NOTES ON THE IGNEOUS ROCKS OF WESTERN ARIZONA." 
By Albert Johannsen. 
PRE-CAMBRIAN CRYSTALLINES. 
The oldest of the volcanic rocks in northwestern Arizona are granites 
(A-6, A-44). 6 According to Lee, they occur helow the effusives 
throughout the entire area, and in the cliffs to the east are overlain 
by the same series of Cambrian rocks which in the Grand Canyon 
section is called the Tonto formation. Over the whole area the fact 
that the granite is an older and not a younger intrusive is clearly 
indicated by the occurrence of dikes and veins within it which do not 
extend through the overlying rocks. The upper surface was planed 
nearly to a base-level and the effusives generally overlie it in hori- 
zontal sheets. 
Much granite occurs in the area. The rock is red to gray in color, 
is usually coarsely crystalline, and is much cut by dikes and veins. 
Here and there in faulted regions, the rock is highly gneissoid, while 
in other places the mass passes, by easy transitions, from the normal 
rock to the schistose. The specimens are very similar in mineralogic 
character, though no thin section of the normal type was examined. 
Megascopically A-6 is fine and schistose, and consists of black and 
white bands of feldspar and biotite. A-44 is dark red in color and 
is very compact. Under the microscope the former is schistose, 
hypautomorphic- granular, while the latter is similar but without 
showing schistosity. In both, the minerals are feldspar, less quartz, 
and still less biotite. Zircon and apatite are accessory. The feld- 
spar is chiefly orthoclase, which is now and then intergrown with 
plagioclase in the form of microperthite ; a little microcline and a 
little oligoclase or andesine occur. The biotite occurs in shreds and 
patches (subparallel in A-6) and is in many specimens considerably 
altered to iron oxide, so that it is black and opaque. Normally I he 
pleochroism of the biotite is a = light green and fcr = dark green. 
Zircon occurs as short, stout, rounded grains, and apatite occurs 
a The specimens here described were collected by Mr. Lee and the. geologic relations air given from 
his observations. The report on the examination of the thin sections was made in 1906. 
>> References are to original specimen numbers; see list . p. — . 
4J)<)(>4— Hull. 352— OS SI 
