MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 429 
no eutectic point. But an eutectic boundary curve exists with 
simultaneous crystallization of plagioclase and diopside. This 
simultaneous crystallization does, however, not take place at a 
constant temperature, but continues some distance at decreasing 
temperatures. With complete equilibrium this drop of tempera- 
ture 1s, however, relatively small, as by the examples above chosen: 
for 50 per cent Diops: 50 per cent Ab,An,, from 1235 to 1200°, and 
for 15 per cent Diops:85 per cent Ab,An, from 1216 to 1200°. 
In deep-seated igneous rocks (with very slow solidification) a com- 
plete, or in special cases not quite but only approximately complete, 
equilibrium takes place between the solid and liquid phases (see 
a separate chapter in the following). And even in the more quickly 
cooled effusive rocks no completely lacking equilibrium appears, 
but an imperfect equilibrium, where the degree of imperfection 
is of a somewhat changeable nature. 
With regard to the relations of crystallization, especially in the 
deep-seated igneous rocks, it is of subordinate importance whether 
the simultaneous crystallization of the final components takes 
place at a constant temperature (eutectic point) or—by a small dis- 
placement of the quantitative proportion between the components 
—within a smal] interval of temperature for a short distance along 
a eutectic boundary curve. Bowen disputes the justification of 
the term “‘gabbroidic eutectic’? which I have previously used. 
I find, however, supported by Bowen’s experimental investigations, 
that this term must be maintained, when we emphasize the fact 
that here we have to do with a short distance on a curve and not 
with a point. 
We shall now examine the crystallization of the pyroxene 
minerals and plagioclase, especially in gabbros and norites, and 
shall commence with the well-known orbicular norite (orbicular 
quartz-norite) at Romsaas, in the Archaen formation, 50 km. south- 
east of Kristiania, described by several earlier investigators, 
especially C. Bugge.* 
Romsaas, which is a small hill rising about 60m. above the 
surrounding gneiss, consists chiefly of quartz-norite, with which 
t Kristiania Vidensk. Selskab, 1906. Here the earlier works of L. Meinich(1878), 
Th. Hiortdahl (1878), and K. v. Chrustschoff (1897) are cited. 
