4 o8 DEVONIAN PERIOD 



shale) has a maximum thickness of 2,600 feet, and thins notably to 

 the north and west, to a few hundred, and in places even to a few 

 score feet. Different names are applied to equivalent formations 

 in various localities. 



Devonian of the West 



Most of the Great Plains region is without Devonian formations, 

 so far as known, and so is inferred to have been land; but the Helder- 

 berg formation is present in the Arbuckle Mountains of Oklahoma, 

 and probably in southwestern Texas. The system has little devel- 

 opment in the Rocky Mountains, but is widespread between the 

 Rockies and the Sierras, though its outcrops are not extensive. In 

 some places, as about Globe, Arizona, the system is much faulted 

 and affected by igneous rock; 1 in others it is bounded by unconformi- 

 ties, both below and above, while in still others its limits are not 

 denned sharply. Where subdivisions of the system have been made, 

 they are not correlated with those of the east. In the Great Basin 

 region, both Onondagan and Hamilton types of fossils are found. 

 Their testimony is to the effect that the basin region was not con- 

 nected with the eastern interior sea in such a way as to allow the free 

 intermigration of marine life. The system is said to be 8,000 feet 

 thick in parts of Nevada, and 2,400 feet in the Wasatch Mountains; 

 but in the Yellowstone Park, it is only 160 feet thick, and not divisi- 

 ble into distinct formations. In the western interior generally, 

 limestone is the dominant formation. 



Devonian formations are known in both northern and southern 

 California, and may be present in many places where the rocks are 

 metamorphosed past identification. The system also is represented 

 in widely separated parts of Alaska. The Devonian faunas of 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 south to Manitoba. The arm of 

 the sea in which these Devonian beds accumulated appears to have 

 extended as far south as northern Missouri (Fig. 358). The fossils 

 of this northwestern Devonian are different from those of the 

 Hamilton fauna east of the Mississippi, and if the beds of the two 



1 Ransome, Professional Paper No. 12, and Bisbee folio, U. S. Geol. Surv., 

 pp. 39-46. 



