142 
ECONOMIC GEOLOGY 
The difficulty of determining the nature of these ores is 
increased by the extreme alteration of the country rocks. 
The general relation of the ores indicates that they are re- 
placement deposits formed under varying but comparatively 
superficial conditions. The simplest type is ore in lime- 
stone or dolomite, remote from igneous rock; the form of 
these ore-bodies recalls that of the replacement ore in the 
limestones of the N.-W. of England. In a second and common 
type the ores lie in hollows with an impermeable base, like 
those in the pre-Pal®ozoic rocks of Lake Superior, due to 
the stoppage of descending solutions. The ore of the third 
type is in wide sheets conformable with the country and 
resting on leptite or halleflinta; this ore may have been 
deposited on the surface of the igneous rock as a ferruginous 
crust; the iron was probably leached from underlying rocks, 
and precipitated on the surface, and may have been supple- 
mented by bog iron ore deposited in pools and enlarged by 
solutions during the metamorphism of the area and the 
intrusion of the granite dykes. 
The non-titaniferous nature of the ores is opposed to 
their igneous origin. Their chemical composition and dis- 
tribution are consistent with the formation of some by the 
processes which deposited the iron ores of Lake Superior 
and the NW. of England, and of others by the processes 
which form iron crusts in arid lands. 
3. ANCIENT SURFACE SHEETS, KiIRUNA—The rich magnetite 
mines at Kirunavaara in Swedish Lapland, despite the re- 
mote position N. of the Arctic Circle (68° N., 20° E., a little 
S. of Lake Tornea), are of commercial importance from the 
high grade of the ore and the easy mining, and are of special 
geological interest from the widespread belief in their ig- 
neous formation. Léfstrand in 1891 suggested that the 
magnetite was a segregation, as at Taberg. That view be- 
came untenable when it was found that the rocks on the 
two sides of the ore differ in age and character. The foot- 
wall is syenite-porphyry of which the upper part shows 
volcanic structures. Above the ore lie quartz-porphyry 
lavas interbedded with tuffs and sediments. The igneous 
origin of the ore has been re-introduced in three forms, 
According to R. A. Daly (1915), the magnetite solidified 
in the quartz-porphyry and sank to its base. According 
to O. Stutzer (New. Fahrb., xxiv, 1907) the ore is an iron oxide