309 



differences between Anticosti and Arisaig are to be referred 

 to differences in the bionomic conditions existing at the 

 time of deposition of the sediments. 



Correlation with the Silurian of the inteiior is equally 

 difficult. As before, the Ross brook faunas furnish the 

 point of departure because the same fossils which have 

 been mentioned above as occurring on Anticosti are also 

 present in the Clinton of New York where also beds of 

 hematite occur which, however, lie below the strata 

 containing Monograptus instead of above as at Arisaig. 

 The only New York forma+^ion which can be correlated 

 with an Aiisaig horizon is the Williamson shale with 

 Monograptus clintonensis and Anoploiheca hemispherica. 

 The presence of these two fossils in the Williamson shale 

 leads to the conclusion that it is the time equivalent of 

 the upper half of the Ross Brook formation. On this 

 view the preceding Sodus green shale, the Furnaceville 

 hematite bed, and the Wolcott limestone would find 

 their time equivalency in the basal portion of the Ross 

 Brook formation and the sandy limestones of the Beech 

 Hill Cove formation. The common Clinton guide fossil, 

 Pentamerus ohlongus, which is so abundant in New York 

 has not been found at Arisaig. 



The Rochester, which succeeds the Clinton of New York, 

 is thought to find its equivalency in the McAdam forma- 

 tion wdth which it has in common the species Monograptus 

 riccartoensis , Dalmanella elegantula, Leptaena rhomboidalis, 

 Camarotoechia neglecta, C. cf. ohtusiplicata, Atrypa reti- 

 cularis, Spirifer crispus, Pterinea emacerata, Bucanella 

 trilohata and Calymene tuherculata or niagarensis. The 

 Moydart formation has generally been considered the time 

 equivalent of the Niagaran and in its faunal development 

 it agrees best with the Waldron and Louisville of the 

 United States. However, these formations have few 

 fossils in common. 



The Stonehouse formation has been variously correlated. 

 Ami and Fletcher considered it as representing the Lower 

 Helderberg, but it is difficult to see any Helderbergian 

 affinities in this fauna. On a basis of its stratigraphic 

 position the formation agrees best with the Guelph, but the 

 two faunas are wholly different. On the other hand the 

 Stonehouse faunas appear to be similar to those of the 

 north European Ludlow, the equivalent of which in Gotland 

 has, according to Lindstrom, Moberg, and others, the 

 common Guelph fossil, Megalomus. 



