102 PETROLOGY OF THE ALKALINE ROCKS 



generation. Microscopically, porphyritic-serial in felspar of the second generation, 

 apparently a soda sanidine. The texture of the rock is holocrystalline, mega- 

 porphyritic, perpatic, rnediophyric, with a pilotaxitic fabric in the ground-mass. The 

 order of consolidation was, first magnetite, then felspar, lastly segirine-augite. 



J. 52 (1466). Phonolitic Trachyte. Another erratic from Cape Royds. — This 

 rock is of an aphanitic appearance and vesicular in nature. It is porphyritic- 

 hiatal, and has a hyalopilitic base approaching strahlenkornig intersertal. It 

 consists of anorthoclase, yellowish-green segirine, and dendritic and pectinate 

 aggregates of grains and rods of iron ore (both magnetite and haematite). Magnetite 

 also occurs sparingly in scattered independent grains, and ksematite as scales. The 

 segirine occurs sparingly as small phenocrysts of the first generation, and more 

 abundantly as idiomorphic microlites, which are commonly bunched together in 

 groups of parallel individuals. This pyroxene is decomposing to ferrite. Inter- 

 stitially occurs a colourless glassy substance, probably of felspathoid composition (see 

 Plate II, fig. 5). 



J. 53 (1960). Phonolitic Trachyte. Erratic from Cape Royds. — This has a texture 

 that may be described as inicroporphyritic-serial, dosemic, vitrophyric. The pheno- 

 crysts are equant and prismatic, consisting mainly of anorthoclase, but a fair number 

 of smaller phenocrysts of pseudoleucite occur in the base. The pseudoleucites 

 are polygonal or rounded, and consist of a mosaic of orthoclase and nepheline. 

 Other similar pseudospherulitic aggregates seem to consist of natrolite and to be 

 secondary after sodalite. The base is a greenish-brown glass, devitrifying with the 

 production of cryptocrystalline characters. Hence this rock is phonolitic trachyte 

 allied to leucitophyre. 



J. 54 (1841). Anorthoclase Trachyte. Erratic from Ferrar Glacier. — This was 

 megascopically an extremely fine grained bluish-grey rock with phaneric phenocrysts. 



Microscopically it is holocrystalline, megaporphyri tic -hiatal, perpatic, and 

 magnocumulophyric (see Plate I, fig. 5). The phenocrysts of the first generation are 

 in groups, and consist of anorthoclase crystals which may envelop subordinate segirine, 

 arfvedsonite, and olivine. The base is coarsely microcrystalline, consisting dominantly 

 of an allotriomorphic granular mass of anorthoclase, the grains of which embrace 

 microlites of albite and segirite as inclusions, and have segirine filling the interstices 

 between them. The structure is similar to that noticed in a Fassifern (Queensland) 

 Trachyte (Proc. Linn. Soc. N.S.W., vol. xxxiv, March 1909). Evidently the cause of 

 the peculiar fabric is that after the albite and segirite had separated out a pasty glass 

 of the composition of anorthoclase was left, and this devitrified in irregular crystal 

 grains which embrace poikilitically the earlier form of minerals. 



P. 254 (5830). Biotite Hornblende Trachyte. Erratic, Cape Royds. — The texture 

 of this rock is porphyritic-serial. 



The phenocrysts consist of biotite and idiomorphic prismatic hornblendes belonging 

 to the normal brown variety, and also lath- shaped sanidine felspars. The base consists 

 of sanidine, greenish needles of segirine-augite, and sparingly magnetite scattered 

 through the rock in idiomorphic grains. 



J. 21 (1930). Phonolitic Trachyte. Inaccessible Island. — This is a reddish baked 

 rock of hypocrystalline pilotaxitic texture. It contains large-zoned anorthoclase 

 phenocrysts. The other minerals are zoned anorthoclases and albites with shadowy 



