IGNEOUS ROCKS. 
49 
The biotite andesite has the same texture and mineral composition 
throughout the different areas. The absence of marginal facies (as 
a basic edge), dikes, and pegmatic veins is especially noticeable. 
METAMORPHISM OF ANDESITE BY THE ORE-BEARING SOLUTIONS. 
Near its contact with the ores for a few feet the andesite takes on a 
greenish-yellow color and dense texture, very similar to that of the 
silicated limestone at the andesite contact. The two are with 
great difficulty discriminated. The grain of the contact phase is much 
finer than that of the main mass of the andesite. The groundmass 
consists of finely crystalline feldspar and quartz, principally the 
former, cloudy with dark clayey-looking material, much of it not 
recognizable, but some of it in the larger masses clearly muscovite 
and rarely chlorite. The contact between the altered phase and 
the fresh andesite is sharp. 
A comparison of the analyses of the fresh and contact phases by 
the circular-diagram method (Pis. XIII and XIV) brings out clearly 
the fact that at the contact the conspicuous change has been the 
introduction of soda. All other constituents show a possible loss, 
though ferric iron has developed at the expense of ferrous iron. In 
the present state of development of the district it is not possible to 
secure specimens from horizons surely below the influence of weath- 
ering. It may be that the oxidation of the ferrous iron here indi- 
cated is really a superposed weathering effect. 
When the normal weathered andesite from the surface is compared 
with the fresh andesite, as in the following table (see also PI. XII, B, 
p. 30), the change is found to be of quite a different character from that 
along the ore contacts. All constituents, including soda, show a 
loss relative to the alumina, as in PI. XII, B. 
Analyses of fresh and altered andesites. 
[Analyst, R. D. Hall, University of Wisconsin.] 
A. 
B. 
C. 
D. 
E. 
Si0 2 
65 29 
11.57 
2.10 
2.67 
2.87 
4.85 
2.10 
518 
.50 
1.82 
.22 
.17 
63. 63 
L5.64 
3. 59 
.93 
2.32 
4.46 
1.70 
5.22 
.40 
1.70 
.15 
.05 
65.80 
14.48 
3.96 
.70 
2.35 
3.19 
3.78 
3.32 
12 
2.71 
.12 
.06 
63.82 
14.28 
2.72 
.81 
5.98 
.70 
3.62 
4.24 
2.30 
1.68 
.04 
.04 
63. 76 
AUOa 
L6 05 
Fe 2 3 
1. 91 
FeO 
58 
MgO 
2.46 
CaO 
4 25 
Na s O 
6.26 
K 2 
2 84 
H2O- 
1 22 
H 2 + 
.93 
P2O5 
.28 
BaO 
None. 
99.34 
99.79 
100. 59 
100.23 
100. 54 
A. Specimen 46612. Fresh andesite east of Granite Mountains. 
B. Specimen 46377. Weathered andesite from Desert Mound. 
C Specimen 46433. Altered andesite near iron-ore contact from Blowout, south of Iron Mountain. 
D. Same nearer iron ore contact. 
E. Specimen 46481. Altered andesite near iron-ore contact from Emma claim on east slope of iron 
Mountain. 
28463— Bull. 338—08- 4 
