THE GENESIS OF ORE DEPOSITS EAo3 
For example, the great iron-ore masses of Kiirunavaara and 
Luossavaara, as described by Daly, have been formed by the 
sinking of early-crystallized magnetite in a quartz-porphyry 
magma. ‘The case of iron ore is, however, exceptional in that it 
is not specially characteristic of any particular magma, but is of 
more or less universal distribution. 
Although this generalization must not be pushed too far, it 
is certainly true in its broad outlines. The reason for it is to be 
found in the presence in, or absence from, the crystallizing and 
differentiating magma of substances capable of combining with 
the metals to form volatile compounds, such as fluorine, boron, 
and especially water. We have every reason to believe that basic 
magmas do not contain much water; at any rate it is possible to 
produce basic rocks artificially from dry melts, whereas with acid 
rocks this cannot be done: some flux is required to insure the 
crystallization of quartz or orthoclase or hornblende. Such fluxes 
collect always in the acid fractions of a differentiate, there lowering 
freezing-points and in particular facilitating the concentration of 
metals in the last residue of the magmatic solution, which must 
always tend toward, if it does not actually reach, the composition 
of the multiple eutectic of the complex solution. In some simple 
cases the eutectic of quartz and felspar is actually reached, as seen 
from the simultaneous crystallization of quartz and felspar in 
- graphic pegmatites. 
The case of iron is a rather exceptional one, since it is found in 
considerable amount in connection with igneous rocks of both acid 
and basic types. The iron-ore masses of Sweden have already 
been mentioned: here the iron occurs as magnetite, occasionally as 
hematite, at all events as iron oxides only, and this is the common 
type in acid rocks. In the basic segregations, however, the state 
of affairs is generally different: in a great many ultrabasic rocks 
we find ferrous oxide combined with chromium as chromite, or 
with magnesium and aluminium in addition as picotite or some 
allied chromiferous spinel. In the felspathic basic rocks, on the 
other hand, the greater part of the iron is in some state of combina- 
tion with titanium, either as ilmenite or as a titaniferous magnetite, 
whatever that may really be. Again, in the basic rocks iron is 
