ORES OF IRON 14] 
pre-Palzozoic. The mines occur to the N.E. of Lake Wener 
and W.N.W. of Stockholm, near the towns of Norberg, 
Persberg, Granjesberg, Striberg, and Dannemorra. The 
field consists of gneiss and schists, with halleflinta, much of 
which was an acid lava, a variety of gneiss known as leptite, 
crystalline limestones and dolomites, and ‘ Skarn,” which 
is a schist containing hornblende, augite, chlorite, garnet, 
epidote, calcite, and quartz. The ore is mainly magnetite 
with about 60 per cent. of iron and I per cent. of phosphorus, 
and it is practically free from titanium. The ores occur in 
sheets or thin lenticles, some of which are more than a 1000 
yards long and 100 yards thick. The ores are often banded, 
and extend parallel to the country. Similar ores occur in 
Norway and owing to their association with igneous rocks, 
F16. 45.—THE MAGNETITE OF THE ADIRON- 
DACKS. 
The magnetite of the Adirondacks showing 
the magnetite (black) replacing the fel. 
spar, P, and quartz, Q, in granite at Cook 
Hill (after Alling, 1925). 
were claimed by Kjerulf and Dahl (1861) as eruptive in origin; 
but as the ore sometimes occurs as a cement between the 
rock fragments it is younger than the enclosing rocks, and 
the simple igneous theory is invalid. Origin by magmatic 
differentiation (Johansson) is equally impossible for ores in 
dolomite, . 
The usual explanation of these ores, based on their apparent 
conformity to the country and banded structure, is that they 
were sedimentary like those of the Mesozoic. H. Sjogren 
attributed the ores to metasomatic action by descending 
solutions ; but as he referred the process to too late a stage 
this view was generally rejected. 
In recent years the ores have been often regarded as 
contact products owing to the associated minerals, which 
are however equally characteristic of regional metamorphism ; 
and the ores are not always associated with igneous rocks 
and some of them are isolated in limestone.