SILURIAN 275 



in the western, part of the continent.""® Eeported occurrences of the 

 Silurian west of the Mississippi valley have been questioned,''" and Weller 

 has stated "that the greater part of this region (west of the Mississippi 

 valley) was above sealevel during Silurian time. This leaves the North, 

 as the only available outlet for the interior epicontinental sea.""^ Re- 

 cently Chamberlin and Salisbury have stated, with a reservation, that "it 

 would appear that a large part of western ISTorth America Avas land during 

 the Silurian period.""^ 



The hypothetical western shoreline of the Silurian sea as drawn by 

 Weller runs northward of Arkansas slightly to the west of the Mississippi, 

 in the United States, and well to the eastward of the Mackenzie river, in 

 British Columbia. Weller has given good reasons for disbelieving that 

 the route of intermigration between the interior American and European 

 Silurian provinces was by way of New York and to the eastward, which 

 would be the geographically most direct one. He has presented the avail- 

 able evidence favorable to the view that the route of intercommunication 

 was by way of Hudson bay, Greenland, and Spitzbergen, and, in the ab- 

 sence of evidence of Silurian faunas in the Northwest, concludes that the 

 path of migration was by this northeastern route. ''^ The discovery of a 

 Middle Silurian fauna in northern and southeastern Alaska requires the 

 shifting of the provisional boundary of the Silurian sea alluded to above at 

 least 1,000 miles to the westward in the northern third of the continent. 

 It indicates also that the Alaskan route, by way of Siberia, as well as the 

 Hudson Bay and Greenland route, must l)e considered as a possible and, 

 in the opinion of the writers, a probable route of intermigration between 

 the European and interior American faunas. 



The absence in the American pre-Silurian rocks of any fauna from 

 which the Silurian fauna could have been derived compels the conclusion 

 that many of its more distinctive elements entered the American Silurian 

 sea from a foreign center of disjjersion. 



The collections of various exploring expeditions have shown the pres- 

 ence of a Silurian fauna at a numlier of points in the Arctic islands which 

 fringe the northeastern part of the continent. For a knowledge of these 

 collections we are indebted largely to Koenig,®* Salter,"^ Houghton,®^ 



"» T. C. Chamberlin and R. D. Salisbury : Geology, vol. 2. Chicago. lOOG. p. 3S4. 



«" Chicago Academy of Science Bulletin no. iv, 1900. p. 17. 

 American Geologist, vol. xviii, 1S06, pp. 31-33. 



01 .Journal of Geology, vol. vi, 1898, p. 696. 



0= Chicago Academy of Science Bulletin no. 4. part i. 1900, p. 17. 



83 Ibid., pp. 12-22. 



"* Observations on the rock specimens collected during the first polar voyage of Cap- 

 tain I'erry. Appendix to Perry's voyage for the discovery of a northwest passage. 

 London, 1824. 



