890 THE GEOLOGY OF THE NANDEWAR MOUNTAINS, 



Constituents: (1) lath-shaped crystals of labradorite (2) 

 j^girine in short prisms showing strong pleochroism in greens 

 and blues, and straight extinction. (3) Idiomorphic prisms oL 

 nepheline and a minute quantity of the same mineral occurring 

 interstitially. (4) Magnetite. (5) Decomposition-products 

 including chlorite, pinite pseudomorphs and kaolin. 



Name : Camptonitic Tinguaite. 



This rock represents a fragment of the earliest consolidated 

 portion of the magma torn from the walls of the reservoir by 

 the rising lava. 



Closely allied to this rock is a fine-grained nepheline phonolite 

 found in a branch (B) of Oakey Creek (N.IO). The acicular 

 felspars are somewhat decomposed and consist of orthoclase and 

 albite. Nepheline occurs in the groundmass. The femic con- 

 stituent is a green nonpleochroic augite with very oblique extinc- 

 tion (about 46°, probably salite); it is surrounded by dark 

 borders. Dusty magnetite, secondary after an amphibole, occurs^ 

 as well as other decomposition-products like kaolin. The com- 

 putation of the analysis gives free quartz in the norm; this is 

 probably due to decomposition and secondary^silicification. 



K12. Loc: Branch (E) of Oakey Creek. (Plate li., fig.l). 



Handspecimen : this rock has a dark green base in which are 

 studded gigantic idiomorphic prismatic felspar phenocrysts often, 

 attaining a length of more than an inch, and a diameter of over 

 half-an-inch; and usually regularly octagonal in cross section. 

 The rock forms sills at the head of the branches of Oakey Creek 

 under Mount Odin. 



Texture holocrystalline, coarsely porphyritic with a hj^pidio- 

 morphic granular base. 



Constituents : the huge felspar phenocrysts consist of acid 

 labradorite and andesine, the extinction angle in symmetrical 

 sections varying from 10° to 25°, but in most cases it is about 

 20°. The refractive index is greater than that of Canada balsam. 

 Twinning is on the Albite law, but pericline twinning is also met 

 with. There are no other phenocrysts. The groundmass consists 

 of a sanidine-like felspar which may be either orthoclase or anor- 



