TERTIARY BASIC DIKE ROCKS. 
93 
510 2 .. 
A1 2 0 3 . 
Fe 2 O s . 
FeO.. 
MgO.. 
CaO.. 
Na 2 0. 
K 2 6.. 
H,0— 
11.. . 0 + 
TiO z .. 
Zr0 2 .. 
Analysis of trachydolerite of the Isabella dike. 
48.76 
C0 2 .. 
17.04 
p 2 o 5 . 
5. 04 
so 3 .. 
3. 47 
Cl... 
4.57 
FeS 2 . 
8. 64 
Cr 2 0 3 
4. 27 
MnO. 
3. 39 
BaO. 
.69 
SrO.. 
1.84 
1.34 
Trace. 
Li 2 0. 
0. 22 
.79 
Trace. 
.01 
. 11 
Trace ? 
.08 
. 15 
.07 
Trace. 
100.48 
A subtype of this group is represented in two dikes southeast of the Eagle mine 
on Bull Hill. They differ from the preceding rocks mainly in holding no olivine. 
On the other hand, their texture, which is intersertal, relates them more closely to 
the basalts. A decomposed rock occurring as a dike in the Ophelia tunnel probably 
belongs here. 
VOGESITE. 
The rocks of the vogesite group are phanerocrystalline, but are generally of 
fine grain. Small rounded grains of light-colored material (analcite) and flakes of 
dark mica are the characteristic minerals of the hand specimen. The rock of the 
Anna Lee dike in the Portland mine contains mica plates up to 1 cm. in diameter. 
Besides the Anna Lee dike, the dike in the Moose mine, one in the Jennie Sample 
mine, the Pinto dike, and perhaps some others are of this type. 
The microscope shows the essential constituents to be orthoclase, hornblende, 
pyroxene, and biotite, with usually some analcite, frequently olivine, and at times 
a little plagioclase. The monoclinic feldspar is abundant as rather irregular laths 
of good size, usually untwinned. Plagioclase is only sparingly present and 
from its low, double refraction is probably albite. Analcite occurs in octagonal 
phenocrysts, also as rounded amygdule-like grains and as groundmass areas, as in 
the Bull Cliff rock. On decomposition it forms stilbite and is sometimes replaced 
by sericite. Brown hornblende is plentiful in long columns bounded by prism, 
clinopinacoid, and orthodome faces. It is often twinned. The pleochroism is not 
very strong. Augite is rather abundant in crystalline grains and a few larger 
phenocrysts. Numerous small foils of deep-brown mica are present. Olivine was 
present in considerable amount, but is now largely changed to serpentine and car¬ 
bonates. Apatite is not common, but shows the same inclusions as in the Bull 
Cliff trachydolerite. Small particles of black iron ore are scattered here and there. 
While the size of the individuals of the several constituents is not uniform and a 
porphyritie appearance sometimes thereby results, the rocks are essentially 
panidiomorphic granular in texture. 
The analysis given below reveals the chemical similarity to the trachydolerites of 
the Isabella and Bull Cliff types. It shows that these rocks are somewhat deficient 
in ferromagnesian and rich in alkalic constituents for the typical vogesite, but the 
discrepancy is not very great and mineralogically they agree very closely. 
13001—No. 54—06-8 
