34 BULLETIN" 77, UNITED STATES NATIONAL MUSEUM. 



In Baffin Land, and apparently elsewhere in Arctic America, the Lower Silurian 

 strata rests unconformably on old crystalline rocks. To the north of Baffin Land, the 

 former are overlain by beds of Niagara or Wenlock age. 



The Trenton faunas, occurring in various places around the insular Archaean nucleus 

 of North America, have much iu common, and this indicates that the conditions at 

 that time were very similar, while the sea was in communication throughout. As 

 yet, however, the distribution of the strata, together with their faunas, are well known 

 only to the south and southeast of the Archaean nucleus, yet that of the west (Mani- 

 toba) and of the northeast (Baffin Land) show direct communication. 



The Baffin Land fauna had an early introduction of Upper Silurian genera in the 

 corals Halysites, Lyellia, and Plasmopora. In Manitoba similar conditions occur in 

 the presence of Halysites, Favosites, and Diphyphyllum. Other Upper Silurian types 

 do not appear to be present. 



The Trenton fauna of Silliman's Fossil Mount, at the head of Frobisher Bay, has 72 

 species, of which 28 are restricted to it. This fauna shows an intimate relationship with 

 that of the Galena of Minnesota, Iowa, and Wisconsin. Fifty-seven per cent of the 

 species of Baffin Land also occur in the Galena of the regions just mentioned. 



The Trenton fauna of Baffin Land shows that the corals, brachiopods, gastropods, 

 and trilobites have wide distribution, and are therefore less sensitive to differing hab- 

 itats apt to occur in widely separated regions. On the other hand, the cephalopods, 

 and particularly the pelecypods, indicate a shorter geographical range. The almost 

 complete absence of Bryozoa ia the Baffin Land Trenton contrasts strongly with the 

 great development of these animals in Minnesota and elsewhere in the United States 

 [p. 175]. 



Most of the fossils upon which Schuchert's studies were based are 

 in the collections of tile United States National Museum, and the 

 opportunity is thus afforded of comparing them directly with other 

 Ordovician faunas and of making more detailed correlations. The 

 results of a second study of these northern faunas lead to the follow- 

 ing conclusions: 



(1) It is improbable that Utica strata occur in the far north, since 

 the only fossil noted of any value in maldng such a correlation is 

 Triarthrus hecM, which is now known to occur also in the oldest 

 Trenton. Of course it is possible that an arm of the sea extended 

 northward during Utica time and allowed the deposition of these 

 particular strata. 



(2) The Trenton age of the principal Ordovician deposits was 

 determined by Schuchert by the percentage composition method. 

 He found that of the 72 species known from the Silliman's Mount 

 locaHty, 54 were conmion to other areas. Comparing these 54 with 

 known horizons in Miimesota, it was found that 10 occur in the Low- 

 viUe, 17 in the Black River, 38, or about 70 per cent, in the Trenton, 

 and 11 in the Cinciimatian. This prevalence of Trenton species 

 seemed to be a good indication of the presence of a single fauna of 

 that particular age. However, this method of correlation is open to 

 some objections and has often been the occasion of error. In this 

 particular example, the fossils were derived from the debris of a con- 

 siderable thickness of strata, and there is just as much if not more 



