EGYPTIAN OAS^ON. - 593 



crystals of plagioclase and augite. Both quartz and sanidins seem to be 

 somewhat of an accessory ingredient Hke the quartz of the quartziferous 

 trachytes. This rock likewise contains no olivine. Under the microscope, 

 plagioclase is seen to be the predominating feldspar. The groundmass is a. 

 globulitic, half-glassy material, which contains peculiar crystalline products 

 of secretion like those found in obsidians and artificial slags. The rock is 

 also comparatively rich in apatite. The peculiar products of devitrification 

 seen in a thin section of this rock will be found illustrated in Vol. VI, Plate 

 XI, fig. 4. 



Below the narrower portion of the canon, v/hich is cut through this 

 dark, peculiar rock noticeable for its red, vitreous, weathered surfaces, spurs 

 of reddish porphyritic rhyolite close into the river on either side for a few 

 miles, and are succeeded by an open Quaternary valley, through which 

 the North Fork flows in a broad, alluvial bottom as far as its junction with 

 the main Humboldt. The hills to the east of this valley show a few out- 

 crops of westerly-dipping quartzites, underlaid by limestones having a strike 

 a little east of north, which were not closely examined, but have been 

 referred to the Weber QuartzAte and the Wahsatch limestone. They are 

 covered high up on their flanks by the nearly horizontal white beds of the 

 Humboldt Pliocene, while to the north the sedimentary rocks are con- 

 cealed beneath the flows of rhyolite. 



The rhyolite of Peko Peak, immediately adjoining the quartzite, has at 

 first glance a strong resemblance to a sedimentary rock. It is, however, a 

 distinctly eruptive body, but contains few crystals except those of quartz. It 

 has a dull-gray, compact, felsitic groundmass, which in places encloses small 

 fragments of green chalcedony, like those which make up the quartzitic 

 sandstone already mentioned. In external appearance, it rather resembles 

 an older eruptive rock. Under the microscope, it is seen to contain an 

 enormous quantity of yellowish-brown grains of ferrite. 



A mesa-like ridge of the white beds of the Humboldt Pliocene sep- 

 arates the lower part of the North Fork from the Elko Valley of the Hum- 

 boldt River. Out of this, southwest of Peko Peak, rises a low rounded hill 

 of white porphyritic rhyolite, whose groundmass has at times a somewhat 

 earthy texture, but in general is very compact, almost like a porcelain. It 



abounds in small distinct crystals of quartz, which for the most part are 

 38 D G 



