W0.1173. ROCKS 1 MOM UNITED STATJCS-M EX IVO liOUNDAEY— LORD. 781 



products of atmospTienV; weathering are .m'milar to thowe of the liorn- 

 bleride. In an extremely veHicular specimen (No. 53294, individual 

 No. 153; the amygdaloid cavities were partially filled by chalcedony and 

 cryptocrystalline quartz. 



Tlie glassy andesite from San Clermente Island (No. .53295, original 

 No. 515; is peculiar in that it contains abundant jjhenocrysts of ensta- 

 tite. The groundmass of this rock consists of a brown andesitic glass 

 filled witli microlitic grains of augite and magnetite and minute lath- 

 Hlisij)ed jjlagioclase crystals. 



Tlie enstatite is of prismatic form and perfectly colorless and shows the 

 effect of extensive magmatic corrosion. The crystals are sunounde<l 

 by a resor7)tion rim of light. green augite grains similar to those of the 

 groundmass. These grains are frequently somewhat elongated and 

 have a tendency to radial arrangement around the core of the parental 

 mineral. 



Inclusions of glass are very frexjuent in the phenocrysts of this rock. 



Two types of andesitic brefx;ia (No. 5.3296, original Nos. 49.5, .520) may 

 l)e mentioned here. One from San Oiego, Mon. No. 258 (No. 495;, a 

 highly altered volcanic arkosc, is extremely ri<;h in (chlorite, ejiidot*;, 

 chalcedony, and secondary biotite; the other, from San Clerment<i 

 Island, Mon. No. 2.58 (No. 520;, consists of fresh fragments of andesite, 

 -coriacfj<^)US basalt, and vitre^Mis rhyolite cemented together by f;^>arse 

 ;,'raiiular calcite. 



h. liuHalt. — Fifty miles west of the liio Orande lii ver, Mon. No. 1 5 (No. 

 ry.yj'Jl). San Bernardino Kiver, Mon. No. 77 (No. 5.3298;. Nogales, Mon. 

 No. 122 (No. .5.3302). Pozo \'erde, " Kl Hanorie,'' Sonora, Mexico, .Mon. 

 No. 140 (No. .53299). (^uitobaquita, Sonoyta Valley, Arizona, Mon. No. 

 172 (No. 533<X)). Tule Mountains, Arizona, Mon. No. 186 (No. 53303). 

 Tucson, Arizona, <;0 miles north of Mon. No. 122 rNos. .53301, .53301;. 



Two distinct types of basalt are rei)resented. One a sc^>riaceous, 

 andesitic variety, rich in feldspar and pfX)r in olivine; the other a nor- 

 mal hoUicrystalline rock, with a preponderance of olivine and augite. 



The specimens vary in caAoi- from dark steel gray (Nos. .53297, r/.VZUH, 

 .53299, 53.300, .53301, original Nos. .50, ^Afi, 420, 449, .533); to a brownish 

 red ^No. 439; ; intermediate shades being brown (Nos. r/Z/jiiUj r}6'M)'6j 

 .53301, 53304, original Nos. 400, 473, .529, .531; or yellowish brr>wn (No. 

 .5.3302, original No. 364) acfjording to the state of i>reservation. 



The andesitic tyi>e is by far the more common (Nos. 50, ,340, 400, 420, 

 439, 449, .529, .5.3.3). It is identical mineralogically and structurally 

 with the young Tertiary basalts from the Snake Kiver Valley and 

 Oreat Basin of the West and re<iuires but a brief description. The dark- 

 gray, sometimes almost black, color of the rock is due to the vitreous 

 groundmass, consisting of an opaque, finely granulated glass baw?, 

 rich in skelet<jn crystals of magnetite, lath-shaped labradorite, and 

 granular augite. The iwrphyritic c;onstitaents are tabular- formed 

 la>iradorite, prismatic augite, olivine, and magnetite. 



