CACTUS RANGE, IGNEOUS ROCKS. 91 
i ccessory minerals. Both quartz and orthoclase phenocrysts show 
undulose extinction, which in certain instances is rather strongly 
developed. The plagioclase phenocrysts and the feldspars of the 
groundmass are turbid through kaolinization. This granite porphyry 
closely resembles some of those of post-Jurassic age. Inclusions of 
a siliceous granite occur in rhyolite at a number of places. 
Pre-Tertiary diorite porphyry. — Two miles north of Antelope 
Spring is a small area of greenish -gray diorite porphyry. The rock 
has a well-developed porphyritic texture; small gray striated feld- 
spars, grayish-green altered hornblendes, and much smaller black 
micas lie in a fine-grained gray groundmass. Under the microscope 
the groundmass appears as a fine microgranitic mosaij of plagioclase 
and some orthoclase. Of the phenocrysts already mentioned both 
plagioclase and hornblende are much altered, the plagioclase being 
sprinkled with epidote, calcite, and zoisite, and the hornblende being 
more or less completely replaced by epidote, chlorite (ripidolite, in 
part), and calcite. Ilmenite and apatite are accessory minerals. 
This rock is practically identical with the older pre-Tertiary diorite 
porphyry of the Lone Mountain foothills. 
Ilornblende-biotite latite. — On the west side of the Cactus Range, 
north of Wellington and south of the median line of the range, low 
rounded bosses of a much altered greenish-gray rock protrude from 
beneath the younger rhyolite. The dull groundmass contains biotite 
plates, white or pale-green areas, apparently altered feldspar, and 
iark-green areas, probably altered hornblende, while the weathered 
surfaces show numerous casts of these phenocrysts. The rock con- 
tains many well-rounded pebbles, some of grayish-white quartzite 
.(Eureka?), others of the diorite porphyry last described. The 
largest pebbles are 3 inches in diameter. The latite appears to have 
lowed out upon an old erosional surface covered by well-rounded 
>ebbles. Under the microscope the groundmass shows as a glass, now, 
jiowever, much altered and composed of epidote, calcite, quartz, and 
>rthoclase. The plagioclase phenocrysts are almost completely 
altered to epidote and calcite, with less chlorite, quartz, and zoisite. 
the hornblende phenocrysts are altered to the same minerals, although 
loisite is as a rule absent. Biotite is replaced by chlorite in associa- 
tion with sagenitic webs of rutile. The phenocrysts and groundmass 
lave thus been altered similarly, although the secondary minerals of 
['he phenocrysts are coarser in grain than those of the groundmass. 
Wherever the contact with the rhyolite was seen the latite appears 
o be the older, a view supported by the absence of rhyolite pebbles 
n it and by the intense alteration and deformation which it has suf- 
fered. Petrographically the latite is rather similar to the later a nde- 
ite of Tonopah, but mineralogically it is more closely allied to the 
° Spurr, J. E., Prof. Taper TJ. S. Geol. Survey No. 42, 1005, p. 33. 
