714 STEINAR FOSLIE 
composition. Chemical calculations, however, show this to be 
impossible, and the differences in the composition of the plagioclase 
point in the same direction. 
We might also consider another sort of differentiation, according 
to which the marginal zone of the massive represents the mean, 
original composition of the magma, intruded by younger femic 
and salic differentiation products. ‘The evident field observations, 
however, contradict such a supposition also. 
Here, therefore, the squeezing theory, as developed by N. L. 
Bowen (loc. cit.), turns out to be a very natural and obvious 
explanation. 
It must be remembered that the process takes place under 
orogenic pressure. While the segregated crystals are only sus- 
pended in the magma, the pressure is static and has no influence 
on the differentiation. From the moment when the outmost shell 
forms a fixed crystal mesh, this shell has eventually to take up 
the mountain pressure, but, of course, at first is not able to do so. 
The pressure then will be dynamic. Following Bowen, this stage 
is supposed to occur when about 80 per cent of the mass is crystal- 
line and only 20 per cent liquid. 
As the volume of the magma is diminished by cooling and still 
more by crystallization, the outer shell will be compressed and its 
remaining interstitial liquid squeezed out. At that advanced stage 
of crystallization, this liquid will contain plagioclase, richer in soda, 
and pyroxene, richer in iron and lime than the already segregated 
crystals, moreover eventually potash-feldspar, free quartz, and 
magmatic water. 
These components accordingly will move, and the direction of 
movement will be inward from the z»nz of dynamic into the zone 
of static pressure, because in the crystallizing zone with still static 
pressure the diminishment of volume will have the effect of releasing 
the pressure. 
It is important to note that the process is here not supposed to 
be a squeezing for a long distance through a crystal mesh. Prob- 
ably it is mainly a differential movement, restricted to the narrow 
transition zone, which moves inward at the same rate as the crystal- 
lization proceeds. 3 
