398 DR ROBERT CAMPBELL ON 



obtained in situ. They vary in colour from grey-brown to dark reddish-brown, and 

 in the hand specimens phenocrysts of felspar, biotite, and less frequently augite are 

 seen embedded in a compact felspathic groundmass. The felspar phenocrysts in thin 

 section have a sanidine-like habit. They are, as a rule, much corroded, and often 

 show a marginal zone of late growth rich in inclusions of magnetite and granular 

 augite. Among the inclusions may be noted also crystals of biotite, needles of 

 apatite, and fluid cavities, the last often arranged in lines parallel to the 100 cleavage 

 of the mineral. The crystals show variable double refraction and fine microcline 

 twinning (PI. XXXVI. fig. l) ; their mean refractive index is about 1'529. They are 

 referred, therefore, to anorthoclase. This determination was confirmed by the 

 values of the extinction angles on cleavage flakes. Bending of the crystals is not 

 uncommon, and secondary limonite-liEematite has been deposited along their cracks 

 and cleavages. 



The predominating femic phenocrysts are biotites, pleochroic from light yellow 

 to deep red, brown, and black, and enclosing small zircons and apatites. The biotites 

 invariably show evidence of resorption, partial or complete. When the crystal has 

 been entirely resorbed it either retains its original form or has undergone disintegra- 

 tion. A green non-dichroic augite also occurs sparingly in euhedral crystals, which 

 sometimes exhibit simple twinning on 100. They enclose small needles of apatite 

 and iron oxides, and rarely crystals of biotite. 



The groundmass shows normal trachytic texture, and consists chiefly of laths 

 of soda sanidine, along with a small development of granular augite and mag- 

 netite. Zircon and apatite occur as accessory minerals. 



(b) Socialite trachyte [G-. 14]. — The collection includes one specimen of sodalite 

 trachyte — a well-rounded stream boulder, greenish-grey in colour, and possessing 

 a noticeable waxy lustre. The rock is sparingly microporphyritic with segerine- 

 augite, magnetite, and zircon — not infrequently aggregated together — sodalite, and 

 brown apatite. The groundmass is holocrystalline and trachytic in texture, its chief 

 constituent being simply-twinned laths of anorthoclase felspar. 



A colourless isotropic mineral whose refractive index is much lower than that of 

 anorthoclase occurs in abundance both interstitially and in broad plates enclosing 

 minute felspars in micropoecilitic fashion (PI. XXXVI. fig. 2). Treatment of a thin 

 section with a very dilute solution of HNO3 and AgNOs confirmed the supposition 

 tli at the mineral was sodalite. 



The ferromagnesian mineral of the groundmass is oegerine-augite, with the ragged 

 ophitic habit usual in the alkali trachytes. 



The accessory constituents include zircon, widely disseminated in characteristic 

 euhedral crystals, apatite in thin colourless needles, and magnetite. 



(c) JEgerine-auyite trachyte [Gr. 8]. — Not unlike the sodalite trachyte in appear- 

 ance on fresh fracture are several angular and subangular blocks of alkali trachyte 

 collected by Dr Pirie from a mass of volcanic agglomerate. The specimens have 



