PHYSICAL AND FAUNAL EVOLUTION 67 



westward of the limestone, and corresponding decrease in the shale; 

 the former increasing from 40 feet at Saratoga to 430 feet at Utica, 

 and to 954 feet at Rochester, while the latter decreases from 1,260 

 feet to 710 feet to probably zero over the same localities (Fig. 9). 

 In the South Mountain region of southern Pennsylvania, the Cham- 

 bersburg limestone of Stones River, Black River, and Lower Trenton 

 age is succeeded by 1,000 feet of gray fossiliferous and dark bituminous 

 shales, with intercalated limestone members in the basal portion 

 which carry a Lower Trenton fauna. The shales contain Leptobolus 

 insignis, Triarthrus hecki, and Utica graptolites, and are succeeded by 

 a sandstone with the fauna of the Eden beds of the Cincinnati 

 region, formerly identified as Utica, but now regarded as younger than 

 that formation. In central Pennsylvania, some 600 feet of Trenton 

 succeeds to Black River, and is followed by 650 feet of Utica shale. 

 In this zone also we have some typical Trenton species, such as 

 Dalmanella testudinaria, Isoteles platycephalus, etc., associated with 

 Triarthrus hecki and other Utica species. The various sections 

 clearly show that along the western border of the Appalachians, 

 dark graptolite shales continued to form in Upper Ordovicic time, 

 while westward from this the Trenlon limestone represents the cal- 

 careous phase of the Utica-Trenton series (see map. Fig. 8). 



THE TRENTON -UTICA GRAPTOLITE FAUNAS 



The Normanskill fauna is succeeded by that of the Magog shales 

 or zone of Diplograptus amplexicauUs — the upper Dicellograptus 

 zone of Gurley. This represents, according to Lapworth, highest 

 Llandeilo or lowest Caradoc, and forms a transition to the true Utica 

 fauna. Many of its species are characteristic of the Hartfell shales 

 (Caradocian) of southern Scotland, though others are equally char- 

 acteristic of the Normanskill and Glenkiln shales. Ruedemann re- 

 gards this fauna as a relict of the preceding one. 



The Didymograptidae have vanished entirely, and the Dicranograptidae 

 almost; only the long range forms, Dicranograptus ramosus and nicholsoni, are 

 still observed, and the Diplograptidae .... hold now almost entirely the field, 

 with the genera Diplograptus, Climacograptus, and Cryptograptus." 



The fauna is best developed near Quebec and at the north end of 

 Lake Memphremagog, only fragmentary representation occurring 



I Op. cit., II, 30. 



