550 C. SCHUCHERT SILURIAN FORMATIONS 



Ordovician. 



Great break. 



Hudson River shales. The contact here is like that to the northeast, with 

 all of the Richmondian absent. The next section to be described, 

 Reedsville, is 42 miles across the strike to the west and in the deeper 

 part of the Appalachian geosyncline. In this interval there appears 

 an additional sandstone formation of great thickness that is correlated 

 with the Richmondian and has a thickness of about 1,590 feet. It 

 lies above the Hudson River equivalents and beneath the Medina 

 (Tuscarora). 



Jacks Mountain, Pennsylvania. — In Kishacoquillas Gap, between 

 Keedsville and Yeagertown, 40 miles northwest of Rockville. This is to 

 the west of Lewistown. See Sogers : Geology of Pennsylvania, volume I, 

 1858, page 473 ; Lesley : Second Geological Survey of Pennsylvania, 

 Pinal Report, volume I, 1892, pages 640, 645, 651-653 ; volume II, 1892, 

 pages 787-799. 



Silurian. Cayugan series. 



Bloomsburg formation. Total thickness of the "red Clinton," according to 

 the Second Pennsylvania Survey, 1,930 feet. Their terminology and 

 erroneous correlations are as follows: (1) Waterlime shale (470 

 feet) ; (2) Salina variegated shale with Ostracoda (358) ; (3) Niagara 

 limestone (3 feet 5 inches) ; (4) Niagara shale (70) ; (5) Clinton 

 Upper Red shale (432 feet 4 inches) ; (6) Clinton Upper lime shale 

 (325 feet 5 inches) ; (7) Clinton Lower red shale (270 feet 8 inches). 



Great break. Most of Niagaran series absent. 



Silurian. Niagaran series. Total thickness about 1,043 feet. A shale forma- 

 tion with iron-ore beds, the typical Appalachian Clinton. 



Clinton formation. The following members of the "Clinton" of the Second 

 Pennsylvania Survey are actually of this age: (1) Clinton Lower 

 lime shale and Clinton Upper olive shale (251 feet) ; (2) Fossil ore 

 beds, ore sandstones, etc. (37.5) ; (3) Clinton Middle olive shale (178) ; 

 (4) Iron sandstones (6.5) ; (5) Clinton Lower olive shale (571). 

 which is sparingly fossiliferous. 

 Actual contact of the Clinton with the Medina not clearly exposed, due 

 to talus, but it appears that there is a narrow transition zone. Be- 

 tween 100 and 200 feet above the Medina occur Chonetes cornutus 

 (rare), Leptocwlia hemispherica, Tentaculites minutus, Beyrichia lata, 

 Calymene clintoni (rare). Ever present in the Lower Clinton are the 

 small ramifying branches of Buthotrephis gracilis Hall, which often 

 can be used in a general way to locate the Lower Clinton. In the 

 "Clinton Middle olive shale," in bed 54 of Dewees' section, there is 

 an abundance of Leptama rhomb oidalis. Chonetes cornutus, Camaro- 

 tcechia, and Calymene clintoni. In the "fossil ore beds" occur Dal- 

 manella elegantula, Chonetes cornutus, Leptama rhomb oidalis, Cama- 

 rotcechia neglecta, and Atrypa reticularis. 



