60 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
nearly always to be seen, but it is obscure or pronounced according to the develop¬ 
ment of the second-generation feldspar. Automorphic development of the feld¬ 
spars of the groundmass produces the typical trachytic texture which is charac¬ 
teristic of these rocks. In several cases the groundmass feldspars are of poor 
crystallographic form, and then usually associated with small nepheline grains, the 
resulting texture approaching microgranular. 
CHEMICAL CHARACTER. 
The chemical nature of the plionolite magma is expressed in the following 
analyses by W. F. Hillebrand, to which is added the analysis by L. G. Eakins, of 
a bowlder found a few miles north of Cripple Creek, and described by Cross. 
Analyses of phonolites from Cripple Creek.o 
I. 
II. 
III. 
IV. 
V. 
VI. 
SiOs. 
58.98 
58.78 
59.00 
58. 64 
58.85 
60.02 
A1;0 3 . 
20.54 
20.03 
20.07 
19.62 
20.06 
20.98 
FejO;. 
1.65 
1.87 
1.58 
2.17 
1.83 
2.21 
FeO. 
.48 
.49 
.05 
.42 
.51 
.51 
MgO. 
.11 
.16 
.10 
.37 
.18 
Trace. 
CaO. 
.67 
.83 
1.05 
1.24 
.95 
1.18 
Na 2 0. 
9.95 
9.36 
8.34 
8.39 
9.01 
8.83 
K 2 0. 
.5.31 
5.50 
5.63 
5.26 
5. 43 
5.72 
H s O-. 
.19 
.31 
.24 
.34 
.27 
1 70 
HoO+. 
.97 
1.57 
2.03 
2.40 
1.74 
f 
Ti0 2 . 
.24 
.29 
.29 
.20 
.25 
Zr02. 
.20 
. 17 
.20 
.09 
.16 
co 2 . 
.26 
.23 
.12 
P»0„. 
.04 
.03 
.05 
.03 
.04 
Trace. 
so 3 . 
.20 
. 12 
.07 
Trace. 
.10 
Cl . 
.288 
.58 
.24 
.14 
.31 
MnO.. 
.26 
. 15 
.12 
.20 
.18 
BaO.-. 
None. 
None. 
Trace. 
Trace. 
Trace. 
SrO.. 
None. 
None. 
None. 
Trace. 
Trace. 
Trace. 
Trace. 
Trace. 
Trace. 
Trace. 
— 
100.07 
100.24 
99.92 
99.74 
99.98 
100.15 
I. Miter Peak, south-southwest of Grouse Hill. 
II. One mile south of Straub Mountain. 
III. dike. Northeast slope of Big Bull Mountain. 
IV. Dike. Southwest slope of Rhyolite Mountain. 
V. Average of the preceding four analyses. 
VI. Between Cripple Creek and Florissant, Colo. Cross, W., Proc. Colorado Sci. Soc., vol. 2, 1887, p. 167. 
A noticeable feature of these analyses is their very close agreement, though 
they represent rocks from separated localities and of markedly different appearance. 
The most striking chemical feature is the richness in alkalies and the low 
amount of bivalent bases. In these respects the Cripple Creek rocks possibly 
surpass the average plionolite. The ratio of potash to soda is rather greater than 
usual. The considerable amount of combined water is accounted for by analcite. 
Of phonolites described from other localities, the most similar are from the Black 
Hills, South Dakota. The rocks from these two areas show many mineralogical 
as well as chemical likenesses. 
a Cross, W., Sixteenth Ann. Rept. U. S. Geol. Survey, pt. 2, 1895, pp. 38, 39. 
