116 PETROLOGY OF THE ALKALINE ROCKS 



Composition. — The phenocrysts are corroded olivine, enstatite, magnetite, and a 

 few idiomorpkic labradorites full of magnetite inclusions. The base, which is almost 

 panidiomorphic in structure, contains brown titaniferous augite in prismatic grains, 

 magnetite, and a smaller amount of basic labradorite microlites. 



J. 9 (1918). Limburgite, Observation Hill. — Texture. — Microporphyritic serial, 

 dopatic, magnophyric. 



Fabric. — Intersertal, diabase structure. 



Composition. — This rock consists of hypidiomorphic grains of titaniferous pyroxene 

 (about 50 per cent.), labradorite microlites (about 35 per cent.), idiomorphic magnetite 

 (about 10 per cent.), and olivine in microscopic phenocrysts (about 5 per cent.). 



J. 23 (1932). Basanitic Limburgite, Cape Barne. — This is a similar rock to J. 9, but 

 more deficient in felspar, wkick amounts to less than 20 per cent. 



Texture. — Porphyritic hiatal, dopatic, panidiomorphic. 



Composition. — Magnophyric phenocrysts of olivine and hypersthene imbedded in a 

 base consisting of titaniferous augite, very basic plagioclase, magnetite, and a colourless 

 interstitial substance with the refractive index and double refraction of nepheline. 



J. 61 (537). Limburgite. — Hand- speci men. — Very vesicular grey rock, porphyritic 

 in magnophyric corroded olivine grains. (See Plate IV, figs. 1 and 2.) 



Texture. — Porphyritic hiatal, dopatic, megaphyric, with divergent radial fabric in 

 base (strahlenkornig). 



Composition. — The phenocrysts consist of olivine and light green pyroxene, both 

 highly corroded. They are bordered with a rim of magnetite granules. 



The base is holocrystalline, and consists of white prismatic pseudomorphs shaped 

 like large felspar laths, which are arranged in a divergent radial manner like the felspar 

 of ophitic diabase, and between these mineral groups lie intersertal masses of 

 titaniferous augite and magnetite in minute idiomorphic grains. 



The divergent radial aggregates appear to be made up of minute idiomorphic 

 pyroxenes, cpiite colourless and non-pleochroic, with a slight interstitial amount of 

 nepheline. The pyroxene may be wollastonite, and the whole may be secondary after 

 an unstable anorthite or soda anorthite. The aggregates are at first sight suggestive of 

 the pinite pseudomorphs after labradorite in liebnerite porphyry, but the secondary 

 mineral is pyroxenic in habit and properties, not micaceous. 



9. MAGNETITE BASALTS 



These rocks are some of the most differentiated portions of the parent kenyte magma. 

 They are dark, heavy, more or less vesicular basalts, sometimes megaporphyritic, 

 sometimes aphanitic. The chief localities for these rocks are Cape Barne and Tent 

 Island. 



Texture. — They are usually microporphyritic hiatal, dopatic, hypo- or 

 hemi-crystalline vitrophyres. 



Constituents. — The phenocrysts are usually extremely corroded, and consist of 

 felspar varying from oligoclase to basic labradorite, more rarely anorthoclase, corroded 

 olivines, augite varying from green a?girine-augite to brown titaniferous augite, 

 magnetite, and idiomorphic apatite in stunted rods. The ground-mass consists mainly 

 of magnetite, though the high power may in places reveal the presence of brown glass 

 interstitially in a mass of dusty magnetite. 



