TERTIARY VOLCANIC ROCKS-PHONOLITE. 
65 
or pale yellowish, isolated or grouped in loose bundles. * * * The needles 
reach a length of 0.1 mm. by an average diameter of but 0.015 mm. * * * In 
certain needles extinction takes place parallel to the prismatic axis, and in such 
cases there is no definite dichroism. In other needles there is an oblique extinction 
and here there is a strong, clear yellow color for the axis nearly normal to the 
prism, while that near c is colorless, as before.”® The mineral is therefore 
probably monoclinic, but owing to absence of definite cross sections in the slides 
examined its optical orientation can not be determined. Cross regarded the min¬ 
eral as probably lavenite, and the unfailing presence of zirconia seems to support 
this view, for zircon does not seem to be sufficiently abundant to account for the 
amount of that oxide found. The weak pleochroism, however, and the impossi¬ 
bility of securing reliable data make any conclusion open to question. 
Two additional minerals of uncertain identity occur in almost the same manner. 
They are sparsely scattered through the groundmass and resemble the irregular 
grains and patches of interstitial aegirine. Both are colorless in the minute grains 
seen, and in both the refractive power is noticeably high. One has moderate 
double refraction and extinction, sometimes parallel and at other times over 40°. 
Its properties suggest a pyroxene, but in its mode of occurrence it does not cor¬ 
respond to that mineral. The other mineral has a similar arrangement of elasticity 
axes, but the double refraction is high. Except for the oblique extinction it resem¬ 
bles zircon. 
A mineral of brownish-yellow color is closely associated and sometimes inter- 
grown with the blue amphibole in slide rock northeast of Copper Mountain. It 
occurs in aggregates of minute grains, which appear to be rudely rectangular. 
The index of refraction is below that of the blue amphibole, while the double 
refraction is somewhat higher and extinction is parallel. Further determination 
was impossible. 
Two unknown minerals occur included in some of the analcite. One is present 
in minute, irregular particles, arranged as streams across the analcite grains or 
crowded near the margin. It has higher index than the analcite and polarizes 
very faintly. The other mineral occurs as exceedingly small needles extending 
into the analcite from surrounding mineral grains. It has rather high index, high 
double refraction, and parallel extinction. 
Besides the alteration products already mentioned in the description of the 
important constituents, carbonates, fluorite, and pyrite frequently occur as sec¬ 
ondary minerals near veins. 
TEXTURE. 
Since texture is very largely dependent on the development of the component 
minerals of a rock, the texture of these phonolites has already been partly described. 
The rocks are liolocrystalline and porphyritic, though phenocrysts are often rare. 
The groundmass consists of feldspar microlites with interstitial grains of sodalite, 
aegirine, and the two unknown minerals of aegirine-like habit. Flow structure is 
a Cross, W., General geology of the Cripple Creek district, Colorado: Sixteenth Ann. Rept. U. S. Geol. Survey, pt. 2,1895, 
pp. 30-31. 
