BY n. I. JENSEN. 901 



between the trachytes and the andesites of the locality. It is a 

 Porphyritic-Hypocrystalline Plagioclase Trachyte. 



N.51. Loc: Dingo Creek near basic laccolite. (Plate L, fig.5). 



Handspecimen white in colour, the rock occurring as a sill. 

 It is compo.sed almost wholly of felspar, which is of two genera- 

 tions, and forms more than 99 % of the bulk. 



Texture holocrystalline, with phaneric phenocr3'Sts and an 

 aphanitic trachytic base. 



Composition : the phenocrysts are clear like sanidine, but the 

 cleavage and extinction angle (S'^-IO") show that they belong to 

 the species anorthoclase; they are twinned chiefly on the Carlsbad 

 plan, but occasionally an albite twin is seen; some crystals show 

 also microscopic poh'synthetic twinning. Occasionally an acicular 

 sanidine crystal is seen enveloped by a crystal of albite. Often 

 again two sanidine laths are twinned so as to form a cross-like 

 staurolite, each of the two individuals being itself twinned on 

 the Carlsbad law. These are probably Baveno Jnterpenetration 

 twins composed of two individual Carlsbad twins. The other 

 constituents, which occur only in minute quantity, comprise 



(1) interstitial rods of lemon-yellow to greenish segirine-augite, 



(2) a few flakes of haematite, (3) occasionally a fragment of 

 riebeckite, and (4) a few fragments of quartz. 



Name: a typical Bostonite, or Sanidine- Anorthoclase Trachyte- 

 Porphyry (c^;. Bostonite from the Lake Champlain District, 

 U.S.A., Kemp & Marsters, Bull. 107, Geol. Surv. U.S.A.). 



N.23. Loc: Kangaroo Valley near Ningadhun. (Plate li.,fig.5). 



Handspecimen a dark basaltic rock containing huge plagioclase 

 phenocrysts. 



Texture holocrystalline, uneven-grained; fabric porphyritic, 

 with microcrystalline hypidiomorphic-granular base. 



Composition : the main constituents are felspar, augite, olivine, 

 and magnetite. The felspar phenocrysts consist of basic labra- 

 dorite; they are highly corroded, and contain magnetite and 

 apatite inclusions. The olivine is likewise very corroded, and 

 shows also partial decomposition to serpentine. The augite 

 occurs as small idiomorphic grains in the base; it is a colourless 



