DIOKITE AND DIOPJTE-PORPHYRY. 255 



ferroinagnesian minerals, which are biotite and a little chlorite, but no 

 pyroxene. Some of the chlorite carries zircon with pleochroic halos, and 

 bunches of rutile needles. Magnetite, zircon, sphene, and stout apatite 

 crystals are the accessory minerals. The rock is a quartz-mica-diorite, 

 approaching granite in composition. It has the same microstructure, degree 

 of crystallization, and mineral composition as the quartz-mica-diorite or 

 hornblende-granite (323) which occurs at Electric Peak. The latter, 

 however, contains hornblende besides biotite, and has no chlorite, which in 

 the rock in Hurricane Mesa may in part replace hornblende. The chemical 

 composition of the rock at Electric Peak has been determined. Its silica 

 percentage is 66.05. 



A still more quartzose and feldspathic variety (1424) forms a vein 10 

 inches wide on the middle spur. It has the general habit of the rocks of 

 this facies, but is lighter colored and carries less ferromagnesian minerals. 

 Its texture is saccharoidal. In thin section it has a granular structure about 

 grade 40, and is composed of quartz, orthoclase, and oligoclase, with biotite 

 and magnetite, very little hornblende, and some chlorite. ■ The accessory 

 minerals are the same as in the previous variety. It is a fine-grained 

 granite, whose chemical composition is shown in analysis 16 on page 

 261. Its microstructure is shown in PI. XXXIII, fig. 3. There are also 

 narrow veins of white rock, composed of quartz and feldspar, with little ferro- 

 magnesian minerals, and numerous small cavities lined with crystals of 

 quartz and feldspar. These veins present the most highly siliceous facies of 

 the rock. 



A porphyrinic form of the very quartzose facies also occurs on the mid- 

 dle spur. It is not noticeably porphyritic in the hand specimen (1423), but 

 in thin section is distinctly so, with a granular quartzose groundmass, grade 

 23. It exactly corresponds in microstructure and degree of crystallization, 

 as well as in mineral composition, to the quartz-mica-diorite-porphyry 

 (331) of Electric Peak. The phenocrysts are biotite, andesine, and quartz, 

 and occasionally orthoclase. Their outlines are nearly idiomorphic, but the 

 quartz in some individuals loses its proper form by merging into the smaller 

 quartzes of the groundmass in the manner described for the rocks at Elec- 

 tric Peak. There is considerable sphene and zircon. Its chemical compo- 

 sition (analysis 15 on page 261) is like that of the rock (329) of Electric 



