MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 323 
this does not exclude that the olivine may have commenced 
crystallizing earlier than the chromite when the latter only 
amounted, for example, to 0.1 or 0.05 per cent. The earlier 
silicate minerals, for example, olivine in olivine-rich gabbros, and 
hypersthene or diallage in hypersthene- or diallage-rich gabbros, 
also often show the together-swimming structure, as in Figures ro, 
fone 20 and 21, and 23. 
The relative commencement of the solidification, especially of 
the minerals that commence crystallizing at a somewhat early 
stage, may often quite easily be decided by the structure. On the 
other hand, the allotriomorphism of a mineral C, against the 
minerals A and B, shows that C only commenced crystallizing 
after an often quite essential part of A and B had already solidified. 
Especially where the later mineral C is present in a small quan- 
tity, its allotriomorphism in connection with its appearance as 
Zwischenklemmungsmasse (or mesostasis) presents an easily recog- 
nizable criterion that it belongs to.a very late stage of the crystalliza- 
tion. But at this late stage the minerals A and B will in many 
cases have continued forming. We refer to the explanation given 
in connection with Figures 17 and 18. 
The simultaneous crystallization of two or more minerals may 
be manifested in various ways. With two simultaneously crys- 
tallizing minerals, each may grow until the individuals of A happen 
to collide with the individuals of B. Or some of the segregating 
mineral A may be deposed on the already solidified crystals of B, 
and some of the simultaneously segregating mineral B on the 
already solidified crystals of A. 
In this manner we may observe crystals of plagioclase with 
quite good idiomorphic contour against the hypersthene or diallage 
and, further, crystals of hypersthene or diallage with quite good 
idiomorphic contour against plagioclase in the same thin section 
of an anchi-eutectic norite or gabbro. In the deep-seated rocks 
it may in such cases often be quite impossible to decide which of 
the two minerals first commenced crystallizing. 
It may also often occur that the two minerals crystallize in an 
intimate intergrowth. In this manner simultaneously crystallizing 
