IGNEOUS ROCKS OF WESTERN ARIZONA. 89 
clase, augite, olivine, and magnetite. The feldspar is labradorite or 
andesine. The olivine occurs in doubly terminated crystals, only 
slightly altered. 
A-46 is very similar to A-23, except that the difference between 
the size of the grains in the groundmass and the phenocrysts is not 
so great. 
Specimens 110, 111, and A-3 are similar to A-45, in having a very 
fine groundmass. The phenocrysts of 110 and 111 are augite and 
biotite in about equal amounts, and less of a completely altered red- 
brown mineral which may have been olivine. The biotite and augite 
are usually altered in part also, and are surrounded by black rims. 
The groundmass consists of lime-soda feldspar laths in subparallel 
position in swirling eddies — a trachytic texture. There are also a few 
small irregular patches in the groundmass which have an index of 
refraction less than Canada balsam and are faintly anisotropic. 
They may be analcite, but are too small to determine. Calcite 
occurs as a secondary product, and magnetite occurs in a few large 
crystals as well as in small grains in the groundmass. 
In A-3 there are more augite phenocrysts than olivine. The latter 
is altered, completely or in cracks, to a red-brown mineral, which 
has an index of refraction much less than that of the olivine and is 
fibrous. It appears to be iron-stained serpentine. Magnetite 
occurs within the olivine in rather large grains and crystals. The 
groundmass consists of laths of labradorite in broad flow lines swirling 
around the olivine phenocrysts. Almost invariably they are twinned 
according to the albite law, and show an extinction angle in the 
neighborhood of 30°. Between the feldspar laths is much augite, 
pale green in color, and occurring in irregular grains, never in well- 
formed crystals or laths. Somewhat less in amount is magnetite, 
which occurs in cubical and irregular sections. Less in amount than 
the magnetite and, like it and the augite, occurring in irregular 
grains between the feldspar laths, is a reddish-brown mineral which 
has an index of refraction apparently greater than the augite. The 
grains are small and irregular, so that the determination is impossible. 
It appears to be partially altered olivine. 
Somewhat coarser in texture under the microscope than the above 
are A-15, A-16, A-24, A-25, A-27, A-37. Of these, A-15 and A-16 
have phenocrysts of olivine, augite, and plagioclase, with hornblende 
and biotite also in A-16. The olivine phenocrysts are generally 
entirely altered to a brownish-red- serpentine and are usually ground 
out in the centers in preparing the section. The groundmass of A-15 
is a dark glass full of small laths of plagioclase, laths of augite, and 
grains of red iron oxide, while A-16 has a groundmass of dark glass 
full of innumerable small feldspar laths and microlites and magnetite 
dust. 
