40 ECONOMIC GEOLOGY
sharply bounded by a fault plane with an impermeable band
of clay. The hanging wall was an irregular ill-defined passage
into gneiss (Fig. 44). The ore included masses of gneiss in
their original positions, showing that the ore had been formed
around them by replacement. The Gellivaara ores belong
to the Lake Superior type, for they are due to descending
water which had become charged with iron—the abundant
biotite in the gneiss is the obvious source—and deposited
it in masses where further descent was blocked by an im-
permeable layer. The ore is a valuable non-titaniferous
magnetite.
In the Adirondack Mountains is another occurrence of
non-titaniferous magnetite in pre-Paleozoic gneiss, schists,
limestones, and plutonic rocks, which has been regarded as
an igneous segregation. The ore is in lenses or pod-shaped
F16. 44.—IroN ORE Deposits aT
GELLIVAARA,
The ore is in biotite-gneiss resting
on a sheet of pug formed by
a fault (F); the hanging wall
is irregular; blocks of gneiss
occur in the ore, in situ.
bodies bent in harmony with the folds in the country rocks.
The association of the magnetite with quartz, fluorite, apatite,
and pegmatite, and its occurrence in any of the local rocks
except the basic igneous rocks—in which igneous ores would
be most likely—render probable a hydrothermal origin, as
advocated by the author in 1924 (Trans. Faraday Soc., xx,
P. 454) and by H. J. Alling (Econ. Geol., xx, 1925, pp. 335-63),
who has shown that the magnetite is of three distinct ages,
that some of it, as in the granite (see Fig. 45), is of late
origin, and has replaced quartz and felspar. He concludes
(ibid., p. 363) that these ores are * magmatic-replacement
deposits due to aqueo-igneous magnetite-rich solutions de-
rived from a differentiating granitic magma."
Middle Sweden—The iron-fields of middle Sweden, where
Swedish iron and metallurgists earned their high reputation,
are in metamorphosed pre-Palzozoic rocks. The rocks and
ore are traversed by granite-pegmatite dykes which are also
