274 BROOKS AND KINDLE PALEOZOIC ROCKS OF UPPER YUKON 



limestone on the Seward peninsula, a very massive bed, is either Silnrian 

 or Ordovician, and in any event the Silnrian is probably represented in 

 some of tlie overlying schistose rocks. 



The possible correlations between these Alaskan Silurian terrains, as 

 well as those of adjacent provinces, can best be expressed in terms of pale- 

 ontology. This will be here attempted, together with some suggestions 

 as to the possible migrations of faunas ; but it should be remembered that 

 the data at hand is far from being complete, and the collections have not 

 yet been studied in detail. 



The Porcupine Biver fauna (see list, page 325) contains species which 

 link it with the Silnrian faunas of both Europe and America. Of these 

 Rhyncliotreta cuneata. var. Spirifer nobilis, and Pentamerus oblongus 

 have long been known in l)oth European and American faunas. The 

 peculiar little twisted brachiopod Streptis greyi has been recognized at 

 but one other American localit)^, however. Williams'^' has reported it 

 from the Saint Clair limestone fauna of Arkansas. In Europe it occurs 

 in the Silurian of Bohemia, in England, and at the island of Gotland. 



The Silurian fauna of the type represented by the species of the Porcu- 

 pine River fauna is known to have a wide distribution throughout the 

 world. It is well known in various parts of Europe and has been recog- 

 nized in regions as remote as China,^® ISTew Zealand," and Australia.^* 



Another, but smaller. Middle Silurian fauna has been found in south- 

 eastern Alaska, on Kuiu island, nearly 1,000 miles southeast of the Porcu- 

 pine Eiver localit}^ This fauna occurs in the midst of a limestone series 

 which appears to be 2,000 feet or more in thickness, much of which seemed 

 to be barren. Upward these limestones appear to terminate with volcanic 

 breccias, while below they pass into cherts and argillites of undetermined 

 age. 



The discovery in the Porcupine section of a distinctively Middle 

 Silurian fauna representing the cosmopolitan fauna of Europe and Amer- 

 ica presents some points of interest in its bearing on the question of the 

 route of intermigration of the European and American Silurian faunas 

 and the location of the hypothetical western shore of the Sikirian sea in 

 America. In recent years some geologists have held that the Silurian 

 rocks were not represented in western America. "Like the older forma- 

 tions of the Silurian, the Niagara has not been identified with certainty 



I 



^ American Journal of Science, vol. 48, 1894, p. .331. 



»" P. von Richthofen : China, vol. 4, pp. 34-74 ; Memoire de la Society Royale des 

 Sciences de Liege, second series, vol. 6, 1876. 



" Quarterly Journal of the Geological Society of London, vol. 41, 1885, p. 199. 



"S Memorandum of the Geological Survey of New South Wales, Paleontology, no. G, 

 1898, 



