120 GEOLOGY OF THE EUREKA DISTRICT. 



All three divisions of the Silurian are found here the Pogonip lime- 

 stone, Eureka quartzite, and Lone Mountain limestone. This orographic 

 block is one of the few mountain ranges made up wholly of Silurian rocks. 

 Nearly all the more elevated portions are formed of Pogonip beds, which 

 gradually pass under the overlying Eureka quartzite, which forms continu- 

 ous bodies to the west and north. The drainage channels running out from 

 the summit are narrow ravines, and, although cutting hundreds of feet into 

 the Pogonip, never, so far as is known, expose the underlying Cambrian 

 strata. It is probable that only the higher Pogonip beds are represented. 

 Abrupt walls of nearly black limestone, characteristic of the upper mem- 

 bers of this horizon, form the sides of these ravines, in many instances the 

 dark rock being capped by overlying beds of white Eureka quartzite, 

 showing that these upper beds were in place. This is especially noticeable 

 to the northwest of White Cloud where the heads of nearly all the ravines 

 occur in the quartzites. Near the summit of the range they cut through 

 nearly vertical walls of quartzite from 200 to 400 feet in thickness. Out- 

 lying patches of quartzite, remnants of erosion, are still to be seen capping 

 the ends of the ridges on both slopes of the mountains. These isolated 

 patches are seldom more than 50 feet in thickness; they lie scattered all over 

 the slopes, many of them being so small and obscure as to be unrepresented 

 on the map. Over the long western slopes detached blocks of quartzite 

 may be found resting on the limestone, showing that while the quartzite has, 

 for the most part, been carried away, the uppermost beds of limestone still 

 remain in place. The Receptaculites beds extend in all directions under the 

 quartzite, paleontology confirming structural evidence of their geological 

 position. All three species of the genus Receptaculites known in the Great 

 Basin have been recognized here, associated with a varied fauna typical of 

 this horizon elsewhere, with the same foreshadowing of Trenton species. 

 The same specific forms occur here that are found underlying McCoy's 

 Ridge and Caribou Hill. A list of the species obtained at Bellevue and 

 White Cloud Peaks will be found on page 53. 



Bellevue Peak is capped with Eureka quartzite which, from here north- 

 ward, stretches in a continuous body to Reese and Berry Canyon. Over 

 this intermediate country it presents much the same general features, a 



