FIELD OBSERVATIONS IN NORTHERN NORWAY 
BEARING ON MAGMATIC DIFFERENTIATION 
STEINAR FOSLIE! 
Norges Geologiske Unders¢kelse, Kristiania, Norway 
Recent years have witnessed a marked development in the 
understanding of differentiation processes. Petrographers have 
tried to get away from the purely theoretical considerations about 
the matter and to harmonize the conclusions from the field observa- 
tions with the results of synthetical experiments on silicate minerals 
and their crystallization obtained by the Geophysical Institute 
at Washington and others. Although these experiments are still 
far from covering all subjects involved, and although the multitude 
of field observations from most parts of the earth are often contra- 
dictory, there seem to be certain lines of development which prove 
_ promising. 
The results now generally converge toward the conclusion that 
an ordinary fluid silicate magma, without concentrated mineralizers, 
is not capable of splitting up into two magmas mutually insoluble 
or with limited solubility. Accordingly, the main part of the 
differentiation processes is transferred to the period of crystalliza- 
tion, resulting in considerable restriction of possibilities. 
According to this supposition, naturally the first mode of 
separation to be considered was that of the heavier crystals from 
the lighter ones and from the still fluid magma by gravitative 
settlement. In a number of instances this sort of differentiation 
has definitely been proved to occur. But it also became obvious 
that this sort of separation could not alone account for a great 
many differentiation processes actually observed. 
The newer theory of squeezing differentiation—quite as well 
in accord with the latest results—seems to be capable of a more 
general application in those very frequent instances where lateral 
1 State mining geologist, Geological Survey of Norway. 
701 
