MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 439 
silicates, indifferently whether hypersthene (Fig. 14), diallage, or 
olivine (see Figs. 23 and 24), for an essential part form a mesostasis 
between the plagioclase (labradorite or sometimes bytownite). 
The question in hand, I have to some extent considered in a 
paper, accompanied by several analyses, published in the Quart. 
Jour. of the Geological Society, 1909, on labradorite-norite with 
porphyritic labradorite crystals from Flakstadden in Lofoten. 
Referring to the quantitative analysis of this rock, given in the 
chapter on anorthosite-norite in Part II, I shall here give a short 
résumé. ‘The entire rock consists of: 
ca. 70.65 per cent labradorite, 55 An, 39 Ab, 6 Or 
Coe One = per cent He,O), 
Gis O70) per cent Heli; 
ca. 10.0 per cent hypersthene 
ca. 10.0 per cent diallage 
ca. 2.3 per cent biotite 
ca. 0.9 per cent apatite 
My. 2 per cent “‘titanomagnetite” 
Relatively to the entire rock, there first crystallized 23 per cent 
prophyritic labradorite of a composition 61 An, 33 Ab, 6 Or, and 
occurring as very large, up to 15-18 cm. long and 6-8 cm. wide, 
crystals, thick tabular along (o10). The remainder, 77 per cent, 
form a coarse-grained groundmass, consisting of: 
ca. 61.9 per cent labradorite, 52 An, 42 Ab, 6 Or 
per cent Fe,O, 
per cent FeTiO, 
i \o per cent “‘titanomagnetite”’ 
9 
ca. 13.0 per cent hypersthene 
° 
fo) 
per cent diallage 
.O per cent biotite 
ca. ©.12 per cent apatite 
In this groundmass the labradorite continued crystallizing, 
and some magnetite (or ‘‘titanomagnetite””) commenced to form, 
of which more below. Only at a somewhat later stage, after the 
quantity of hypersthene and diallage had risen somewhat above 
30 per cent, the pyroxene commenced forming. 
The hypferitic (or ophitic) texture of plagioclase crystals, with 
tabular development along (o10), appears in the gabbro rocks only 
when the latter contain at least about 55 per cent plagioclase 
(labradorite). The laths of plagioclase show partial idiomorphism 
