44
ECONOMIC GEOLOGY
and regarded the hot water as derived from the syenite ;
but the pneumatolytic minerals are very exceptional, and
the water probably acted at a lower temperature. The
explanation which on a visit to the field in 1907 appeared to
the author best to explain the facts is that the ore was de-
posited as a surface sheet of hematite by water that perco-
lated through the syenite-porphyry, and that the sheet of
ore had been thickened on the under side by secondary replace-
ment, probably when the apatite veins and occasional
tongues of ore were deposited in the quartz-porphyry. This
explanation is consistent with the gradual passage of the ore
into the syenite-porphyry, with the sharp separation from the
overlying quartz-porphyry, with the phosphoric and practi-
cally non-titaniferous nature of the ore, and with the occur-
rence of the pebbles of the lode in the base of the overlying
quartz-porphyry.
Superficial sheets of ore, especially those of organic origin
(cf. p. 145), are characterized by a high proportion of phos-
phorus just as the igneous ores are characterized by their
richness in titanium. The bulk of the Kiruna ores is esti-
mated to contain from 1} to 2 per cent. of phosphorus, and
this argument for its organic origin has been emphasized by
W. H. Herdsman. Deposits of such antiquity in an area
which has undergone such changes and earth-movements
naturally show marked differences from analogous modern
deposits. The apatite has been dissolved and redeposited in
streaks in the ore and as veins and replacements in the
quartz-porphyry. At the same time some iron has been
dissolved and redeposited in places as tongues protruding
into the quartz-porphyry. These have been described by
Per Geijer as dykes, but that explanation is inconsistent with
the pebbles, of which the evidence seems the more weighty.
Tur BEDDED IRONSTONES—AQUEOUS PRECIPITATES—
About 80 per cent. of the known supplies of iron ore are inter-
bedded in sedimentary rocks. They include the British
Jurassic ores which have played a leading part in the dis-
cussion on these ores owing to their accessibility, early
economic importance and variety; the Liassic minette of
Lorraine, which is the largest reserve of iron ore in Europe ;
the Clinton oolitic hamatites of Ordovician age in Alabama,
which occur in a basin about 50 miles by 10 miles in area
