SUCCESSIVE DEPOSITS IX THE APPALACHIAN REGION 413 



trilobite Triarthrus becki is a ease in point. In the Cincinnati region it 

 occurs in the Fulton shale of Upper Utica age, 5 feet thick. It is absent 

 in the Lower Eden or Economy zone, 80 feet thick, but reappears in the 

 Southgate zone, 120 feet thick, this reappearance being, according to 

 TJlrich, about 150 feet above the Fulton. In New York, on the other 

 hand, my students and I have collected this trilobite from the Upper 

 Utica as well as the Frankfort shale. Indeed, it occurs in the basal 10 

 feet of the Martinsburg shale (Utica shale in the widest sense), which is 

 of the horizon of the Lower Trenton. Limited thus to two restricted 

 horizons in Cincinnati, this trilobite has a wide range where the dark 

 muds of the Utica type have a wide range and where presumably the 

 conditions of its existence were maintained. 



If we now turn to the other outcrops of the Bald Eagle formation, we 

 find a steady decrease in the thickness of this formation, botli to 'the 

 north and the south in Bald Eagle Ridge. Thirty miles to the northeast, 

 at Bellefonte Gap, the thickness has diminished to 550 feet. This dimi- 

 nution is brought about by the failure of the lower beds and a consequent 

 overlap of the higher, as indicated by the relation of the upper beds to 

 the succeeding red sedimentation. These upper beds are greenish gray 

 somewhat ochery and micaceous sandstones, with intercalated greenish 

 shales, while the lower series, 170 feet in thickness, is a hard gray sand- 

 stone without pebbles. It is thus seen that the formation has not only 

 thinned, but has also changed in character away from the region which 

 must be regarded as nearest to the source, of supply. The Hudson series 

 at the base of the section in Bellefonte Gap, has a total thickness of about 

 1,000 feet. Personal observation has convinced me that it is absolutely 

 continuous stratigraphically with the Upper Trenton limestone of this 

 section, which is 603 feet thick, according to Collie. 20 The Trenton lime- 

 stone here is characterized by Trematis terminalis, Trinucleus concentric 

 cus, Dalmanella testudinaria, and many other typical Trenton fossils I „ 

 e upper beds the black shales appear as intercalated members becom- 

 ing more important upward with the failure of the limestones Finally 

 ^lack shales predominate, with only a few thin limestone beds in the 

 basal part. Dalmanella testudinaria and Tsoteles platycephalus still 

 ,occur, but the shales are especially characterized by Triarthrus becki 

 .which, according to Collie, ranges through the lower 300 Peel of these 

 shales. The next 350 feet of the series consists of ferruginous shale. 

 Alternating with fissile black and brown shales, containing nodules of 

 '^Pyrit es; the se in turn are succeeded by brown, red, and gray soft 



30 Bull. Geol. Soc. Am., vol. 14, p. 412. 



