ANCIENT CRYSTALLINE ROCKS-OLIVINE SYENITE. 
53 
Si0 2 .. 
ai 2 6 3 . 
Fe 2 0 3 
FeO.. 
MgO. 
CaO.. 
Na.O. 
Analysis of muscovite schist. 
. 51.88 
K 2 0. 
. 23.86 
Ti*0. 
. 7.24 
PoO- . 
. 1.89 
. 1.43 
. .21 
. .68 
Loss oil ignition.. 
5. 55 
.70 
.07 
6.05 
99.62 
OLIVINE SYENITE. 
A rock of considerable petrographic interest occurs on Iron Mountain, in the 
northwest corner of the district. It is just outside the boundary of the area shown 
in the first Cripple Creek map, and is not shown nor mentioned in the Pikes Peak folio. 
Only a portion of the mass is included within the area mapped in this report, and its 
southern and eastern limits are concealed by slide and alluvium. There is reason to 
believe that on the north a sharp contact exists with the Pikes Peak granite, which 
is probably the older. This rock in its most common development appears as a 
dark-brown or reddish-brown, rather coarse-grained aggregate of Carlsbad twins 
of feldspar, with smaller amounts of a black mineral with pyroxenic cleavage and a 
dark-brown mineral of rather vitreous luster and uneven fracture. 
The feldspar is almost exclusively microperthite—an intergrowth of orthoclase 
and albite—usually in Carlsbad twins, and commonly partially kaolinized. 
Quartz occurs very sparingly in irregular interstitial grains, which, nevertheless, 
sometimes show corrosion. Individuals of pyroxene up to half a centimeter in 
size exhibit at best only rude crystal form. The mineral is generally brownish 
yellow of varying intensities, but near the border frequently becomes greenish 
and sometimes deep green. Pleochroism is very slight, except in the green por¬ 
tions, and even there it is not marked. The angle of extinction is between 40° 
and 45°, but it was impossible to tell which one of the bisectrices lies nearest to 
the vertical axis. Cleavage is particularly good for pyroxene, and cross sections 
show, besides the prismatic, a pinacoidal cleavage sometimes of even better develop¬ 
ment. The second cleavage is probably parallel to the orthopinacoid, though 
this could not be determined with certainty. These properties and the character 
of the rock indicate that the mineral is diallage. It is more resistant to alteration 
than the other constituents. Olivine is present in almost the same amount as 
the pyroxene and in grains but little smaller. Good crystal faces are almost wholly 
lacking, but numerous individuals give a suggestion of the characteristic olivine 
outline. The mineral appears almost contemporaneous with the pyroxene, yet 
holds numerous crystals of apatite and grains of iron ore, which are uncommon 
in the bisilicate. The color ranges from a faint pink to a fairly strong but clear 
and transparent brown. The mineral is nearly isochroic. Besides the usual 
irregular cracks, two sets of cleavages are present, at right angles and of different 
degrees of perfection. The rough surface attests to the high index of refraction 
and the interference colors are less brilliant than usual because of the high double 
refraction of the mineral. The axial angle lies in the basal pinacoid, is smaller 
than for ordinary olivine—giving interference figures whose hyperbolas curve 
