DIKES SOUTH OF SYLVAN PASS. 31 1 



small amounts. Coarser-grained modifications occur (1552 to 1554) that are 

 fine-grained diorite, the average-sized crystals being about 2 mm. in length. 

 They consist of nearly idiomorphic labraclorite, very little quartz, consider- 

 able brown biotite intergrown with brown hornblende, and uralitized 

 pyroxene. In one rock (1554) there is a little augite still unaltered. 

 Apatite is abundant in long, stout prisms, with cross fractures, and some- 

 times bent or broken. The rocks belong to the group of mica-pyroxene- 

 diorites, and are similar to some of the diorites of Electric Peak. 



The very fine-grained granite or granite-porphyry (1536, 1537) consists 

 of nearly idiomorphic crystals of labradorite and andesine, about 1 mm. 

 long, with smaller grains of brthoclase and quartz, besides considerable 

 biotite and magnetite ; also small apatites and zircon. There is a little chlo- 

 rite and calcite. It is a question whether the rock might not be more 

 properly classed as quartz-mica-diorite. It is very similar to the quartz- 

 mica-diorite (321) from Electric Peak, which has 67.54 per cent of Si0 2 , 

 and which may well be classed as granite. 



DIKES SOUTH AND SOUTHEAST OF SYLVAN PASS. 



South and southeast of the neighborhood of Sylvan Pass there are 

 dikes and intrusions and surface flows of massive rocks which by their 

 composition and petrographical habit, as well as by their mode of occur- 

 rence, appear to belong to the eruptions we have just been describing. It 

 is noticeable, however, that they differ from the intrusive rocks in the imme- 

 diate vicinity of Sylvan Pass in the degree of crystallization attained by 

 the groundmasses of the rocks. They are still finer grained. A number 

 of dikes have been observed along the ridge southwest of the headwaters of 

 Middle Creek. A large dike cuts the southern slope of the ridge between 

 Mount Doane and Mount Langford, and trends north-northwest with a hade 

 of 60° NE. It is about 300 feet wide in its lowest exposure, growing 

 narrower near the crest of the ridge. It consists of hornblende-andesite 

 (1559, 1560), with abundant small phenocrysts of hornblende and smaller 

 feldspars in great abundance. The groundmass of the rock is light gray 

 and lithoidal, with many small cavities or pores. At the contact it is 

 darker colored. In the center of the dike it is holocrystalline, with slight 

 micropoikilitic structure, while near the margin it is almost glassy, with 

 crowded microlites, though in places even here it is holocrystalline. The 



