ATEYPA PEAK. 125 



Castle Mountain may, for sake of convenience, be taken as the northern 

 limit of the Fish Creek Mountains. From Castle Mountain to Reese and 

 Berry Canyon no beds come to the surface other than the quartzites. Here, 

 however, a sudden change takes place, the canyon occupying a line of 

 southeast and northwest faulting with the quartzite on one side dipping at a 

 low angle to the west, and the Lone Mountain limestone on the opposite 

 side, but without any distinct line of bedding. From the head of Reese 

 and Berry Canyon the limestone crosses over a low saddle to the head of 

 Lamoureux Canyon, following the latter ravine until it makes an abrupt 

 bend to the south. The limestone may be traced eastward around the base 

 of Atrypa Peak, thence westward again with an irregular course as far as 

 Spring Valley. In this area the underlying limestone belongs, for the most 

 part, to the Silurian, but in one or two places the beds assigned to the 

 Devonian on lithological grounds rest directly upon the quartzites abutting 

 against them almost at right angles. The division between the Silurian 

 and Devonian in this region is an arbitrary one, but in most instances the 

 passage from the white saccharoidal limestone of the former into the strati- 

 fied gray beds of the latter is the same here as elsewhere in the District. 



Atrypa Peak. Nowhere in this area is there any place which permits of a 

 measurement of the Silurian rocks, but the region of Atrypa Peak, the cul- 

 minating point, affords excellent sections across the Nevada limestone, the 

 beds presenting nearly uniform dips and strikes. This imposing mountain 

 is formed almost wholly of Devonian limestone, the name of the peak 

 being derived from the abundance of Atrypa reticularis found on its slopes. 

 Two sections for comparative purposes were made : one, directly across the 

 strata on the southeast slope of the peak, the other on the high ridge 

 extending westward lying between the peak and the head of Lamoureux 

 Canyon. The latter section will be found on page 67. 



Where the sections include the same geological horizons they agree 

 closely in details, but the one taken across the slope of the peak gives a 

 much greater thickness of Silurian rocks, whereas the ridge section ex- 

 tends higher up into Devonian strata. The fossiliferous shaly belt (No. 5), 

 in the section east of Lamoureux Canyon, is easily traceable across the 

 ravine to Atrypa Peak and may be taken as a base for comparing the 



