240

but evidences of stratification in the latter were not found. A more acidic
intrusive, probably granodiorite, is exposed a few hundred yards from
the showings, but its relation to the diorite was not ascertained.

OCCURRENCE OF THE MAGNETITE

Magnetite is found in two different occurrences in this deposit, as
vein-like masses and irregular replacements in the volcanic rocks, and as
veins, small masses, and spotty impregnations in the diorite. The out-
crops extend over a longitudinal distance of 400 feet. Only three small
exposures of it were found in the tunnel, but it is interesting to note that
no magnetite was found near the face of the tunnel which lies 120 feet
underneath the bluff exposure. No structure is visible within the mag-
netite exposed on the bluff, and the contact seen on its western face between
magnetite and diorite is by no means a definite one, as there is a gradation
between solid magnetite on the north, through small, irregular-shaped
masses to veinlets and scattered grains to the south. These veins, con-
sisting of quartz with chalcopyrite, bornite, and magnetite (the latter
mainly in the diorite walls), have a steep northerly dip, giving a pseudo-
gneissic appearance to the diorite, and suggesting a nearly vertical structure.
On the other hand, this apparent vertical structure does not extend down-
ward to the tunnel level, and there is no other feature of the whole deposit
indicating a vertical attitude. Within the tunnel immediately northwest
of the first bend, there occurs a stringer or layer-like deposit of magnetite
exposed in both walls in nearly horizontal attitude, with a slight dip into
the tunnel. It is mixed with garnet, pyrite, and pyrrhotite, is about 30
feet in length, and varies in thickness from 6 to 18 inches. Whether this
has any relationship to the structure of the deposit is not certain, but it
might be suggested that a shallow dip of the voleanic rocks to the east or
southeast might explain the occurrence of this tabular mass, as well as
account for the absence of magnetite in the face of the tunnel.

CHARACTER OF THE MAGNETITE

Only very small amounts of solid magnetite are exposed in this deposit.
Most of it is badly mixed with garnet, tremolite, actinolite, pyrite, pyrrho-
tite, and altered volcanic rocks; the remainder is scattered through the
diorite, and associated with quartz stringers containing chalcopyrite and
bornite.

ORIGIN OF THE DEPOSIT

The magnetite is a replacement of the volcanic rocks and of the diorite,
manifestly by tenuous solutions that attacked the ferromagnesian minerals
in preference to, and before, the others. The deposit does not seem to
have the usual characteristics of the contact metamorphic type, and it
seems strange that a mass of limestone, so close by, should have escaped
replacement. The copper mineralization was at a later stage, following
quite clearly the deposition of magnetite.

The source of the solutions is not known. There may be two genera-
tions of magnetite—one in the volcanics due to the influence of the diorite
dykes or sills, and another in these intrusives, due to the influence of larger
masses of diorite elsewhere or at greater depth. It is also possible that
all the magnetite may be of one generation, supplied by emanations from