594 THE GEOLOGY OF THE WARRUMBUNGLE MOUNTAINS, 



the analyses do not show any notable excess of carbon dioxide or 

 combined water above what we find in the arfvedsonite rocks 

 (compare N.59a, an arfvedsonite rock, and N.55, a ferrite rock). 

 The two kinds of trachyte frequently occur close together with 

 a sharp line of demarcation between them but exposed to exactly 

 the same weather-agencies and equally resisting to the blows of 

 a hammer. From these facts I conclude that vapours containing 

 HF and HCl have wholly or partly decomposed the arfvedsonite 

 molecule subsequent to its formation, at the same time oxidising 

 the FeO in it toFeoO^, and removing the TiO.^, NaoO and ZrOo, 

 redepositing them elsewhere in the rock as rutile and zircon; and 

 a later cj'-cle of activities restored in part these constituents, 

 forming a rim of arfvedsonite, or perhaps more frequently the 

 the ingress of vapours took place before the complete consolida- 

 tion of the magma, and, as soon as they re-escaped, crystallisation 

 proceeded in the normal way. 



In a similar way the frequent occurrence of an envelope of 

 Eegirine round soda-hornblende, or of soda-hornblende round 

 aegirine, may be explained. The former is probably due to the 

 exhaustion of the mineralising vapours present in the first period 

 of consolidation, the latter to the introduction of mineralisers 

 after crj'stallisation had commenced and gone on for some time 

 without their aid. 



W.16. Loc: base of Timor Rock. 



1. Handspecimen grey in colour with bluish-black specks, 

 giving the whole rock a bluish-grey colour. 



2. Texture : like "W.SS, but hypocrystalline, containing a 

 yellowish interstitial glass which gelatinises with acid and stains 

 with malachite green. It may contain cryptocrystalline nepheline. 



3. Composition : felspar similar to that in W.38 is the chief 

 constituent. Brown amphibole and ferrite are almost wholly 

 wanting. The blue amphibole occurs in moss-like aggregates, 

 ^girine is almost absent. A little limonite occurs as a decom- 

 position-product. Round some of the felspar phenocrysts there 

 is an isotropic white mineral w^iich may be slightly decomposed 

 nepheline. 



