492 R. H. RASTALL 
indicator of the possible presence of the platinum metals. In the 
basic rocks also we find large masses of iron ore commonly rich in 
titanium, either as ilmenite or as titaniferous magnetite. The 
presence of titanium is specially characteristic of the basic group. 
Another type of great scientific interest and commercial importance 
is seen in the great segregations of nickeliferous sulphides found in 
connection with norites and gabbros, as at Sudbury, Ontario, and 
Insizwa in Griqualand East. Whatever may be the actual mecha- 
nism of the Sudbury nickel deposit, its close connection with the 
norite intrusion will hardly be denied, and at Insizwa the con- 
centration of the sulphide by some form of differentiation seems 
clear. In association also with basic rocks, especially with gabbros, 
are found great masses of pyrite, as at R6rds in Norway, and the 
cupriferous sulphides of the Huelva district (Rio Tinto and others) 
also belong here. 
In all these cases of the occurrence of ores in basic magmas 
one point is worthy of special attention, namely, that the ores 
occur either as disseminations in the rock itself (platinum and 
chromium in part) or as segregations separated from the magma 
either by gravity, by diffusion to the margin, or by separation of 
immiscible liquids. ‘The ore bodies therefore lie either within the 
intrusion as disseminations, schlieren, or irregular patches, or as a 
definite layer at its base, or as separate intrusions, often laccolithic 
in form. Here there is no separation of the ores in pegmatites and 
all the innumerable varieties of mineral veins, as with.the acid 
rocks, while pneumatolytic effects are subordinate, or more com- 
monly entirely absent. In conformity with this, metamorphic and 
metasomatic effects are also much less marked. The most charac- 
teristic type of basic pneumatolysis is scapolitization, which 
appears to correspond to tourmalinization in acid rocks, while the 
concentration of apatite In some instances may also be referred 
here: 
Here another generalization of fundamental importance may 
be made: the ore segregations of the basic rocks are usually the 
first fraction of the magma to crystallize, those of the acid rocks 
are usually the last fraction. In neither case is the statement 
invariably true, but exceptions are as a rule due to special causes. 
