ORES OF IRON 139

igneous rocks of the Keweenawan. All the rocks are pre-Paleozoic,
 as the Keweenawan is correlated with the
Scottish Torridon Sandstone. The ore is generally hematite,
and contains usually between -02 and -05 per cent. of phosphorus
 ; it occurs in very varied positions, which have one
feature in common. The ore-bodies rest on an impermeable

Fig, 42.—LakE Superior IRoN Ores. -
The Mesabi type (after H. Garde, Bik. ¥ern-Kontorets Ann., 1918). The
ore occurs as a widespread sheet beneath an old land surface, and
overlies and is partly interbedded with iron-bearing chert,
lL Quartzsite,
Old land surface.
Iron ore.
Iron-bearing chert (taconite).

bed which may be a synclinal in slate or serpentine, or it
may, as at Mesabi, fade off in irregular tongues into ferruginous
 or greenalite chert. The iron ore has been formed
by replacement and deposited where solutions, which had
dissolved iron from the overlying red sandstones of the
Keweenawan or from basic igneous rocks, or from ferruginous
 or greenalite cherts, were stopped in their descent
(Fig. 43). The bulk of the iron probably came from an oolitic
Fra. 43.—Lake Superior ORES.
The Lake Superior ores formed by replacement
 bounded below by an impermeable
surface. From Cuyuna, Minnesota,
(after Van Barneveld). The rocks consist
 of cherty iron carbonates, amphibolite,
 slate, diabase dyke, DD. The
ore is in solid black.

)

chert series in which the ore was a chemical precipitate as in
the Mesozoic and Clinton bedded ores.
The Gellivaara and Adirondacks—The famous ore-field of
Gellivaara in Swedish Lapland contains massive bodies of
magnetite in biotite-gneiss. - The Ifoot-wall of the chief
body at Gellivaara that was being worked at my visit was