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370 V. DOLMAGE.

relation of the galena to the bornite is also indicative of the
bornite being primary. The small scattered and rounded grains
of galena in the bornite resemble very strongly the type of struc-
ture seen in zinc blende, which is not known to ever have had a
secondary origin. The occurrence of bornite specks in zinc
blende, and also in magnetite, is still further evidence in favor of
its primary origin.

Aside from the chalcocite, there is no mineralogical evidence
that secondary processes have played a part in the formation of
this ore. Pyrite is entirely absent from the ore-bearing bed
itself, and from any of the nearby country rocks, since it is con-
fined to the metamorphic zone. And without this mineral or
marcasite to supply the iron sulphates for lixiviation, it would be
difficult to obtain enrichment in this type of deposit. Such min-
erals as chalcedony and opal, which are strongly indicative of en-
richment processes, are not present.

In the case of this deposit, too, the geological conditions are
very strongly opposed to the enrichment theory. Here is a bed
only 20 feet thick, with a dip of only 18 degrees, buried under
1,250 feet of barren tuffs whose original thickness must have
been much greater. It is exposed to the surface only on a ver-
tical wall of a glacial cirque formed long since glacial time, and
contains ore shoots of, in places, almost solid bornite and chal-
cocite. In the absence of a leached zone above, but instead, an
overburden of 1,250 feet of barren tuffs, where could the enrich-
ing metal come from? Supposing this were a chalcopyrite de-
posit so enriched, and then consider the amount of copper which
would have to be brought from the overlying rocks. Chalco-
pyrite contains 34.5 per cent. copper, and a unit volume of bornite
and chalcocite in equal amounts, contains about 67 per cent. cop-
per, therefore, since replacement takes place volume for volume,
the replacement of chalcopyrite by a mixture of this kind, which
is very similar to the present case, would involve almost doubling
the original amount of copper.

There is also much good evidence for believing the chalcocite
of the High Grade claim to be of primary origin.