112 PETROLOGY OF THE ALKALINE ROCKS 



The pseudoleucite and nosean crystals contain inclusions of black magnetite and of 

 honey -yellow to deep brown wohlerite. (Plate IV, figs. 3 and 4.) 



The base is a miniphyric vitrophyre consisting of colourless glass in which minute 

 needles of alkali- felspar, microlites of titaniferous augite and dusty magnetite are 

 scattered broadcast. The texture of this rock is porphyritic serial in leucite and 

 nosean with ocellar fabric. From the chemical composition the colourless glassy 

 material of the base should have the composition of nosean. 



6. CLASS OF BASALTS POOR IN OLIVINE 



These rocks possess the usual textures and mineral composition of basalts. Cape 

 Barne was a particularly prolific source of them. They vary from fine-grained to 

 aphanitic in grain size. 



J. 6 (1915). Intersertal Augite Andesite, Cape Barne. — Hand- specimen. — Vesicular 

 brownish rock with the appearance of andesitic basalt, no phaneric phenocrysts. 



Microscopic Texture. — Porphyritic serial in felspar which is irregularly distributed 

 in prismoid and equant microscopic phenocrysts, consequently very uneven- grained; 

 hypocrystalline with intersertal fabric tending towards ophitic, groups of augites being 

 present which extinguish together. The phenocrysts consist of oligoclase, but the 

 smaller laths seem to be anorthoclase, the composition of the felspar varying between 

 these two according to the size of the crystals. The other minerals are idiomorphic 

 magnetite grains, a few stray olivines, hypidiomorphic grains and rods of brown 

 titaniferous augite and interstitial glass. 



The pyroxene was the last mineral to crystallise. The position of this rock is 

 intermediate between trachyte, andesite, and basalt. 



All the other basalts poor in olivine are likewise from Cape Barne. 



J. 2 (1912). Aphanitic vesicular rock with ocellar structure. — Texture. — 

 Hypocrystalline, hyalopilitic. (See Plate V, fig. 3). 

 Constituents. — Labradorite in laths, 50 per cent. 

 Faint brown augite and brown glass, 40 per cent. 

 Magnetite, 7 per cent. Olivine, 2 per cent. Apatite, 1 per cent. 



J. 4 (1914) is a similar rock, but contains slightly more olivine. (See Analysis, Table 

 III, D, p. 114, and Table V, No. X, p. 122.) 



J. 10 (1919). This rock has a higher felspar percentage, and has numerous prisms 

 of olive-brown to reddish-brown hypersthene interspersed with the augite laths and 

 magnetite of the base. Phenocrysts are rare, but dark patches of opacite occur, which 

 are decidedly suggestive of derivation from basaltic hornblende. The hypersthene 

 of the base also points that way. 



J. 58 (1358). This rock is intermediate between J. 4 and J. 10. 



J. 19 (1928). Texture. — Microporpkyritic serial, vitrophyric, with vesicles shaped 

 and arranged so as to give an ocellar structure to the rock. 



Composition. — This rock differs from those just described mainly in containing 

 a higher percentage of magnetite and black glass. Small olivines are sparingly present, 

 and are highly corroded. Brownish augite occurs as small phenocrysts in the 

 devitrifying base. 



