THE DEVONIAN PERIOD 569 



abundantly represented in widely separated parts of Alaska. 1 

 The Devonian faunas f the coastal region, like those of the Great 

 Basin, are Eurasian in their affinities. 



Middle Devonian in the northwest. A considerable area of De- 

 vonian which has sometimes been called Hamilton is found in the 

 basin of the Mackenzie River and southward to Manitoba. 2 The 

 great arm of the sea in which the Devonian of this area accumulated 

 appears to have extended as far south as northern Missouri (Fig. 

 409). Whether this arm of the sea antedated the Hamilton epoch 

 is uncertain. 



The fossils of this northwestern Devonian are different from 

 those of the Hamilton formation of the east (Illinois to New York) , 

 and if the beds of the two regions were contemporaneous, as they 

 may have been, they seem to have been deposited in waters which 

 were not connected. The union was probably prevented by a 

 narrow belt of land running south-southwest from Wisconsin to 

 Missouri, somewhat as shown in Fig. 409. Till late in the Hamilton, 

 this land seems to have constituted a barrier between the eastern 

 interior sea and a northwestern sea which stretched from Missouri 

 . on the southeast, through the Mackenzie basin to the Arctic Ocean 

 on the north. Toward the end of the Hamilton epoch, this barrier 

 seems to have been removed sufficiently to allow the waters and the 

 life on opposite sides to mingle freely. 



Areas Where the Devonian Comes to the Surface 

 While the Devonian system is widely distributed it does not 

 appear at the surface over large areas. The reasons for its limited 

 exposures are substantially the same as those for the limited ex- 

 posures of earlier systems, and have been explained. 



The absence of the Devonian strata in certain situations is 

 significant. Thus between Iowa and Indiana, Devonian formations 

 do not appear at the surface between the Silurian on the north and 

 the Carboniferous on the south. The absence of Devonian beds 



1 Brooks, Bull. Geol. Soc. Am., Vol. XIII, pp. 256-261, and Professional 

 Paper, No. 1, U. S. Geol. Surv., p. 211; Schrader, Bull. Geol. Soc. Am., Vol. 

 XIII, p. 241; and Professional Paper, No. 20, U. S. Geol. Surv., pp. 62-67. 



2 Whiteaves, Am. Geol., Vol. XXIV, 1898, pp. 210-240. 



