[GNEOUS ROCKS OF WESTERN ARIZONA. 83 
the plagioclase laths. None of the original mineral remains, and there 
is occasionally a center of calcite. Possibly they are pseuclomorphs 
after hornblende. The magnetite occurs in irregular and corroded 
grains and is usually surrounded by a rusty rim. By incident light 
the surface appears black and metallic, but with rusty patches, the 
smaller grains sometimes having the entire surface rusty. The 
groundmass is anisotropic and full of alteration products, and con- 
sists of xenomorphic laths and patches of feldspar, much altered to 
kaolin, sericite, and chlorite, and of magnetite grains and dust. It 
has the appearance of having originally contained small lath-shaped 
feldspars. Whether there was also originally orthoclase is indeter- 
minable. Kaolin occurs as a secondary product in semiopaque 
patches in the groundmass and in the feldspar phenocrysts. 
RHYOLITE SERIES. 
Resting upon the andesite at Black Mesa, and in other places 
directly upon the granite, is a thick series of rhyolites, rhyolite flow 
breccias, and tuffs, which reach a thickness of 3,000 feet or more. 
They are all clearly older than some of the andesites (107, A-18) and 
older than all of the basalts. In the field the general appearance of 
the rhyolite is white to pink, making a sharp contrast with the under- 
lying granite and the overlying andesites and basalts. Usually the 
flows are massive, often brecciated, and often very coarse agglom- 
eratic, with angular blocks having a maximum diameter of several 
feet. In places these included fragments are rounded and the bed- 
ding of the tuffs is horizontal. No attempt was made to separate 
the rhyolite into distinct flows, as might be possible with more detailed 
study. The areas at which the specimens studied were collected 
being in many cases far removed from the others, it is impossible to 
correlate the relative geologic positions. 
The rhyolites may be divided into four groups — flow breccia, ash, 
normal rhyolite, and spherulitic rhyolite. 
In general the megascopic appearance of the rhyolite flow breccia 
is white to pink, with included irregular dark-brown fragments, usually 
small. Under the microscope the appearance of all (101, A-13, A-19, 
A-35) is very similar. The rock consists of a glass full of bubbles, 
often in flow lines and sometimes devitrified. It is full of irregular 
and broken fragments of orthoclase, quartz, sometimes a little biotite, 
mierocline, apatite, plagioclase, and hornblende, several or all, and 
usually some iron ore. Some of the specimens are altered and con- 
tain, besides iron oxide, much calcite. Besides the mineral frag- 
ments contained in the glass, there are irregular dark and altered 
inclusions of andesite, which clearly indicate that there was a pre- 
vious eruption of a basic rock. No correlation is possible, however, 
