236 INDEX TO THE STKATIGRAPHY OF NORTH AMERICA. 



J 15. EASTERN MISSOURI AND SOtTTHERN ILLINOIS. 



According to Savage ^"^^ the Silurian is represented in southern Illinois only 

 by thin-bedded dark-gray limestone with chert overlain by h^avy-bedded pink or 

 mottled limestone, the whole 29 to 75 feet thick, to which he applied the name Clin- 

 ton. Savage states that the fauna corresponds to that obtained from the Clinton 

 (so called) of Dayton, Ohio. 



In eastern Missouri there is a white oolitic limestone followed by buff dolomite, 

 10 to 30 feet thick altogether, which may represent the Niagara *^^^ or may be of 

 Devonian age. It rests on Maquoketa shale (Ordovician) and is overlain by the 

 Louisiana limestone (Mississippian) . The occurrence is not shown on the map 

 accompanying this volume. 



J-K 11-12. NEVADA AND UTAH. 



In the Eureka section, Nevada, the Eureka quartzite (Ordovician) is according 

 to Hague ^^^^ unconformably followed by " steel-gray, almost black gritty limestone" 

 and "dark bluish-gray limestone" about 300 feet thick, which contain Trenton 

 fossils. There is no lithologic division but a gradual passage — 



into light-gray siKceous limestone, with a peculiar saccharoidal texture, in places becomuig 

 almost white and wholly without bedding. On the surface the hmestones weather brown and 

 buff, their light colors throughout a great vertical range standing out in strong contrast with the 

 other massive beds of the Paleozoic. It weathers in rounded outlines, breaking with an irregu- 

 lar fracture and presenting a monotonous appearance wearisome- to the eye. Rock of this char- 

 acter makes up by far the greater part of the horizon, and then by slow, imperceptible changes 

 it becomes darker in color, with more and _more tendency to develop planes of stratification, 

 and gradually passes into the overlying limestone of the Devonian. 



As already mentioned, an unconformity exists between the Eureka quartzite and the Lone 

 Mountain hmestone. There is therefore no direct evidence in the district of the thickness of the 

 Umestone. The average thickness of strata exposed has been taken at 1,800 feet, but it is 

 probable that this is under rather than over estimated, and at Lone Mountain they attain a 

 somewhat greater development, at least 2,000 feet being exposed. * * * 



Above the Trenton no good grouping of fossils has as yet been discovered until the Devonian 

 rocks are reached. The other portion of the Silurian limestone presents a most forbidding aspect 

 for the preservation of organic remains, and although diligent search has been made throughout 

 the horizon it was rewarded only by finding a few imperfect corals^ belonging to the species 

 Halysites catenulatus, which is so characteristic of the Niagara of the East, and here found in what 

 should be its true geological position. They have a wide range and occur nearly 1,500 feet above 

 the summit of the Eureka quartzite. The same coral has been obtained from Lone Mountain 

 and White Pine, and in both these latter localities associated with the genus Zaphrentis. 



See also Walcott ^" on the paleontology of the Eureka district. 



The distribution of the Lone Mountain limestone (now regarded by Ulrich as 

 all Ordovician and so treated by Emmons ^^'^ in a recent report) or its equivalent 

 strata has been traced chiefly by Spurr '''"^ and by Weeks (in an unpublished paper), 

 from whose maps the formation is sketched. 



A recent note on the occurrence of a Silurian fauna in Utah has been given by 

 Kindle.*'**' The section corresponds with that at Eureka, the fauna occurring in a 

 magnesian limestone 200 to 300 feet thick, below which is a much darker limestone 

 of undetermined age and above which lie dark magnesian limestones of Devonian 



