282 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



Generalized section of the pre- Mississippian strata in southwestern Illinois — Continued. 



J-K -11-12. GREAT BASIN, NEVADA AND UTAH. 



In the Eureka section, Nevada, the Lone Mountain hmestone (Ordovician) 

 passes by imperceptible gradations upward into the Nevada limestone, which is 

 followed by the White Pine shale, both assigned to the Devonian by Hague and 

 measuring about 8,000 feet. Hague ^^^^ describes the relations as follows : 



The Lone Mountain and Nevada limestones taken together present an immense thickness 

 of beds, lying between the Eureka quartzite and the White Pine shale. Together they measure 

 about 7,800 feet in their broadest development. The division into Silurian and Devonian is 

 based mainly upon palfeontological grounds. The transition in sedimentation from character- 

 istic Silurian to unmistakable Devonian is so imperceptible that a boundary between them is 

 impossible to establish, and, as is usually the case where the beds form a continuous, conforma- 

 ble limestone series, a line of separation based upon faunal changes must always remain more 

 or less arbitrary. Lithologically, in their broader features, the Silurian and Devonian lime- 

 stones are quite distinct; it is only in the intermediate beds that no line can be drawn. The 

 light-gray and white siliceous beds that form the masS of the Lone Mountain present a wide 

 vertical range, and in these beds are occasionally seen obscure impressions of Niagara corals, 

 and in other localities, in similar rocks not much higher up in the series, occur Atrypa reticularis 

 and other forms foreshadowing the Devonian. It is known that the characteristic Lone Moun- 

 tain beds carrying Halysites catenulatus extend for nearly 1,500 feet above the Eureka quartz- 

 ite, and that beds easily identified by their organic remains bring the Devonian down to about 

 6,000 feet below the summit of the great limestone belt lying between the Eureka quartzite 

 and the White Pine shale. Halysites and Atrypa reticularis were never found associated 



