. Bulletin of the Geological Society of America. Geology. SILURIAN 275 in the western, part of the ;"® 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 ;"^ Re- cently Chamberlin and Salisbury have stated, with a reservation, that "it would appear that a large part of western ISTorth Ameri
. Bulletin of the Geological Society of America. Geology. SILURIAN 275 in the western, part of the ;"® 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 ;"^ 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 ;"^ 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
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