TERTIARY VOLCANIC ROCKS-SYENITE. 
87 
As in the preceding-rocks, pyrite replaces magnetite, and carbonates and fluor¬ 
ite are introduced by metasomatic action along veins. 
The texture is in general typically granular, but occasionally becomes sub- 
porphyritic. The textural gradation to latite-phonolite has already been described. 
CHEMICAL CHARACTER. 
The analyses given below make clear the chemical nature of these rocks and 
their close relation to the latite-phonolites: 
Analyses of syenites from Cripple Creek. 
w 
I. 
II. 
III. 
IV. 
SiO;. 
54. 43 
54. 34 
51.98 
56.01 
A1 2 0 3 . 
19.01 
19.23 
17.94 
17.92 
FejOj. 
2.85 
3.19 
3.85 
4.20 
FeO. 
1.93 
2.11 
3.37 
2.52 
MgO. 
.99 
1.28 
2.88 
2.04 
CaO. 
4.33 
4.53 
5.62 
4.80 
Na 2 0. 
0.92 
6.38 
4.63 
4.92 
KjO. 
5.07 
5.14 
4.50 
4.21 
HjO —. 
.31 
. 14 
.72 
.31 
H 2 0+. 
1.68 
1.17 
2.09 
1.10 
Ti0 2 . . 
.96 
1.09 
1.34 
1.20 
ZrC >2 _ - . 
.04 
.07 
.03 
.02 
co 2 . 
.14 
Trace. 
Nor.e. 
P 2 0,. 
.25 
.27 
.67 
.55 
so 3 .. . 
.42 
.07 
None. 
None. 
Cl . 
.22 
.28 
Trace. 
Trace. 
.07 
.41 
.04 
MnO. . . 
.08 
.08 
.08 
.13 
.21 
.24 
.19 
.16 
.21 
.16 
.11 
.06 
Trace. 
Trace. 
Trace. 
* 1 
100.07 
99.77 
100.32 
100.19 
I. Latite-phonolite, south slope of Bull Cliff (IV of table on p. 79). 
II. Syenite, near Longfellow mine, west of Vindicator mine. See Cross, W., Sixteenth Ann. Rept. U. S. Geol. Survey, 
pt. 2, 1895, p. 45. 
III. Syenite, Portland mine, 500-foot level. 
IV. Latite-phonolite, Portland mine, 600-foot level (I of table on p. 79). 
Analyses I—III by W. F. Hillebrand; IV by George Steiger. 
II. This specimen is light gray and finely granular, with occasional larger 
grains of feldspar and numerous prisms of pyroxene showing a slight flow structure. 
In thin section the feldspars are found to be mostly ortlioclase, with some micro- 
perthite, considerable albite, and a little which is perhaps more calcic. Sodalite 
occurs in good-sized grains, some of them well crystallized, others of irregular form; 
it is commonly decomposed. The analysis indicates that a little nosean is also 
present. Analcite is rather plentiful in clear grains of irregular outline commonly 
associated with feldspar. Extremely minute needles, perhaps of stilbite, penetrate 
these grains. The occurrence of the analcite is the same as in the latite-phonolites. 
No nepheline can be distinguished, though Mr. Cross thought that certain turbid 
areas represent it. The pyroxene has the colors and low extinction angle of aegirine- 
augite. It is often surrounded by a zone of greenish-brown hornblende, which 
I 
