64 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
angle of ±73° with the vertical axis, and a negative orthodome which is less promi¬ 
nent and somewhat steeper. Low clinodomes are sometimes present, and in a few 
instances a negative orthodome is seen which is approximately perpendicular to 
the vertical axis. 
Cross sections show that the axial plane occupies the normal position, in the 
plane of symmetry, and that one of the optic axes lies very near to the vertical 
axis. The maximum extinction angle observed was a:c=+43° 30', a lying in 
the obtuse angle The optical angle is therefore large, but the axis of greatest 
elasticity may bisect it acutely. Pleochroism is marked, as follows: a = deep 
grayish or greenish blue, b = dull olive, with sometimes a tinge of purple, c = light 
yellow or greenish yellow. a>b>c, absorption along a and b being nearly 
equal. The index of refraction could not be determined definitely, but is about 
1.65 to 1.70. The double refraction probably never exceeds 0.025. Axial disper¬ 
sion is noticeable, but its character was not determined. The foregoing descrip¬ 
tion applies to what seem to be the most distinctive and characteristic sections.- 
The mineral appears to vary in composition, however, and in some sections the 
angle of extinction ranges from 20° at the periphery to over 40° at the center. 
Other well-formed crystals show uniform extinction a:c = about 25°. In such 
sections the absorption colors are a little lighter and of slightly different character, 
a = grayish blue, b = dull olive, c = light yellow. This last variety corresponds to 
the description given by Cross. 
Its association throws considerable light on the chemical character of this 
mineral. It frequently occurs, like aegirine, as ragged patches filled with inclusions 
of nepheline and feldspar. At other times it is a more or less fibrous aggregate. 
It sometimes surrounds, sometimes is surrounded by individuals of aegirine. 
Minute poikilitic grains of aegirine are abundant, and in numerous cases a definite 
micropegmatitic inclosure of aegirine may be observed. It appears, therefore, 
almost beyond question, that this amphibole is chemically related to aegirine. 
So far as known no one species yet described corresponds to this mineral. 
Its properties suggest a combination of two amphibole molecules analogous to 
aegirine-augite. Crocidolite is similar in some respects, but arfvedsonite agrees 
most closely, particularly with the variety of lower extinction, and may easily be 
one of the combining molecules, while the other one is unknown. 
Oilier minerals .—Titanite occurs in the small lozenges or the lath-sliaped 
forms it often assumes in phonolites. It is brown, shows strong absorption, 
and is often twinned parallel to the orthopinacoid (100). Apatite forms small 
hexagonal prisms and needles. A little magnetite is present in small grains, 
which often approach octahedrons. In some cases a grain of magnetite is inclosed 
at the center of an aegirine individual. A few foils of brown biotite of strong 
absorption occur in part of the Trachyte Mountain mass. Very small grains or 
rounded crystals of zircon may be seen in several rocks. Scattering grains of olivine, 
apparently as inclusions, and now showing resorption, occur in a rock collected by 
Cross from Rhyolite Mountain. 
In several of the Cripple Creek phonolites is found a mineral whose identity 
is not certain. It has a development and distribution similar to aegirine, from 
which it is not always to be distinguished. ‘‘It occurs in minute needles, colorless 
