sandstone, estimated at 500 feet, the thickness measured 27 



miles to the northeast in the Milesburg Gap. A series of white, gray, 



and pink sandstones with thin shale partings. 

 Ordovician. 



Break in section. 



Juniata formation of the Richmondian, about 1,090 feet. Can be seen in 

 Juniata Gap along wagon road east of Tyrone City. The detailed 

 sequence is described by Piatt on pages 17-18 of the cited report. It 

 is a series of dominantly reddish, cross-bedded, hard and soft, muddy 

 sandstones interbedded with thin zones of a sandy shale. There are 

 occasional thin zones of a greenish color, and many mud-cracked 

 layers with some of the cracks nearly a foot deep. The lower 250 

 feet are coarser-grained sandstones. 



Bald Eagle sandstones, apparently 1,320 feet thick. Thin-bedded light 

 greenish sandstones interbedded with green sandy shales and an occa- 

 sional red shale zone, the whole becoming more and more thin-bedded 

 and shaly downward, passing into the 



Martinsburg dark shales, about 900 feet. 



Cumberland-Keyset' region, Maryland-West Virginia. — -Along the Po- 

 tomac Kiver, 75 miles south of Tyrone. See Schuchert : Proceedings of 

 the U. S. National Museum, volume 26, 1903, pages 413-424, and Mary- 

 land Geological Survey, Lower Devonian volume, 1913. 



Devonian. 



Upper Oriskany or Ridgley sandstone, about 260 feet. 

 Lower Oriskany or Shriver chert, about 90 feet. 



Break. All of higher Helderbergian absent. 



Devonian. Helderbergian series. 



New Scotland member, about 60 feet. 

 Keyser member, from 270 to 300 feet. 



Probable break. 



Silurian. Cayugan series. 



Tonoloway member, about 630 feet. 

 Wills Creek member, about 663 feet. 



