SILURIAN KOCKS. 47 



knowledge of the structure at the other localities, and in the Oquirrh Range 

 the Olenellus shales are known to be cut off by a sharp fault from the Upper 

 Cambrian. 



By reference to the Eureka section it will be seen that the Olenellus 

 horizon is nearly 2,500 feet below the top of the Prospect Mountain lime- 

 stone, where there comes in a fauna showing a mingling of Middle and 

 Upper Cambrian forms. At the base of the Hamburg limestone, 1,600 feet 

 higher in the strata, the true Potsdam fauna of Wisconsin and Minnesota 

 is abundantly represented by a characteristic grouping. By comparing 

 these lists of fossils from the different horizons, it will be seen that in this 

 group, at the top of the Hamburg limestone, there are found seven species, 

 which first occur at the top of the Prospect Mountain limestone. They 

 pass up through the beds at the base of the Hamburg limestone and, together 

 with five additional species obtained for the first time from the latter hori- 

 zon, come up to the close of the epoch, making in all twelve species common 

 to the top and bottom of the Hamburg limestone. Three species obtained 

 from both the base and the summit of the limestone are identical with 

 forms from the Potsdam sandstone of Wisconsin Hyolitlies primordiaUs, 

 Dicellocephalus osceola, Ptychaspis minuta. Another, Lingula manticula, first 

 described by Dr. C. A. White, 1 from the Schell Creek Mountains, Nevada, 

 has here at Eureka a wide range, extending from the Prospect Mountain 

 limestone through the Hamburg limestone and shale and well up into the 

 overlying Pogonip group of the Silurian. 



SILURIAN EOCKS. 



Rocks of the Silurian period at Eureka fall readily into three epochs. 

 From our present knowledge, it would be a somewhat difficult matter to 

 subdivide them still further, except upon fine distinctions founded upon 

 paleontological grounds, which might not hold good over any large area 

 of country. These three divisions correspond with the lithological character 

 of their sediments, two heavy masses of limestone with a sharply defined 

 intervening bed of quartzite. This quartzite is a highly altered sandstone, 

 much purer in composition than the Cambrian quartzite below or the sili- 



1 U. 8. Geographical Surveys West of the 100th Meridian, vol. iv, Paleontology, part 1, p. 5-'. 



