MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 521 
components) are consequently related to each other as the com- 
ponents with high and low melting-points in the binary mix-crystal 
system No. IV (cf. Fig. 36). 
PYROXENE AND BIOTITE, RESFECTIVELY HORNBLENDE, IN 
THE GABBRO ROCKS 
We have previously treated one case, viz., the quartz-norite 
from Romsaas (see pp. 434-35) where hypersthene first crystallized, 
but later stopped forming as the remainder of the ferromagnesian 
silicate in the magma entered into dzotite. In another series of 
norites and gabbros with relatively much orthorhombic or mono- 
clinic pyroxene and usually with only 2-5, seldom up to 8-10 per 
cent of biotite, we find the latter (see, for example, Fig. 13) partly 
grown into the exterior parts of, and partly grown on to, the pyrox- 
ene, indicating that the biotite belongs to a somewhat later stage 
of crystallization than the pyroxene. 
Primary hornblende is lacking in numerous Norwegian norites 
and gabbros, but appears in others, most often, however, only in a 
quantity of 5-ropercent. ‘This primary hornblende, which always, 
or nearly always, is brown or greenish-brown, and probably through- 
out contains some titanic acid, often shows a parallel intergrowth 
with the orthorhombic or monoclinic pyroxene, in such a manner 
that the hornblende appears at the periphery of the pyroxene. 
Or the hornblende may have grown as independent individuals on 
to the pyroxene. This signifies also for the hornblende a stage of 
forming later than for the pyroxene. The explanation probably 
is that the original small contents of H,O in the gabbroidic magma 
was concentrated in the residual magma by the solidification of 
a greater or lesser part of the pyroxene, so that biotite, respectively 
hornblende, was able to individualize. This will be discussed in a 
later chapter. 
| OLIVINE, Mg,Si0,:Fe,SiO, 
The melting-point of pure Mg,SiO,, according to the determina- 
tion at the Geophysical Laboratory, Washington, is 1890°. The 
melting-point of Fe,SiO, lies, according to approximate determina- 
tion, at ca. 1100°. Doelter gives ca. 1065°. Mg,SiO, and Fe,SiO, 
form a continuous mix-crystal series. 
