PETROLOGY-ROCKS OF THE CRIPPLE CREEK VOLCANO. 
Ill 
When these areas are computed into percentages the following result is obtained: 
Relative abundance of the Cripple Creek volcano rocks. 
Phonolite. 
Latite-phonolite. 
Biotite trachyte. 
Syenite. 
Bull Cliff trachydolerite 
This shows the surprisingly small importance of the basic rocks, and though the 
figure for the Bull Cliff rock may be somewhat too high, it makes very little differ¬ 
ence. The leucitophyre has, of course, been omitted, but the probably very small 
amount which was ever present would have little effect on the whole. 
In order to obtain from these figures the composition of the average rock of 
the volcano, it is necessary to make another assumption, viz, that the analysis of 
each rock type in cases where only one analysis has been made and the average 
of the several analyses where several have been made represent truly the average 
composition of the respective rock types. It may be said in the first place that 
material for analysis was chosen particularly to typify the group to which it belonged. 
In the case of those rocks of which only one analysis has been made, their relative 
amount is so small that the possible error arising from an unrepresentative analysis 
will have little effect on the total composition. In each case where several analyses 
have been made, it is believed that the average of these is fairly typical of that rock 
division. In the phonolites, which, as above shown, make up nearly three-fourths 
of the total, the five analyses are so nearly identical that in that case certainly the 
average analysis is a fair representation of the composition of the phonolites. 
By taking the above respective percentages of the average analyses of each 
rock type, an analysis is obtained which represents the average rock of the volcano. 
Composition of average rock of the Cripple Creek volcano. 
Per cent. 
73.505 
23.907 
1.023 
1. 125 
.362 
Per cent. 
Isabella trachydolerite. 0. 039 
Vogesite.008 
Monchiquite. .031 
100.000 
• 
I. 
II. 
III. 
I. 
II. 
III. 
58.23 
58.50 
0.975 
S0 3 . 
0.13 
0.13 
19.65 
19.75 
.193 
Cl. 
.26 
.26 
Fe 2 C >3 . 
2.16 
2.18 
.014 
FeS 2 . 
.01 
FeO 
.86 
.87 
.012 
Cr 2 Oj. 
Trace. 
Trace. 
MgO . 
. 47 
.47 
.012 
MnO. 
. 17 
.17 
CaO 
1.74 
1.76 
.031 
BaO. 
.05 
.05 
8.29 
8.34 
.134 
SrO. 
.02 
.02 
k 2 o 
5.27 
5.31 
.056 
Li 2 0. 
Trace. 
Trace. 
h 2 o 
.27 
1.61 
1.62 
.090 
99.99 
100.06 
Ti0 2 
. 43 
.43 
Less O for Cl. 
.06 
.06 
. 13 
. 13 
C0 2 
. 10 
99.93 
100.00 
P 2 Oo. 
.07 
.07 
. 
I. Average rock computed from analyses. 
II. Average rock with pyrite, carbon dioxide, and hygroscopic water excluded and recalculated to 100 per cent. 
III. Molecular proportions of II. 
