COLLECTED AT CAPE ROYDS 



175 



consist of dark uralite with a very small extinction angle. The only conspicuous 

 phenocrysts are felspar much kaolinized and affording no very satisfactory measurements. 

 Some appear to be orthoclase untwinned or twinned after the Carlsbad law. Others 

 are certainly plagioclase with Carlsbad, albite, and pericline twinnings; the measure- 

 ments indicate very acid oligoclase. In addition there is a good deal of hazy perthitie 

 intergrowth of the felspars and also fringes of granophyre round the felspar grains. 

 Chemical C omposition. — The following is an analysis of this rock by 



Messrs. Burrows, B.Sc, and A. B 



Si0 2 



TiOo 



A1 2 3 



Fe^Og 



Feb 



MgO 



CaO 



K,0 



Na 9 



H o + 



h;o- 

 co 2 



MnO 

 P2O5 



Walkom, B.Sc. 



68 

 



17 

 

 3 

 

 1 

 3 

 4 

 

 

 

 



•00 



•16 



•28 

 •07 

 •56 

 •37 

 •67 

 •59 

 •08 

 •46 

 •23 

 •27 

 •05 



Trace 



99-79 



447. Granophyre 



Macroscopic Characters. — Fine-grained purplish- pink rock, of very even 

 grainsize with a few larger black grains of biotite scattered through it, and occasional 

 phenocrysts of pinkish felspar 2 mm. by 1 mm. 



Microscopic Characters. — Somewhat porphyritic rock with even- grained, 

 holocrystalline base, the average grainsize being about "35 mm. 



The fabric is micro -graphic. The chief constituents are quartz, felspar, and biotite 

 with minor constituents. There are idiomorphic-phenocrysts of felspar. These are 

 mostly untwinned, but some show Carlsbad twinning, and some lamella? after albite 

 and pericline laws. No satisfactory extinctions were obtained, but an examination 

 of relative refractive indices indicates that the felspar is oligoclase. All the grains are 

 somewhat decomposed and are surrounded by secondary outgrowths of orthoclase 

 identical in character with the felspar of the groundmass, and, like it, subject to grano- 

 phyric intergrowth with quartz. 



The orthoclase of the groundmass is in quite irregular grain ; sometimes these are 

 solid throughout, sometimes solid at the centre with a fringe of granophyric structure 

 round the edges, or granophyric throughout, or, lastly, there may be a quartz nucleus 

 with a granophyric fringe, but this is rarer. The felspar throughout is somewhat 

 clouded by decomposition products. 



Quartz is mostly interstitial, though, as above noted, it sometimes forms the nucleus 

 of a granophyric intergrowth. 



Biotite occurs in very ragged flakes up to 1 mm. by 0*2 mm. It is all somewhat 

 altered so that the pleochroism is from dark greenish-yellow to opaque. Some flakes 



