88 RECONNAISSANCE OF PART OF WESTERN ARIZONA. 
OLIVINE BASALTS. 
Resting upon the andesite" upon the rhyolite, or even upon the 
granite, and also upon and within Quaternary gravels, depending upon 
the amount of erosion that has taken place, is a series of olivine 
basalts — the youngest lava in the region. There are at least three 
flows of this, but it is impossible, from the amount of field work 
done, to correlate the different sheets in different parts of the area. 
Five or six varieties can be distinguished under the microscope, but 
they may be but different phases of the same rock, a greater or less 
development of olivine or a coarser or finer ground mass forming 
the various types. This difference in appearance may be seen even 
in two sections (as 110, 111) which have been collected near to- 
gether from undoubtedly the same flow and which show different 
characters. Five types may be distinguished, according to the fine- 
ness of the groundmass: (A) 110, 111, 3, 45; (B) 15, 16, 24, 25, 27; 
(C) 37, 46; (D) 22, 23; (E) 30, 34. 
Occasionally the field relation can be seen; thus, 24 is from a sheet 
overlying 25; 15 is from a sheet intruded into the rhyolite and lifting 
it; and 16 is from a sheet above the rhyolite. 
In the field the general appearance of the basalt is black or very 
dark red. In general it is darker in color than the underlying ande- 
site and is often vesicular and ropy. Where it rests upon the rhyolite 
it forms sharp cliffs. In many places it forms flows in erosion val- 
leys, and though actually lower in position, is higher geologically. 
Megascopically the hand specimens are compact and vary in color 
from black through dark gray to dark brownish red. Occasionally 
there is a somewhat banded arrangement — that is, alternating bands 
of a lighter color. Under the microscope the texture of all the speci- 
mens is porphyritic, with a groundmass generally trachytic and with 
the small plagioclase laths in subparallel position. In general the 
phenocrysts of the rock are olivine, or, when other minerals occur, 
they are greatly in the minority. In four specimens, however (22, 
23, 45, 46), the phenocrysts are chiefly plagioclase. 
In specimens 22 and.A-23 the plagioclase phenocrysts are greatly 
in excess of the olivine, and the latter are much in excess of the 
augite. The groundmass is much coarser than A-45 and consists 
of plagioclase laths and areas, less augite grains, and still less magne- 
tite. The feldspar is labradorite. Olivine is very much altered to a 
red-brown mineral which may be iddingsite or an iron-stained ser- 
pentine. Some of the olivine is fresh and shows simply an altered 
rim or altered center. 
A-45 is perpatic, has a much finer groundmass than either A-22 
or A-23, and does not contain prominent augite grains in the ground- 
mass. The constituents, in the order of their abundance, are plagio- 
