ORE DEPOSIT THEORIES. Did 
examination, and in part by the nature of the occurrences 
themselves. Thus, although the majority of cases of mag- 
matic differentiation show a gradual passing over from one 
type of rock into the other, sometimes this is not the case, 
and there is indisputable evidence that after the migration 
of the material, movements have taken place in the still 
molten rock, and irregular intrusions of one type into the 
other have been formed. In these cases the plane of division 
between the two types is often quite sharp. In other cases 
intrusions of the marginal type into the adjoining country 
rock occur. 
Segregations of Titanic Iron Ores in Gabbro. 
Although a large number of these deposits are known, 
comparatively few of them have been worked with success. 
This is due to their usually high percentage of titanic acid, 
‘which, besides being a drawback to their metallurgical 
treatment, also reduces the percentage of iron. The latter 
is often further reduced by a percentage of silicates, 
especially ferro-magnesian silicates, such as hypersthene, 
dialage, and olivine, so that they are generally somewhat 
poor, or only moderately rich in iron.. In spite of such 
drawbacks these ores, on account of their dimensions and 
the consequent cheapness of production, have in several 
localities given occasion to somemining. Asatypical deposit 
of this kind, the well-known deposit at Taberg, in Sweden, 
may be mentioned. The ore, which is known as “ mag- 
netite olivinite,” consists mainly of titano-magnetite, with 
some olivine, and in parts a little biotite and strongly basic 
plagioclase. The latter are absent from the high-grade ore. 
Step by step the passing over from the normal olivine 
hyperite, which is comparatively poor in magnetite and 
olivine, to the plagioclase bearing magnetite-olivinite, and 
finally the pure magnetite-olivinite may be traced. Here, 
as in all other deposits of titanic iron ore, the concentra- 
tions have been made in the centre of the eruptive mass, 
not at the margins. A great number of similar deposits 
are known in Norway and Sweden, Finland, the United 
States, Brazil, New Zealand, &c. All of them present 
essentially the same features. They are invariably con- 
nected with basic eruptive rocks, with a maximum of 55 to57 
per cent. of silica. Usually the rock is of plutonic origin, 
seldom dyke-forming, and never effusive. They are always 
found in the central portions of the eruptive mass, and 
usually a gradual passing over from the normal rock to the 
normal ore may be observed. 
