90 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
Black iron ore is very plentiful, and in the absence of spliene must be titaniferous 
to a considerable extent. (See analysis.) 
Small interstitial particles of turbid appearance and brown color occur scattered 
sparingly through the groundmass. They possess weak single refraction and give 
aggregate polarization in low colors, and without doubt are devitrified portions of 
an original glass bases. 
The texture of this rock is therefore hypocrystalline porphyritic. The holo- 
crystalline portions are generally intermediate in character between the trachytic 
texture of the phonolites and latite-phonolites, and the intersertal texture of the 
basalts. Small portions of the groundmass are microgranular. 
CHEMICAL CHARACTER. 
The composition of this rock is shown in the analysis, by Hillebrand,® given 
below. 
Analysis of trachydolerite from Bull Cliff. 
Si0 2 .. 
AhOj . 
Fe 2 0 3 . 
FeO.. 
MgO . 
CaO.. 
Na 2 0 . 
k 2 o.. 
h 2 o- 
H„0+ 
Ti0 2 . 
49.84 
ZrO z 
17.78 
co 2 . 
5.86 
PA 
2.62 
so 3 . 
3.02 
CL. 
7.35 
MnO 
5. 20 
BaO 
3.04 
SrO. 
.34 
Li 2 0 
2.02 
1.43 
.03 
.52 
. 76 
None. 
Trace. 
.21 
.22 
. 18 
Trace. 
100 . 42 
A comparison of this analysis with No. Ill on page 87 reveals the close chemical 
relation of the trachydolerite with the less alkalic of the syenites. 
SUMMARY. 
The occurrence of analcite as a groundmass constituent in a role similar to 
that of glass places this rock in Pirsson’s “analcite group,” 6 along with the mon- 
chiquites which it somewhat resembles/ 
From what has been said of its field occurrence and of its mineral and chemical 
composition, it appears probable that this trachydolerite is intermediate in age as 
well as in character between the latite-phonolites and syenites, on the one hand, 
and the latest products of eruption, the basic dike rocks, on the other. 
BASIC DIKE ROCKS. 
GENERAL DESCRIPTION. 
Cutting all the other rocks of the district, and hence representing the final 
phase of eruptive activity, are certain dark intrusives which occur as dikes and sills 
from a few inches to several feet in thickness. They are most abundant in the 
a Bull. U. S. Geol. Survey No. 228, 1904, p. 163. 
b On the monchiquites or anabite group of igneous rocks: Jour. Geol., vol. 4, 1896, pp. 679-690. 
c Cross, W., Sixteenth Ann. Rept. U. S. Geol. Survey, pt. 2, 1895, p. 37. 
