BY H. I. JENSEN. 891 



thoclase. This occurs as microlites. Idiomorphic segirine augite 

 grains decomposing to chloritoid, apatite needles, greenish 

 micaceous fibrous decomposition-products of the pinite group, 

 finely divided magnetite, haematite, a little nepheline and cancri- 

 nite are also present. 



Name : Labradorite Porphyry. Magmatic name, Andose (see 

 Tables i. and ii.). 



N.26. Loc: Branch (C) of Oakey Creek. 



Handspecimen a green, somewhat decomposed coarse-grained 

 rock. 



Texture holocrystalline. even-grained, hypidiomorphic-granular. 



Composition : the main constituent is felspar in hypidiomorphic 

 crystals which have been largely saussuritised, chloritised and 

 kaolinised; it contains as inclusions magnetite, stout needles of 

 apatite, and small idiomorphic grains of fluorite. The alteration 

 of the rock has led to the production of a certain amount of 

 secondary felspar in needles within the original phenocrysts. 

 The original felspar was probably an albite surrounded by a zone 

 of microperthite, decomposition having effected the exterior more 

 than the interior. Ptepresenting what was originally augite and 

 conforming to its crystalline outline, we have aggregates of 

 chlorite and green chloritoid, sometimes serpentine as w^ell. 

 These occasionally contain a nucleus of uralite. Remarkable 

 skeletons of ilmenite and magnetite are present in abundance. 

 A little nepheline occurs, and decomposition-products after 

 nepheline are fairly common. These include katapleiite, cancri- 

 nite, liebnerite pseudomorphs, etc. Rutile needles are very 

 abundant. The other decomposition-products present are kaolin, 

 chlorite, chloritoid, serpentine, haematite, saussurite, etc. 



Name : Altered Augite-Nepheline-Syenite near Laurdalite. 



N.14. Loc: Branch (A) of Oakey Creek. 



This rock occurs as a sill. In microscopic structure and 

 mineral composition it closely resembles N.8 and N.9, and is 

 therefore a sulvsbergite, or nordmarkite-porphyry. 



Remarks. — The alkaline rocks dealt with so far are all derived 

 from the sills underlying Mount Odin. Underlying the whole of 



