100 PETROLOGY OF THE ALKALINE ROCKS 



soda-hornblendes, etc., occurring in the manner described in the previous slide. Some 

 of the segirite grains exhibit a tendency to uralitic alteration to arfvedsonite. The 

 main point of difference between this slide and the previous lies in the fact that the 

 former contains a considerable amount of colourless isotropic interstitial material which 

 is quite amorphous and frequently studded with inclusions. This has the characteristics 

 of nosean or sodalite, gelatinises with dilute acids and stains readily. The fabric of the 

 rock is pilotaxitic, and the texture may be described in general terms as holocrystalline, 

 megaporphyritic, perpatic, mediophyric. The specimen and slide bear close resemblance 

 to some pseudoleucite and nosean trachyphonolites from the Warrumbungle 

 Mountains. (For Analysis see Table I, A, p. 98.) 



J. 27 (1936). In texture and composition this rock is very similar to J. 1 and J. 13. 



It differs mainly in containing sparing phenocrysts of olivine. The yellowish-green 

 aegirine-augite behaves ophitically to the felspar and shows frequent decomposition to 

 uralitic arfvedsonite and serpentine. Ferrite is also a common decomposition product. 

 The dominant felspars show faint polysynthetic twinning in addition to Carlsbad 

 twinning under the high power, and have an extinction angle of 8°, which shows them 

 to be anorthoclase fairly rich in the oligoclase molecule. Several other specimens, 

 J. 29 and J. 30, exhibit the same characteristics in micro-section. 



J. 32 (1742). This is a similar rock in texture and composition. It contains a 

 number of isotropic patches, penetrated by felspar laths in a stellate (divergent radial) 

 manner, giving parts of the slide a strahlenkornig structure. The patches seem to 

 consist of sodalite. The inclusion in this specimen proved to be an olivine segirine 

 orthophyre or sanidinite. Another rock, J. 43 , is similar. A little nepheline and minute 

 interstitial crystals have also been identified in these rocks by staining methods. 



All these typical Mount Cis rocks are dark and silky, vesicular, and slightly 

 porphyritic as viewed megascopically. Under the microscope they preserved a texture 

 described as holocrystalline or nearly so, megaporphyritic, perpatic, mediophyric, with 

 a pilotaxitic to trachytic fabric. They contain usually a fair amount of felspathoid, 

 which may exist as any of the minerals nepheline, nosean, or sodalite. They are 

 therefore trachyphonolites or phonolitic trachytes, and are all closely allied to 

 Washington's Cumal Phlegrose (see The Roman Comagmatic Region, also Table I, G, 

 p. 98). 



Other Trachytes. — Of these the most interesting types were : 



J. 7 (1916). An oligoclase trachyte included in kenyte breccia from a parasitic cone 

 on Mount Erebus. 



Megascopic characters. — Light grey compact porphyritic rock ; felspar phenocrysts 

 about 2 mm. long of tabular and prismatic habit. 



Microscopic characters (see Plate I, fig. 6). — Hypocrystalline, megaporphyritic to 

 microporphyritic serial, dosemic ; ground-mass about 30 per cent., consisting of 

 anorthoclase, oligoclase, lilac augite, apatite, magnetite, other iron ores, nepheline, 

 cancrinite, brown glass. The fabric is of a vitrophyric intersertal type, the phenocrysts 

 consisting chiefly of megascopic, prismatic, and tabular oligoclase-andesine, and smaller 

 microscopic anorthoclase and olivine crystals, both of which species vary in size from 

 almost megascopic to the minute size of the nepheline augite and magnetite crystals 

 of the base. 



Oligoclase occurs chiefly as phenocrysts which are highly corroded and which show 

 both optical and mechanical zoning. Twinning, Carlsbad, albite, and pericline. The 



