The orthoclase basalt is a dark-gray to nearly black rock, in places rather dense and fine grained and in others medium grained. A por-phyritic texture is locally present. The rock is composed of augite, olivine, biotite, orthoclase, iron ores, and a little aegirite. Weathering or partial weathering renders the rock lighter in color.
The peridotite agglomerate is a dark-greenish rock altering to a dark reddish on partial weathering. It is composed of olivine, largely altered to serpentine in places, enstatite, a little diopside, and iron ore, with much yellowish serpentine filling. More or less limonite staining is present in some specimens. The olivine and serpentinized olivine occurs in rounded grains and in fragments of brecciated grains through the serpentine. Portions of the peridotite contain inclusions of rounded and angular fragments of foreign material, as quartz or sandstone. Rounded pebbles of granular olivine or peridot are also present as inclusions. Some of the peridotite was observed to contain many rounded and corroded grains of transparent peridot from pin-head size up to those as large as a pea. Small emerald-green diopside crystals are also scattered through the peridotite.
The different rocks present different degrees of resistance to weathering and consequently occupy varying positions with respect to the topography. The several ridges extending from the hard sandstone boundaries into the basin of volcanic rocks are composed chiefly of the more resistant orthoclase basalt. These ridges also contain areas of peridotite and monzonite porphyry in places. The latter two rocks appear in the lower ground and in some of the low, rounded hills in the valley. One of these hills, about 100 feet high, north of the center of the basin, is formed by peridotite agglomerate, resembling kimberlite, and monzonite porphyry, which have resisted erosion longer than the surrounding rock. This hill is now being rounded off into angular talus and gravel slopes by weathering.
Peridot is found more or less plentifully at several places in the valley. Some of these are at the foot of the hills or ridges of peridotite and others are on the flat valley floor. Specimens were gathered in the talus and wash at the foot of the ridge back of Navajo Charlie's house, especially below outcrops of peridotite agglomerate. In the valley wash a half to three-quarters of a mile northwest of the house
E eridot was found in several bare, sandy places. Some of the ant-ills, 1 to 2 feet high, in one of these patches were found to be built up of over 75 per cent of peridot grains. The remainder consisted of garnet, quartz, rock fragments, diopside, etc. These grains range up to 4 millimeters in diameter, and are carried in from the surface over an area of many square feet around the ant-hills. They are not brought from the ground underneath the hills as is thought by some persons. The nests are built above ground and are covered with the grains of mineral. The ants use no selective method, but take the most available grains. The richness of the ant-hills in peridot therefore indicates the abundance of that mineral in the soil. Larger grains of
} peridot suitable for gem purposes are not found on the ant-hills, but loose in the soil. From the occurrence of the peridot near and below the peridotite agglomerate outcrops and the presence of gem-quality peridot in good-sized grains in this rock, it is evident that the gem is derived from the agglomerate. The abundance of small grains of olivine or peridot both in the same soil as the large grains and in the peridotite, combined with the tendency to disintegration of the latter, also strengthens this view.