THE ORDER OF CRYSTALLIZATION IN IGNEOUS 
ROCKS 
N. L. BOWEN 
Massachusetts Institute of Technology 
The order of crystallization in igneous rocks is necessarily 
determined from the end product, the solid rock. When thin 
sections of the holocrystalline rocks are examined the constituent 
minerals show certain relations of outline to each other that give 
some information as to the order of crystallization. The relations 
of outline referred to include: the idiomorphism of one mineral 
against another, the indentation of one mineral by another, and 
the complete inclosure of one mineral by another. It is one of 
the objects of this paper to inquire into the conclusions that may 
safely be drawn from observing these relations. 
Fig. 1 is a diagrammatic section of a crystal of, say, plagioclase 
which incloses a few crystals of another mineral, say, magnetite. 
Only the outer rim A of this crystal 
can be safely assumed to be of later 
crystallization .than the magnetite. 
The inner portion B may be of 
later crystallization but, on the 
other hand, may be earlier. The 
fact that magnetite is situated 
only in the peripheral portions 
would make one suspect the truth 
of the latter possibility, but from 
this selfsame crystal of plagioclase 
an endless number of sections could 
be cut in which there would be no 
evidence of this peripheral placing of the magnetite. Some sections 
would contain magnetite scattered throughout, even in the most 
central portions. S of Fig. 1 indicates the direction of such a 
section cut in a plane normal to, or at some high angle with, the 
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