128 
ECONOMIC GEOLOGY OF SILVEKTON QUADRANGLE, [bull. 182. 
ill 
From this analysis and the microscopical study the mineralogicai 
composition nmy be calculated as follows: 
Mineralogicai composition of monzonite-porphyry. 
Constituent. 
Albite (molecule). 
Quartz 
Orthoclase (molecule) 
Chlorite 
Calcite 
Sericite 
Constituent. 
Titanic iron ore 
Magnetite and rutile 
Pyrite 
Apatite 
Per cent. 
1.5 
1.2 
All the lime shown in the analysis is required to form calcite with 
the carbon dioxide, so that the plagioclase, whatever its original com- 
position, must at present contain very little lime and is calculated as 
albite. The appearance of the thin section indicates that the chem- 
ical determination of the pyrite may be a little low and that a portion 
of the iron, here calculated as titanic iron ore, should really be estimated 
as pyrite. 
On the north side of the National Belle knob a mass of this same 
porphyry has been altered by metasomatic processes to a very light- 
gray rock in which the feldspars are transformed to dull kaolin-like 
aggregates while the quartz phenocrysts are apparently unchanged. 
A few minute veinlets, filled with kaolin, occur in the hand specimen. 
The groundmass is gray .in color, compact in texture, and abundantly 
sprinkled with minute crystals of pyrite. This alteration is evidently 
directly connected with the deposition of the National Belle ore bodies, 
and is merely one phase of the metaniorphism which lias given rise 
to the siliceous mass in which the ore occurs. 
Under the microscope rounded and embayed phenocrysts of quartz 
containing minute fluid inclusions, and pseudomorphous aggregates 
after feldspar, are seen to lie in a finely crystalline, granular ground- 
mass. The feldspar phenocrysts have been changed to pseudomorphs 
of kaolin, diaspore, and quartz. The diaspore is intimately associated 
with the kaolin, in which it often occurs embedded in ragged, shred- 
1 ike areas. Near it there is sometimes a small amount of a colorless 
isotropic mineral with a fairly high refractive index, which has thus 
far defied identification. The groundmass is a finely crystalline mosaic 
of quartz and kaolin with small scattered crystals of pyrite and occa- 
sional crystals of rutile and apatite. Sericite was not noted, but is 
perhaps not entirely absent. 
A chemical analysis of this rock is given under II, on page 127, and 
although the two rocks are not from the same mass, there can be little 
doubt that they were both originally of the same character. 
It may be that the nearly identical contents in alumina shown by 
the two analyses is merely a coincidence, but it is probable that it 
