BY H. I. jp:nsen. 873 



Of these subdivisions the first two have very many minerals in 

 common, and show other points of similarity. The next three 

 divisions are also closely related to one another, numerous inter- 

 mediate forms linking them, They are also related to (a) and (b). 

 The last subdivision has no relationship at all to the others. 



B. The Hypabyssal Series, consisting of : — 



(a) Ali-syenite-porphyries (alkaline syenite-porphyries), in- 



cluding syenite-porphyry, ceratophyre, bostonite, solvs- 

 bergite, grorudite. 



(b) Felsp ithoid rocks such as pulaskite-porphyry. 



(c) Augite-porphyrites and teschenites. 



(d) Bronzite-peridote-porphyrites (monchiquitic lampro- 



phyre). 



Subdivisions (a) and (b) of the Hypabyssal Series are closely 

 related to one another, but not to (c) and (d). The intrusions of 

 the last two rock-types are later than Permo-Carboniferous, and 

 probably also Post-Triassic, but not Post-Cretaceous, for they are 

 nowhere seen interbedded with or capping sediments of Cretaceous 

 age; but we find them underlying the fluviatile deposits of theNamoi 

 at the Narrabri bore, and these deposits probably commenced to 

 form in the Cretaceous. Again, eroded masses of them are met 

 with at the level of the Cretaceous peneplain (Mole Cycle), but 

 the lavas and tuffs of these rocks have been denuded away and 

 only the hypabyssal forms left. 



All the other rocks described from this region, both hypabyssal 

 and volcanic, with the exception of the Carboniferous rhyolites 

 and rhyolitic tufis of Maule's Creek, and perhaps the Boggabri 

 rhyolites, may be looked upon as differentiation-products of the 

 same magma. The Boggabri rhyolites, tuffs and pitchstones are 

 doubtful both as to age and relationship. 



The latest basalts in the Nandewars are calcic, but often free 

 of olivine, and are best looked upon as the basic residuum of an 

 alkaline magma. 



The augite-porphyrites and bronzite-porphyrites of Cretaceous 

 age are often rich in analcite, and may therefore also be related 

 to the alkaline series, but this is extremely doubtful. 



