+ eats

GORE ES LE BReVIsINS Ol PELKVWVAlDIStRICE. 365

to the chalcocite and their relative ages cannot, at present, be

learned. The unusual structure of this bornite is shown in Plate
XXITI., C. A band of bornite of uniform width in a chalcocite
field follows a vein of hematite faithfully but is separated from it
by a narrower margin of chalcocite. It also shows a similar
band of bornite following the gangue contact, and separated from
it by a similar band of chalcocite. Small spikes of chalcopyrite
project into the bornite; they appear as white in the figure. A
probable explanation of the structure is that the bornite bands
are residual remnants after the replacement of the rest of the
bornite by chalcocite, these being protected from the replacing
solution by the solution pressure of the iron of the nearby
hematite.

Chalcopyrite does not occur in the ore excepting in the minute
spikes just mentioned, and as a few stray grains in the quartz.

Native silver can be seen in all of the specimens of the ore in
which calcite is present. The chalcocite is nearly always blue
when near the native silver. It was never observed in contact
with the white chalcocite and its age can only be determined in-
directly. In one specimen it was replaced by hornsilver.

Argentite is only found surrounding the silver and is not
plentiful. It is later than the native silver, and was also seen
filling cracks in the chalcocite.

Malachite and azurite occur incidentally as secondary minerals.

Hematite, the characteristic and ever-present mineral in this
district, is present in large amounts and is most plentiful where
epidote and calcite occur, especially in the epidote band through-
out which it is disseminated only in the form of small blades.
Wherever the epidote has penetrated the ore, the hematite is
- closely associated with it, but it has itself penetrated much farther.
Microscopically small veins of it are seen in every specimen. These
veins (see in Plate XXIII, C) have a strong tendency to follow
the chalcocite-gangue contacts. These lines probably offered the
least resistance, and were, therefore, usually selected by the iron-
bearing solutions, but one also finds the veinlets running across
the chalcocite sometimes cutting off small peninsulas of the chal-
cocite which jut out into the quartz. Another habit of the hema-

: :
#]
4
4
Pe
: v