ENVIRONMENTAL AND HISTORICAL FACTORS. 81 
coarser types of groundmass; pyroxene seems to occupy a subordinate 
position. Accessories are magnetite, olivine, and apatite. The pheno- 
crysts of pyroxene are fresh, rather dark colored for this mineral, and 
slightly pleochroic. They show high extinction angle and the other 
well-known characteristics of augite. The feldspars are of a rather basic 
type of labradorite, as shown by the extinction angle on the albite twin- 
ning plane. They are filled with black inclusions of the groundmass, 
sometimes radially arranged. Faint zonal extinction has also been noted. 
Olivine, when present, presents the same characteristics as in the olivine 
basalt described above. It is not at all abundant and in some slides is 
absent. For this reason, perhaps, some investigators might prefer to 
classify this type as a basaltic pyroxene andesite. 
AMYGDALOIDAL BASALT. 
Most specimens of this rock are too badly altered to admit of very 
satisfactory study. Megascopically it is a non-porphyritic rock, only 
an occasional crystal of pyroxene being easily recognized in the fresher 
pieces. It varies from yellowish-gray to reddish-brown, colors without 
doubt due to alteration. It contains in places numerous rounded cavi- 
ties which have become more or less filled with siliceous matter in the 
form of banded agate, chalcedony, jasper, and smoky quartz. Fine 
geodes of brilliant quartz crystals have occasionally been observed. The 
cavities frequently have a shell of agate, the interior being either empty 
or filled with calcite and siderite. 
Microscopically the rock consists of a groundmass of andesitic texture 
containing conspicuous crystals of feldspar and pyroxene with very sub- 
ordinate olivine. In some specimens the feldspar and pyroxene are 
quite fresh, but the olivine and more or less of the groundmass seems 
always to have altered to yellowish decomposition products. 
A similar rock occurs in another portion of the field. It is rather finer 
grained, although quite variable in different portions of the same mass, 
dark brown in color and, like the type just described, has given rise to 
considerable secondary silica in the form of red jasper and agate. Some 
parts of the mass show quite conspicuous feldspar phenocrysts close 
together and evenly arranged. Other specimens show porphyritically 
only reddish and yellowish alteration products of the ferro-magnesian 
minerals. The main mass, however, is quite dense and of even texture. 
RHYOLITIC TUFF. 
This is a light-gray rock, originally consisting of volcanic ash, but now 
consolidated into a mass of uniform texture and of sufficient strength to 
be used extensively as a building material. It contains numerous inclu- 
sions, mostly in the form of pumiceous material but sometimes of darker, 
more basic fragments which have given rise to concentric rings of striking 
appearance. 
