96 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
0.3 to 0.8 mm. in size, while the more abundant younger grains range from 0.2 mm. 
down. Practically no difference can be distinguished in the character of the two 
generations. Many of the olivine grains possess excellent form, but some are of 
irregular shape. They vary in size from 0.1 to 1 mm. Serpentine, carbonates, 
and iddingsite are the common alteration products. Biotite is of common occur¬ 
rence in the groundmass. The biotite of the Gold Sovereign dike is yellowish red, 
pleochroic to nearly colorless, and gives brilliant interference colors. A few small 
crystals of apatite and many small grains of magnetite are present as accessories. 
In texture these rocks are analogous to liypocrystalline-porphyries in which 
analcite is present instead of glass. 
Chemically they are richer in alkalies than many monchiquites, as an analysis 
shows. But, on the whole, they correspond very well with that group and better 
than with any other. 
Analyses of monchiquites. 
I. 
II. 
I. 
II. 
Si02.-. 
44.08 
43. 74 
Cl. 
0.04 
Trace. 
AI 2 O 1 . 
12.80 
14.82 
FeS 2 . 
.26 
4.58 
2. 40 
S. 
.10 
FeO. 
3. 72 
7.52 
Cr 2 0 3 . 
.05 
MgO. 
7.22 
6.98 
MnO. 
. 14 
CaO. 
11.21 
10.81 
BaO. 
. 13 
Na 2 0. 
2.97 
3.06 
SrO. 
.06 
K 2 0. 
3.31 
2.90 
Trace. 
H 2 O-. 
H 2 O 4 -. 
. 77 
2.35 
| 2.94 
99.97 
100. 23 
Ti0 2 .’. 
1.43 
2.80 
Less O for Cl. 
.01 
C0 2 . 
P 2 O 5 . 
SO 3 . 
4.14 
.70 
.01 
1.50 
. 64 
99.96 
I. Monchiquite, Block 8 mine, Cripple Creek. W. T. Schaller, analyst. 
II. Monchiquite, Rio do Ouro, Brazil. Rosenbusch, Elemente der Gesteinslehre, Stuttgart, 1901, p. 244, analysis 8. 
It was evidentlv this class of rocks which Stevens called limburgites. In his 
«y ~ 
description it will he noted that Kemp, who made the determination, spoke of the 
plienocrysts as present in “a clear, colorless, isotropic base, either glass or anal¬ 
cite.” “ If the base were glass, the rocks would be limburgites, but since it is 
analcite they belong with the monchiquites of Pirsson.*' 
The dike from near the end of the Raven tunnel belongs to the same series of 
intrusions and is probably composed of a rock of similar character to that which 
Cross collected from the Appie Ellen shaft and which, from the analysis on page 50 
of the 1894 report, lie calls a nepheline basalt. He notes that the rock was con¬ 
siderably decomposed and that treatment with cold dilute hydrochloric acid dis¬ 
solved nearly all of the soda. It seems quite probable that the soda was carried 
not by nepheline but by analcite and its decomposition products, like stilbite, 
especially since Cross’s determination of nepheline by the microscope was not very 
positive. 
a Trans. Am. Inst. Min. Eng., vol. 30, 1900, p. 763. 
b Jour. Geol., vol. 4, 1896, pp. 679-690. 
