598 ALKALINE PETROGRAPHICAL PROVINCE OF E. AUSTRALIA, 



probably induced the isomorphous form segirine to commence to 

 separate at an early period. The segirine phenocrysts are much 

 more perfect than those of felspar, and are seldom corroded to 

 any marked extent. This is because the mineral combination 

 within them did not tend to become unstable on diminution of 

 pressure. 



The curious feature about the soda-pyroxenes and soda-amphi- 

 bole of these rocks is that they commenced to form early, hrjt 

 they continued to separate out right up to the very last phase in 

 consolidation. Hence the last products of solidification are the 

 curious feathery and mossy poikilitic aggregates of felspar with 

 segirine-augite or arf vedsonite. 



The simultaneous formation of felspar and segirine is probably 

 due to the presence of nuclei and crystals of these minerals 

 formed for the reasons already given. Their presence would 

 prevent overcooling, and lead to the continuous separation of 

 these two minerals as long as the necessary chemical constituents 

 existed in the magma. As the felspar, by virtue of mass-action, 

 consolidated more rapidly than the segirine, there was a tendency 

 for the magma to become enriched in the pyroxenic constituent,. 

 and for a eutectic mixture of the^e minerals to form in the last 

 phase of consolidation. Their presence as phenocrysts prevented 

 overcooling of the eutectic so that finally, when the temperature 

 fell low enough, they both crystallised out together as a crystalline 

 intergrowth. These points, too, explain the rarity of glass in 

 the alkaline trachytes. Glasses only occur where the lava is 

 suddenly cooled and greatly mixed up with breccias. Interstitial 

 glasses are also comparatively rare, and represent the suddenly 

 cooled eutectic mixture of segirine and felspar, the final aggre- 

 gates of which are usually wanting in hypocrystalline lavas. 



The presence of water and of mineralisers like HF, HC1, Zr0 2 

 and Ti0 2 probably also assisted in maintaining a kind of eutectic 

 relationship between the felspar and the segirine. 



When nosean and pseudoleucite are present, they commenced 

 to form in the interval between the formation of the phenocrysts 

 and the consolidation of the base. In some of the rocks of a 



