10

A special feature of almost all the deposits is the sharpness of their
outlines. Within spaces of a few inches or a few feet, unaltered or only
slightly altered country rock gives way to highly altered material or to
almost solid masses of magnetite and secondary silicates. Where limestone
(usually crystalline) is the host the contact between pure limestone and
the solid bodies of magnetite, garnet, etc., in many cases is a sharp line.
Away from this boundary the limestone for an indefinite distance is wholly
unaltered, except in so far as it has been rendered crystalline. Within the
areas of alteration, masses of only partly destroyed country rock are not
infrequent. Apparently the action of the mineralizing agents was con-
fined, in any one place, to comparatively narrow limits and although it
seems to be a general rule that the deposits do not occur singly but
occur in many places over districts miles in length, yet, as numerous
examples indicate, the limits of the individual masses seem to bear a
direct relation to various locally developed, major geological structures.
An important conclusion deducible from various facts detailed in the
two succeeding chapters is, that no individual deposit extends to great
depths and that an extensive outcrop does not always imply a consider-
able vertical thickness. It is also concluded that in any given district
the discovery of a single body of magnetite is an almost certain in-
dication that other bodies exist or did exist nearby either vertically or
horizontally or in both directions, but there are no known grounds for
supposing that the undiscovered bodies le immediately adjacent to the
known deposit. A third main conclusion is that if in any given case the
attitude of a mineral body is not clearly apparent it is not safe to assume
that the mass extends in any direction more than a few feet beyond its
observed limits, whether these be against drift or solid rock. A further
widely applicable generalization is that the mineralogical composition and,
therefore, the iron content, of the individual deposits, is variable. Con-
siderable parts of every deposit consist of low-grade materials and the dis-
tribution of the high-grade material appears to follow no predictable course.

The foregoing remarks apply to the usual type of magnetite deposit
encountered along the Pacific coast, but some of the bodies there found, as
on Pitt (locality 3) and Porcher (locality 2) islands, and the west side of
Seymour inlet (locality 14) differ from the ordinary type in that they are
more of the nature of impregnations and have developed within compara-
tively narrow though in some cases long zones, in schistose sediments and
volcanics. In places the impregnations consist of massive bodies of magne-
tite, in other places the iron oxide oecurs in streaks and vague patches.
These impregnations lie close to the edges of granitic bodies and appreciable
amounts of secondary silicates, including garnet, are present. Such deposits
are considered to be a special phase of the more typical contact-metamorphic
deposits.

The definitely known magnetite deposits occur at intervals along the
whole length of the Pacific coast of British Columbia and have been found
in southern Yukon along or close to the eastern edge of the Coast Range
batholithic area. In addition to the publicly known deposits, there is
substantial evidence that many others have been found, and it seems
almost certain that many more deposits, a majority of which are doubtless
of no value, remain still undiscovered on the islands and mainland coast
within areas of sedimentary and volcanic strata adjacent to the edges of
granitic bodies.