B98 THE GEOLOGY OF THE NANDEWAR MOUXTAIXS, 



N.43. Loc. : at the coal seam near the basic laccolite, Dingo 

 Creek. 



Handspecimen a reddish porphyritic vesicular rock, which 

 forms a sill a few yards thick. 



Texture holocrystalline, porphyritic, with a moderately even- 

 grained vesicular base showing flow-structure and typical 

 trachytic fabric. 



Composition : the phenocrysts are tabular and somewhat 

 corroded, and show Carlsbad and Baveno twinning; they consist 

 of orthoclase, and are interpenetrated with acicular microlites of 

 another felspar, apparently albite. The base consists of acicular 

 anorthoclase microlites, rods of haematite distinctly secondary 

 after a?girine and blue am pinhole; a little unchanged riebeckite 

 and ?egirine; and some primary bsematite. A few fragments of 

 quartz appear interstitially, and zircon occurs both as inclusions 

 in the phenocr3'sts and as a constituent of the base. Nepheline 

 does not appear to be present. Magnetite occurs in small 

 amount. 



iSTame : Solvsbergite. 



N.44 is a similar rock which comes from another similar sill 

 on Dingo Creek. The felspar of this rock contains curious wavy 

 bands and circles of another mineral, apparently quartz, inter- 

 grown with it. Both quartz and nepheline occur very spai-ingly 

 in the base. 



N.46 is another somewhat similar rock from a columnar mass 

 a couple of miles N.W. of Ningadhun. This rock is also holo- 

 crystalline, and has a camptonitic fabric. The constituents are 

 sanidine (perhaps anorthoclase), segirine, primary haematite and 

 riebeckite (or arfvedsonite). The mass is probably laccolitic. 



The three rocks last-mentioned are typical of the great bulk of 

 the trachyte of the Nandewars, and should perhaps be classed as 

 solvsbergites. Many of the really volcanic rocks- of the flows 

 are similar in structure and composition. The fresh rock has a 

 flesh-colour due to primary haematite; on decomposition the 

 colour changes to brick-red. 



