152 IF. H. Twenhofel — Silurian Section at Arisaig. 



marks the base of his Niagara. Strike N". 55° W. ; dip 

 10°-35° W. 



Estimated thickness 66 feet. 



Chonetes tenuistriatus, Rhynchonella cf. robusta, Anoplo- 

 theca hemispherica, Cormdites distans. 



11. Greenish-gray shales with some much shattered lenticu- 

 lar fine-grained sandstones. Zone ends at a small gulch in the 

 cliff, that is here forty or more feet high and crowned by 

 stratified glacial material. Strike K 54° W. to N. 73° E. ; dip 

 34° W. to 10° E. 



Estimated thickness 63 feet. 



Very f ossiliferous : Monograptus clintonensis, Hetiolites gein- 

 itzianus, Orhiculoidea tenuilamellata, Dalmanella elegantula, 

 Leptwna rhomboid alis, Chonetes tenuistriatus, Camarotoschia 

 cf . obtusiplicata, Wilsonia cf . sajfordi, Anoplotheca hemispher- 

 ic, Comulites distans, Dalmanites, Conularia. 



12. Dark -gray shales, weathering rusty, with, very few hard 

 bands. Beds much disturbed. 



Thickness thought to be near 80 feet. 



Fossils abuudantj particularly graptolites: Monograptus clin- 

 tonensis, Leptcena rhomboidalis. Chonetes tenuistriatus, Ano- 

 plotheca hemispherica, Modiolopsis (?) cf. primigenius, Caly- 

 mene, and Dalmanites. 



Higher Niagaras Zones 



Silurian Division III, or McAdam Formation. 



Following zone 12, the highest member of the Clinton, there 

 is a fault of considerable importance, the eastern limb of which 

 has been elevated and a part of the strata lost. The fossils of 

 zone 13 are markedly different from those below and yet a 

 number of species, chiefly pelecypods, range on both sides of 

 the fault. Apparently the throw has not been extensive. 

 The species of zones 13 to 27 are not many in number ; but 

 indicate, rather distantly however, the Rochester shale. This 

 time equivalence is best seen in the presence of Monograptus 

 cf. 'riccartoensis, Dalmanella cf. edgelliana, Camarotcechia 

 neglecta, C. cf. obtusiplicata, Spirifer crispus, and first 

 appearance of large Atrypa reticularis. Other species that 

 help to confirm this correlation could be mentioned, but most 

 of these have a longer upward range. In fact, all the zones 

 about the fault have an indigenous fauna that gradually changes 

 into those of the higher beds. Division III is believed to coin- 

 cide, in the main, with the McAdam formation of Ami 

 (1901: 301), in part with the Niagara or Division E3 of 



