594 ALKALINE PETROGRAPHICAL PROVINCE OF E. AUSTRALIA, 



Such rocks occur in the Glass House Mountains (Beerwah 

 type), the Warrumbungles (Timor Rock type), Mt. Flinders- 

 (summit type), &c, &c. 



(c) Then we have the andesitic and phonolitic types of trachyte. 

 These are of a dark colour, microcrystalline, and aphanitic. 

 Phenocrysts are usually, but not always, present, and may consist 

 of either microperthitic felspar, anorthoclase, pseudoleucite, 

 nosean, biotite (rare), magnetite or pseudobrookite (rare). The 

 groundmass is holocrystalline, but very fine grained, and has 

 usually a trachytic texture with well marked flow-structure* 

 The common minerals in it are idiomorphic magnetite grains, 

 idiomorphic or hypidiomorphic segirine or segirine-augite (as rods 

 or needles), idiomorphic felspar laths and needles, and allotrio- 

 morphic interstitial felspar, also allotriomorphic felspathoid (rare). 

 Feathery segirine aggregates do not often occur, but may some- 

 times. This rock-type is particularly abundant in the Warrum- 

 bungles, but occurs also associated with other types in all the 

 other regions mentioned. 



(d) Basic alkaline lavas. — These rocks do not universally 

 accompany the trachytes, nor do they exhibit very constant 

 textural characters. In the Glass House Mountains proper no 

 basic rocks exist, and those of the Blackall Range adjoining are 

 not distinctly alkaline. In the Mittagong district no definite 

 relationship exists between the basalts and trachytes, and in the 

 Nandewars the affinity is not nearly so marked as in the Warrum- 

 bungles. In the last-mentioned region the trachydolerites and 

 more basic rocks are characterised by porphyritic structure, 

 inclusions of coarse dolerite lumps in a finer base, richness in 

 iron and titanium minerals, and a typical pilotaxitic fabric in 

 the base, with often a tendency to ophitic structure. 



D. MlNERALOGICAL EVIDENCE OF CONSANGUINITY. 



There are certain minerals which almost invariably occur in 

 the acid and intermediate members of these rocks. 



1. One of them is anorthoclase, which may or may not be accom- 

 panied by microcline-microperthite. The anorthoclase phenocrysts 



