KAXSOME. 
METAMORPHISM OF THE COUNTRY ROOK. 
127 
the rock in all probability contains over 90 per cent of silica, and the 
evidence of thorough silicification, with removal of nearly all of the 
bases save a little iron to form pyrite, is unquestionable. It shows 
that under certain conditions even the alumina may be removed, and 
that the assumption as to the constancy of this constituent in unal- 
tered and altered rock must be used with caution. 
An intrusive porphyry, belonging* with the monzonite-latite group 
of rocks, is fairly abundant in the Red Mountain region, where ii 
forms relatively small masses of circular or elliptical plan. It has 
been intruded in the form of plugs or bosses into the volcanic rocks, 
and, like the latler, has undergone metasomatic alteration. Where 
not g -eatly altered — as, for example, at an exposure by the roadside a 
lew hundred yards south of the Yankee Girl mine — the rock is con- 
spicuously porphyritic. showing phenocrysts of pink orthoclase up to 
three-quarters of an inch in length, smaller crystals of white plagio- 
clase, and anhedrons (crystalline grains) of quartz, all lying in a 
compact, greenish-gray groundmass. 
I T nder the microscope the lock shows considerable decomposition. 
r l ne orthoclase is fairly fresh, but the plagioclase (apparently in part 
acid oligoclase) is partly sericitized. Areas of calcite and chlorite 
with a little pyrite are apparently pseudomorphous after augite or 
diopside. The groundmass is a rather cloudy, fine-granular aggre- 
gate which seems to consist chiefly of quartz and orthoclase, but con- 
tains considerable calcite and chlorite, with some apatite, a good deal 
of finely disseminated pyrite, and some titanic iron altering to leu- 
coxene. A chemical analysis of this rock is given in Column 1 in the 
follo\. ing table: 
Analyses of monzonitic porphyry. 
II. V Stokes, analyst.] 
i lonstituent. 
I. 
II. Constituent. 
I. 
II. 
SiO,. - 
A1 2 3 - 
Per cent 
58. 78 
13.62 
1.7(1 
2.27 
3.29 
4.31 
3. 24 
4. 06 
. 25 
1.57 
.99 
3.49 
Per cent. 
73.01 P 2 O fi - 
13.97 SO, 
Per cent. 
.53 
None. 
None. 
Trace. 
,, 2 
.05 
Trace. 
1.56 
Per cent. 
.33 
Notdet. 
Fe.>O s 
None. 
None. 
CroOa 
None. 
FeO 
NiO 
None. 
MgO 
CaO 
None. 
.31 
.04 
.06 
.58 
4.18 
MnO 
BaO 
Trace. 
.04 
Na«0 
K>0 
SrO 
Trace. 
LioO 
Strong tr. 
HoO 
FeSo 
5.62 
H,0+ 
99.73 
99.34 
TiO, 
.60 
None. 
C0 2 
I. Monzonitic porphyry, from near Yankee Grirl mine. 
II. Metasomatically altered facies of foregoing, from near National Belle mine. 
