AN ANALCITE-BASALT FROM COLORADO 685 
at the time of discovery as indicating a short dike parallel to the 
ridge, but no contact was seen, owing to detritus. Although the 
rock had a somewhat unusual habit, it was not supposed to be 
materially different from many other basaltic dikes which had 
been observed cutting the fragmental series referred to. It turns 
out on examination, however, that this rock of The Basin is an 
analcite-basalt, quite unlike any other rock collected in the entire 
district. 
The rock is dark and very fresh looking, with many small 
crystals of augite and olivine, and a white mineral occurring in 
roundish grains, all these averaging about 1™™ in size. A few 
augite prisms are larger, and terminations exhibiting the usual 
pinacoidal twinning were seen. The white mineral, by its rounded 
grains and the absence of cleavage, presents the only marked 
deviation from the normal habit of the neighboring plagioclase- 
basalts. A black, aphanitic groundmass holds the phenocrysts. 
Under the microscope augite, olivine, and magnetite possess 
a development common in basalts. Augite of pale, yellowish 
green color, and very faint pleochroism, occurs in phenocrysts, 
which are usually almost free from inclusions, but a few aygre- 
gates of similarly colored grains are full of irregular or sack- 
shaped glass inclusions. There is commonly a narrow outer zone 
to the purer augite, in which inclusions of glass and magnetite 
are seen, and the substance of this outer rim has a faint purplish 
tinge, like that of the smaller groundmass grains. An analysis 
of the pure augite is given below. 
Olivine appears in moderate abundance, of usual habit, and 
is the only mineral showing any sign of alteration. Perhaps 
half of the olivine is serpentinized with pale brown biotite leaves 
inclosed in the serpentine of the most altered grains, as an 
apparent secondary product. Primary dark, reddish brown bio- 
tite also appears very sparingly. Magnetite and apatite have 
the customary development. 
A considerable part of the rock is colorless and isotropic in 
polarized light; a much smaller part is colorless but doubly 
refracting, and is mainly assignable to three species of feldspar. 
