BY H. I. JENSEN. 591 



In the nosean and pseudoleucite phonolites tie soda-amphibole 

 tends to cluster round the felspathoid minerals. This seems to 

 indicate that mineralisers abstracted SiO, and NaoO from the 

 felspar molecules, and at the same time added SO 3; the NagO 

 they deposited in the jegirine-augite molecules with ZrOo, TiOg, 

 r and CI to form arfvedsonite; and the SiOo passed off as fluoride. 



The other minerals sometimes, but not invariably, present in 

 the trachytes comprise magnetite in primary idiomorphic grains, 

 a very dull black or brownish-black mineral, probably pseudo- 

 brookite or rutile, minute zircon needles and more rarely minute 

 stunted rods apparently apatite. 



The order of consolidation generally followed was as follows: — 



1. Accessories (magnetite, zircon, 



apatite, pseudobrookite (?), etc.) 



2. Felspar 



(a) Orthoclase (sanidine) 



(b) Albite 



3. .^girine-a.ugite 



4. Soda-amphiboles 



Nosean, when present, crystallised out early, practically simul- 

 taneously with the felspar phenocrysts; nepheline, when present, 

 was one of the very last substances to consolidate. 



This order of consolidation I will hereafter refer to as the 

 normal order for these trachytes. 



W.215. Loc: Scabby Rock. (Plate xxx., fig.3). 



1. Handspecimen : this is a light bluish-grey fine-grained rock 

 which bears close resemblance to the Glass House Mountain 

 comendites in handspecimen. 



2. In microscopic texture it may be described as hypocrystalline, 

 with a glassy interstitial matter cementing even- sized grains of 

 the constituent minerals. The fabric is almost orthophyric. 



3. Composition : the main constituent is/elspar, most of which 

 occurs in elongated prismatic and isometric grains. As already 

 mentioned in the introductory notes on the minerals, it consists 

 of microline microperthite (anorthoclase). Some of the crystals^ 



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