600 ALKALINE PETROGRAPHICAL PROVINCE OF E. AUSTRALIA, 



crysts. If not protected in this way, it has generally been 

 resorbed. 



The importance of mineralisers in giving to the alkaline tra- 

 chytes and comendites their special characters cannot be over- 

 looked. To them it is largely due that the whole ground-mass 

 has been enabled to crystallise out ; they, too, have filled inter- 

 stices and miarolitic cavities with opal and chalcedony. Pneu- 

 matolytic minerals are of frequent occurrence, filling interstices, 

 such as fluorspar, topaz, wohlerite, laavenite, etc.; and in this 

 connection it must be remembered that arf vedsonite and riebeckite 

 themselves are of pneumatolytic origin, Murgogi considering HF 

 essential for their production. Perhaps the best evidence of the 

 action of pneumatolytic agents is seen in some haematite trachytes 

 and comendites, which consolidated as alkali-amphibole rocks, but 

 circulating vapours immediately after consolidation broke up the 

 arf vedsonite (or riebeckite) molecule, oxidising its FeO to Fe 2 3 

 and removing its Na 2 and Si0 2 to other parts of the rock, where, 

 with magnetite, they formed aegirine, and, with titanite, they gave 

 wohlerite. 



In some cases the same mineralising agents have restored some 

 of the soda and silica to the haematite, forming round it a rim of 

 aegirine and riebeckite. Instances of these occurrences have been 

 given by me in former papers. 



In all these cases the haematite is distinctly pseudOmorphous 

 after alkali-hornblende, and this fact, in conjunction with the 

 freshness of the rock in the cases specially described, shows that 

 the occurrence is due to pneumatolysis just after consolidation, 

 and not to weathering. 



The other facts concerning the consolidation of the alkaline 

 rocks of Australia are essentially the same as those which obtain 

 in other igneous rocks; and, being well known, they need not be 

 specially discussed here. 



The frequent parallel lamellae of crystalline quartz in anortho- 

 clase phenocrysts of comendites (micrographic phenocrysts) may 

 also have been formed by these mineralising vapours acting along 

 solution-planes. 



