BY H. I. JENSEN. 



893 



The order of consolidation in these rocks usually was as 

 follows : — 



1. Magnetite : ilmenite 



2. Plagioclase 



3. Apatite 



4. Hornblende 



5. Orthoclase 



6. Augite (^girine) 



7. Nepheline 



ISr.25. Log.: base of Ningadhun Rock. 



Handspecimen white in colour with a few dark specks; light 

 in weight, probably due to an abundance of minute vesicles; 

 soft and friable like sandstone. 



Texture holocrystalline ; very fine-grained, microcrystalline ; 

 with trachytic fabric. 



Composition : the main constituent is felspar, which forms 

 about 90% of the rock; it occurs of both prismatic and tabular 

 habits, the phenocrysts being chiefly sanidine of tabular habit, 

 the remainder being partly sanidine and partly anorthoclase, the 

 latter showing under the high power a delicate striation due to 

 multiple twinning on various laws. Carlsbad and Baveno 

 twinning are both common. Phenocrysts having an hourglass 

 appearance between crossed nicols are probably Baveno fourlings. 

 Next in order of abundance comes aegirine, which occurs both as 

 corroded phenocrysts surrounded by decomposition and corrosion 

 rims and as finer acicular crystals in the base. A deep blue 

 hornblende (arfvedsonite or riebeckite) is represented fairly 

 plentifully as minute highly pleochroic rods. Yellow and reddish 

 iron ores and chlorite occur as decomposition-products. No 

 nepheline or quartz is recognisable, but the rock stains slightly 

 with malachite-green after gelatinisation with dilute acid. 



Name : Trachytic ^girine Trachyte. Magmatic name, 

 Phlegrose. 



Note on N.30 : the specimen analysed from Ningadhun is a 

 similar rock. It differs from N.25 only in that segirine is 

 relatively more abundant, riebeckite rather less so, occurring 



