108 PETROLOGY OF THE ALKALINE ROCKS 



naked eye. They are mostly sempatic, the large phenocrysts amounting to from 

 40 to 60 per cent, of the mass. 



Rocks of this type come chiefly from Turk's Head, Cape Royds, and the Dellbridge 

 Islands. 



J. 11 (1920). Plagioclase Kenyte, Turk's Head. (See Analysis D, Table II, p. 110.)— 

 This is a grey rock with very large tabular phenocrysts, the thicker of which are 

 occasionally made almond-shaped by corrosion. 



Texture. — Megaporphyritic, sempatic, magnophyric. The grey aphanitic base is 

 pilotaxitic, and varies from micro- to crypto-crystalline. (See Plate III, figs. 2 and 3.) 



Constituents. — The most abundant constituent is plagioclase of the species 

 oligoclase, often surrounded by a zone of albite or anorthoclase. This occurs only 

 as phenocrysts. The other phenocrystalline minerals are olivine, magnetite, and brown 

 titaniferous augite in corroded grains, and stout rods of apatite. The pyroxene has 

 an extinction angle of 25°, and is probably a titaniferous a>girine- augite. 



The base is practically holocrystalline, the only isotropic constituent being a clear 

 colourless interstitial residuum, which probably consists of analcite or sodalite together 

 with some nosean as indicated by analyses. The other constituents are : 



(1) Felspar laths with Carlsbad and fine lamellar twinning, and the refractive 

 index near that of Canada Balsam, probably anorthoclase. 



(2) /Egirine- augite in minute stunted rods and grains. 



(3) Black grains of magnetite. 



(4) A few idiomorphic nephelines. 



(5) A brown barkevicitic hornblende in scattered grains, and also in mossy 

 aggregates, composed of minute rods. 



(6) Allotriomorphic leucite crystals with characteristic inclusions. 



(7) A few riebeckite grains. 



Of these constituents the felspar is predominating in amount. 



This rock, which may be termed oligoclase- kenyte, is closely akin to the 

 anorthoclase- kenyte (J. 44), though in macroscopic structure and appearance there is 

 a notable difference, especially in the habit of the porphyritic felspars. 



J. 55 (500), P. 66. Erratic, Cape Royds. — Texture. — Porphyritic serial and 

 glomeroporphyritic serial, sempatic, minophyric. The base is a hemicrystalline, 

 decomposed, vitrophyre. 



Constituents. — Oligoclase in small phenocrysts, still smaller phenocrysts of 

 magnetite with haematite and limonite as decomposition products, light brown 

 titaniferous augite with an extinction angle of 34°, very corroded grains of enstatite, 

 haematite secondary after a brown basaltic hornblende, and a few idiomorphs of nepheline. 

 These form the phenocrysts; the base is very decomposed, and contains abundant 

 felspar microlites and skeletons of secondary iron ores. Felspathoid minerals appear 

 to have been originally present. 



This rock has close affinity with phonolitic oligoclase trachyte, and with the 

 Kulaites described later. 



J. 46 (1955). Kenyte Agglomerate, Turk's Head. — A dark fragment in the agglomerate 

 was sectioned. It was serial porphyritic in oligoclase of prismatic habit, and with 

 Carlsbad, albite, and pericline twinning. Optical anomalies, such as shadowy 

 extinction, occur in the phenocrysts, and are due to strain. Small olivine phenocrysts 

 are also present. The base is mostly non-crystalline, and consists of innumerable dusty 



