102 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
Si0 2 ... 
A1 2 0 3 .. 
Fe 2 0 3 .. 
FeO... 
MgO.. 
CaO... 
Na 2 0.. 
k 2 o... 
H„0 - 
IL,0 + 
Average analysis of Cripple Creek granite. 
74.90 
Ti0 2 . 
12.89 
PA- 
.58 
F ... 
.91 
MnO 
.07 
BaO. 
.74 
Li 2 0. 
3. 10 
5.92 
.15 
.55 
Less O for F 
0 . 12 
.02 
.31 
Trace. 
Trace. 
Trace. 
100. 26 
.13 
100.13 
On comparison with analyses of typical granites of other localities,.he found 
that these Colorado granites are (1) rich in silica, (2) deficient in the sum of iron, 
magnesium, and calcium oxides, (3) normal in regard to the amount of total alkalies, 
but rich in potash and lienee correspondingly poor in soda, and (4) unusual because 
of their content of fluorine which is present as fluorite. To this the writer would 
add (5) that they are low in alumina. 
OLIVINE-SYENITE GROUP. 
The mineralogical evidence, as revealed by the microscope, makes it certain 
that an intimate genetic relationship exists between the pyroxene granite, pyroxene 
syenite with accessory olivine, olivine syenite, olivine gabbro and its doleritic 
contact facies, and the anorthosite dike of Iron Mountain, as well as nearly or quite 
all the diabase dikes of the district. Chemical investigations of these rocks have 
not been made, but it may be said that, in the order above given and excepting 
the anorthosite, they contain decreasing amounts of silica, alumina, and the alkalies, 
and increasing proportions of iron and lime. There is reason to believe that the 
content in magnesia reaches a limit in the olivine gabbro and is somewhat smaller 
in the contact facies of that rock and in the diabase. Even in the granite the 
amounts of silica, alumina, and alkalies are probably below the average for granite, 
while in the olivine gabbro and the diabase iron, magnesia, and lime are certainly 
high. Phosphoric acid is prominent in the olivine gabbro. The anorthosite, 
presumably the youngest rock of the group, probably contains rather low silica, 
high alumina and lime, and low soda. 
This group, which is younger than the granite group, is thus characterized on 
the whole by low silica and alumina, high iron, magnesia, and lime, and probabty 
a notable amount of alkalies. 
ROCKS OF THE CRIPPLE CREEK VOLCANO. 
For various reasons the rocks of the Cripple Creek volcanic group have been 
more thoroughly studied than those of the foregoing groups. Their distribution, 
being in general limited to the immediate vicinity of the volcanic center, has been 
conducive to obtaining a comprehensive and accurate idea of their mutual relations 
on the surface. The numerous mine workings, of which the majority are located 
in these volcanic rocks, add the great advantage of a third dimension to the geolog¬ 
ical relations seen on the surface and afford opportunity to collect fresh material 
which would not otherwise have been obtained. The interesting and compara¬ 
tively unusual character of the individual rock types has led to more or less careful 
investigation of each. 
