56 H. S. Williams — Different types of the 



is now rated as above the Hamilton shales and below the Bed- 

 ford shales this upper Devonian of Eastern Ohio is from 400 to 

 2,000 feet in thickness, thinning westward (See Professor Or- 

 ton's Preliminary Report on Petroleum and Gas, 1887, p. 26). 



When we reach the central part of the interior area we find 

 the Devonian represented by limestones running up into fine 

 argillaceous shales, resting on upper Silurian limestones which 

 in numerous places are of Niagara age and, in the southern 

 border of the region, are more or less siliceous, and hold fossils 

 of the later Silurian time, as in the Delthyris shales of Missouri 

 which are, doubtless, as late as Lower Helderberg time. This 

 central area lacks the black shale and runs up immediately into 

 Sub-carboniferous limestones, calcareous shales and sandstones, 

 and the total representatives of the Devonian are scarcely 200 

 feet thick. 



The Western Devonian Area. 



I take the Nevada section of the Eureka district as typical, 

 since this has been carefully developed by the labors of Hague 

 and Walcott. (See Walcott's Monograph, Paleontology of the 

 Eureka district, U. S. Geol. Survey, 1884). 



The peculiarities of this section are as follows : 



Lying unconformable upon a thick series of limestone beds, 

 representing the Trenton and, at the top, the Niagara of eastern 

 sections, comes the Nevada Limestone, 6,000 feet thick, indis- 

 tinctly bedded and siliceous below, and becoming massive toward 

 the top with intercalated beds of shale and quartzite. The 

 same fauna runs from bottom to top, but with some change in 

 part of the species. In the lower 500 feet the fauna is dis- 

 tinctly lower Devonian, and in the terminal 500 feet it is as 

 distinctly allied with the upper Devonian of the east. Through- 

 out there are found species which in the typical eastern sections 

 are restricted to particular zones. In its species it shows closer 

 relationship with the Iowa Devonian than with the more eastern 

 faunas, containing two species (seep. 265) that have been found 

 far to the north in the Mackenzie River Basin, i. e. Orthis 

 McFarlini and Rhynchonella eastanea, (N. 67° 15' long. 126° 

 W.) Overlying this limestone is the White Pine Shale, a 

 black shale, estimated at 2,000 feet in thickness, running into 

 red and brownish sandstones and arenaceous shales, with some 

 plant remains and a sparse fragmentary fauna which closely re- 

 sembles in general character the fauna of the similar upper 

 Devonian black shales of the eastern continental area. 



In these western sections there is a remarkable difference in 

 the range and habit of species. " Some species," as Mr. C. D. 

 Walcott has shown, "have reversed their relative position in the 

 group as they have been known heretofore, and others have a 



