538 Jo Hea VOG Ts 
pyroxene, orthorhombic:monoclinic pyroxene, diallage:primary 
hornblende, hypersthene:biotite, are not subject to extensive 
variations. We may find a little variation sometimes in one and 
sometimes in the other direction, but this may be due in part to 
inaccurate determinations. But all in all, we here have approxi- 
mately the same MgO:FeO proportions in both minerals. We 
especially emphasize that no mineral is characterized by a constant 
relative enrichment either of MgO or FeO. Lesser variations, with 
regard to the MgO:FeO proportion, by two or still more ferro- 
magnesian silicates, crystallizing from the same magma, may be due 
to a ‘series of factors, of which we may mention the horizontal 
distance between the liquidus and solidus curves (or the difference 
between the a:b proportion in the first crystallized mix-crystal 
and in the liquid phase); the degree of equilibrium between the 
solid and liquid phases; the electrolytic dissociation. 
A small horizontal difference between the liquidus and solidus 
curves, and a nearly complete equilibrium between the solid and 
liquid phases will cause nearly the same MgO:FeO proportion 
between the segregated ferromagnesian silicates and the magma, 
and consequently also between the ferromagnesian silicates 
mutually. 
As well in olivine as in orthopyroxene and diopside-heden- 
bergite, the Meg-silicate is concentrated in the first mix- 
crystal. By more or less incomplete equilibrium between the 
liquid and solid phases—as in the dike and effusive rocks—we may 
expect a relative enrichment of MgO in the mineral which first 
commenced crystallizing. With two ferromagnesian silicates we 
may generally expect a more evenly distributed MgO: FeO pro- 
portion among deep-seated rocks with complete or nearly com- 
plete equilibrium between liquid and solid phases than among 
dike and effusive rocks. 
Addition.—Also in ilmenite a little MgO enters, viz., as MgTiO . 
In this manner ilmenite No. 54f from a peridotite (with about 
0.12 FeO:0.88 MgO in the entire rock) shows not less than 8.68 
per cent MgO, or 0.64 FeO:0.36 MgO. The ilmenite from the 
labradorite rock near Ekersund (with about 0.5 FeO:0.5 MgO 
