NORTHWESTERN PANGASINAN. 275 
of Sual. On the Marshall claim the copper is associated with 
pyritic lenses in the silicified fractures mentioned under gold. 
The origin of these lenses is not-altogether certain, but their 
“occurrence indicates that they have resulted from the shattering 
of the zone and the work of circulating waters. It is to be noted 
that pyrite occurs disseminated throughout much of the andesite, 
and its origin there is purely primary as an original crystallization 
from the magma. Faint traces of copper can be detected in this 
disseminated pyrite. Immediately surrounding these lenses, the 
andesite has been decomposed into a clay. This undoubtedly is 
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Fic. 5.—North-south section of workings on Marshall group. 
due to the oxidation of the pyrite, forming acidic waters which 
attacked and kaolinized the surrounding rock. The occurrence 
of gypsum and calcite crystals is indicative of this reaction. 
The lenses were sampled and analyses revealed copper, a little 
gold, antimony, and zinc. The gold is associated with copper in 
chalcopyrite; the antimony is combined with the copper in the 
mineral tetrahedrite; and the zinc occurs as sphalerite. The last 
mineral often occurs as black crystals lining cavities in the py- 
rite, and its occurrence there is purely secondary. As shown 
by Plate IV, a little prospecting work has been done on the 
