ROUKDTOP MOUNTAIN. Ill 



Black ehertv bands and beds of black quartzite form a characteristic 

 feature of the horizon. One of these siliceous beds on the crest of the 

 ridge may be followed for a long distance without any break in the con- 

 tinuity and is sufficiently well marked to form a characteristic feature of 

 the ridge. 



Evidence of the age of these beds, based upon their organic remains, 

 rests mainly upon the material obtained from the limestone immediately 

 overlying the Secret Canyon shale. Fossils are known to occur, more or 

 less well preserved, in a number of places, but the most satisfixctory locali- 

 ties are found just north of the Geddes and Bertrand dike and innnediately 

 west of the divide separating Secret Canyon from New York Can}on. All 

 the species obtained are identical with those collected from the same hori- 

 zon noith of Ruby Hill. Midway up the west slope of Hamburg Kidge, 

 and nearl)" due west from Roundtop, sevei'al species with much the same 

 grouping occur in a dark, compact limestone — a locality which, if thor- 

 oughly examined, might possibly yield a rich fauna. The Hamburg shale 

 forms a well marked horizon, but, being harder and more compact, yields 

 less readily to erosion, and, in consequence, is less easily determined by 

 topographical features than the same horizon northwai-d. It may be 

 traced from the extreme southern end of the ridge northward across the 

 broad west spur of Roundtop, until abruptly cut off by the rhyolite body 

 which occupies Glendale Valley. The Pogonip limestone has much the 

 same north and south limits, rising gradually out of the rhvolitic tuffs at 

 the base of Gray Fox Peak on the south, and terminating in a high wall 

 which forms the west side of the u})per Glendale Valley. 



Roundtop Mountain.— Roundtop Mouutaiu is almost wholly made uj) of 

 Pogonip limestone, and offers the best exposure of the series of beds 

 characteristic of this horizon to be found in the southern part of the Eureka 

 District. On the spur running out to the west fi-om the top of the moun- 

 tain, and in an arenaceous limestone immediately above the Hamburg 

 shale, a few organic remains were obtained, belonging to a characteristic 

 grouping which marks the transition from Cambrian to Silurian, found in 

 several other localities at the ])ase of the Pogonip. On the southern spur 

 of Roundtop, in beds dipping from 65° to 70° eastward, a small but 



