ORTHOCLASE-GABBEO. 249 



grade 43, nearly that of the coarsest rock found at Electric Peak. The 

 minerals are the same as in the last variety, but the ferromagnesian silicates 

 are more abundant, especially biotite. The pyroxenes do not carry many 

 microlitic inclusions. The micrographic structure in scattered patches is 

 pronounced. Olivine is absent from the more crystalline members of this 

 series of specimens, and the chemical analysis (No. 8 on page 2(30) shows that 

 the most crystalline portion of the rock is more siliceous and feldspathic 

 than that nearer the margin of the core. These rocks are orthoclase-gabbro 

 and orthoclase-gabbro-cliorite, approaching monzonite. 



Along the crest of the spur toward the northwest the gabbro becomes 

 less coarse and more porphyritic, with abundant tabular feldspars (1397), 

 and assumes the phase of crystallization represented by 1392, already 

 described. 



Another series of specimens represents the gabbro just south of the 

 lake at the head of the west gulch. The coarsest- grained variety (1430, 

 1431), near the outlet of the lake, is dark colored and granular, but not 

 saccharoidal. It is like 1395, the coarsest-grained form in the series first 

 described, in degree of crystallization and microstructure, but contains more 

 ferromagnesian silicates, and its chemical analysis (No. 5 on page 260) 

 places it between the extremes of the other series. There is much augite 

 with pinacoidal cleavage and characteristic needle-like inclusions, and less 

 hyperstkene, but numerous serpentinized olivines. There is considerable 

 biotite and magnetite, a little orthoclase, and very little quartz. Its micro- 

 structure is shown in PI. XXXIII, fig. 1. 



The finest-grained member of this series is porphyritic and medium 

 grained. The plagioclases are dusted with black dots and rods, the margin 

 of the crystals and the smaller grains being free from them. The dots 

 appear to be magnetite in large part, for when recognizable grains of mag- 

 netite occur in the feldspar they are surrounded by halos of clear feldspar 

 substance. 



Slightly different modifications of the gabbro occur in various parts of 

 the core, some of which have been analyzed chemically, and should there- 

 fore be described in more detail than would otherwise be necessary, in 

 order that the analyses may gain greater significance. The gabbro becomes 

 still coarser grained down the west gulch, and a short distance below the 

 lake is quite micaceous (1412). This rock and a large body of gabbro- 



