80 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
ties, but often rather cellular and even scoriaceous in structure. In 
portions which have become altered these cavities have frequently been 
filled with agate, gypsum, or calcite, minerals resulting from the breaking 
down of the complex silicates. Due to the same reactions, perhaps 
aided by arid conditions, the black basalt has in places become irregu- 
larly covered with thin films of white calcareous material. Reddish 
modifications due to oxidation of ferrous silicates are not common, though 
the black mass frequently presents dark, reddish-brown surfaces as the 
result of superficial alteration, aided perhaps by a concentration of basic 
material on the surface. 
Under the microscope the following constituents may be recognized: 
Plagioclase feldspar, pyroxene, olivine, magnetite, an occasional needle of 
apatite, and rather light-brown isotropic material. The feldspar appears 
entirely in the form of lath-shaped crystals frequently arranged in flow- 
lines and crowded against the olivine crystals. They average about 
0.2 mm. in length and a few hundredths of a millimeter in width. The 
olivine appears as isolated rounded grains rarely over 0.5 mm. in diam- 
eter. Only occasionally does it show its characteristic orthorhombic 
crystallization. Its most common decomposition product in this type of 
basalt is red ferritic material. This frequently permeates the whole 
crystal, but is also often found as a halo surrounding it. In the lavas 
underlying the rhyolitic tuff, this mode of alteration has extended to 
_ such an extent as to make the olivine, which is rather abundant here, 
quite conspicuous even to the naked eye. Pyroxene appears as exceed- 
ingly small grains occupying the spaces between the feldspar laths. It 
rarely has developed into crystals of sufficient size to show very charac- 
teristic features. Without polarized light it is sometimes impossible to 
distinguish it from interstitial glass. Magnetite is present as black, 
unaltered crystals scattered throughout the slide. 
PLAGIOCLASE BASALT. 
This type of basalt consists of a dense, dark-gray to black ground- 
mass, in which are scattered numerous and unusually large phenocrysts 
of plagioclase, with an occasional crystal of lustrous black pyroxene. 
The feldspar sometimes appears as faulted and brecciated crystals fre- 
quently reaching a length of 20r3cm. They are quite fresh and the albite 
twinning can often be observed without the microscope. Angular out- 
lines are the most common, but there are specimens in which the crystals 
have been resorbed by the magma yielding rounded forms. The pyroxene 
appears as isolated crystals, usually about 4 mm. in length and separated 
by intervals varying in different specimens from 2 to 5 cm. 
In thin sections the ground mass is found to be made up chiefly of 
feldspar crystals varying in size from microlitic growths in the denser 
varieties to individuals which can easily be seen by a hand lens in the 
