MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 527 
non, viz., lath-shaped labradorite individuals protruding into the 
olivine, and the latter shows no sign of idiomorphism even against 
the labradorite. The olivine chiefly fills the intervening spaces 
between the plagioclase laths in the same manner as in the anortho- 
site illustrated in Figures 23 and 24. We refer, for instance, to 
Figures 48 and 49 where the original structure, however, is partly 
effaced by the here quite strongly developed reaction rims between 
the olivine and the labradorite. In these olivine-poor olivine 
hyperites a greater or lesser part of the plagioclase must accord- 
ing to the structure have crystallized earlier than the olivine. 
On the basis of the crystallization sequence we draw the 
conclusion that the individualization boundary at high pressure 
between the olivine (about 1Fe,SiO, .2Mg,SiO,) and basic plagio- 
clase lies at little olivine and much plagioclase. As an estimate we 
may set the individualization boundary (by weight) at 0.15 
Oliv.:0.85 Ab:Anz,o0.25 Oliv.:0.75 Ab:An,, and probably about 0.35 
Oliv.:o.65 An. That these statements are approximately right is 
verified by a study of the ‘“‘orbicular gabbro” from Debesa, Cali- 
fornia, described by A. C. Lawson, which carries orbs with changing 
shells (challotes) of radially arranged olivine together with bytown- 
ite. ‘This structure must depend on a simultaneous crystallization 
of the two minerals, that is to say, a crystallization along a eutectic 
boundary-line. ° From the quantitative analysis we calculate the 
composition of this boundary at about 0.34 Oliv. (0.35 Fe.SiO,. 
0.65 Mg,siO,):0.66 Plag. (AbisAns,, consequently about Ab;An;.) 
The quartary system Ab:An:Mg,Si0,:Fe,SiO, separates at 
high pressure into two individualization fields, viz., Ab+An and 
Mg.SiO,+Fe,SiO,. We may here apply the same general con- 
siderations as for the system Ab+An and CaMgsi,06+ CaFeSi.06 
(see p. 442). 
OLIVINE AND PYROXENE 
Olivine and monoclinic pyroxene.—Forsterite, Mg,SiO, (melting- 
point=1890°) and diopside, CaMgSi,O¢ (melting-point =1391°), 
form, according to N. L. Bowen,? a eutectic, 88 per cent diopside: 
12 per cent forsterite with melting-point =1386°, consequently 
t Univ. of Cal. Publications, Dept. of Geol., Vol. III (1904). 
2“The Ternary System Diopside-Forsterite-Silica,’”? Amer. Jour. of Sci., Vol. 
XXXVIII (co14). 
