546 C. SCHUCHERT SILURIAN FORMATIONS 



terials of a delta and they often cut more or less deeply into the red 

 shales. The latter are much cleaved and slaty, and there are many 

 zones with vertical holes that are interpreted as due to capillary 

 pipes through which the ground waters were raised to the surface, 

 due to the dry climate of the time. In the upper portion there is, 

 rarely, an argillaceous limestone with hemispheric bryozoa. Toward 

 the bottom sandstones are more prevalent and appear to make the 

 base of the Salinan series. 



Great break. All of the Niagaran absent. 



Silurian. 



Shawangunk quartzites (see plate 21, figure 2). The succession is not all 

 exposed. The Second Pennsylvania Survey gives the thickness as 

 1,565 feet, but Grabau (loc. cit.) gives it as 1,900 feet. Twenty-three 

 miles to the northeast he says it has thinned to 1,500 feet. The for- 

 mation begins at the base with cleanly washed, white, coarse-grained, 

 heavy-bedded, somewhat conglomeratic quartzites. The rest of the 

 formation consists of thin-bedded, less cleanly washed, dark greenish 

 sandstones, with innumerable zones of vein quartz conglomerates 

 (about 80 per cent of the formation). The pebbles vary between .25 

 and .50 inch in diameter. Associated with the quartz pebbles are 

 others of a black shale in flat pieces, and in sizes up to 5 inches. 

 These are of a intraformational character, as they are derived from 

 the mud layers broken up by the storm waves or the undertow. 

 There are also interbedded many dark green to black shale zones that 

 vary in thickness up to 2 feet or more. All in all, the Shawangunk 

 here resembles that of Otisville, only it is darker in color and with 

 far more and larger intraformational shale pebbles. 

 Arthrophijcus alleghaniense can be had on the New Jersey side at 225 

 feet above the base just above a black shale bed 10 inches thick. 

 Another zone occurs 10 feet higher, and apparently also at 75 feet 

 above the base, where the first black shale bed appears. Rogers 

 (1858) reports this fossil very rarely in the upper portion of the 

 Medina here. Van Ingen states (Memoir 14, New York State Mu- 

 seum, 1912, page 417) that at about 735 feet above the Hudson River 

 shales, or in about the middle of the "White Medina conglomerate 

 No. 2" of the Pennsylvania Survey, he collected an abundance of 

 eurypterids (the Otisville fauna) in thin seams of black shale. The 

 species are Eurypterus maria, Dolichopterus otisius, Stylonurus cf. 

 myops, Eughmilleria shawangunk, and Pterygotus cf. glooiceps. The 

 same eurypterids occur in the Swatara Gap, Lebanon County, Penn- 

 sylvania, associated with Arthrophijcus alleghaniense (ibid., pages 

 418-419). 



Ordovician. 



Great break. All Richmondian absent. 



Hudson River shales. At the southeastern end of the gap on the Pennsyl- 

 vania side is shown the contact between the Hudson River series and 

 the Shawangunk. The contact is of the disconformable type. The 



