BY H. I. JENSEN. 887 



Composition : the phenocrysts consist of basic andesine, olivine 

 and greenish diopside. The felspars are sometimes zoned and 

 show albite, pericline and Carlsbad twinning, and contain 

 numerous inclusions of magnetite in glassy matrix. The olivines 

 are slightl}' corroded, and the diopside is greatly corroded and 

 resorbed. 



The groundmass is even-grained and very fine-grained, and 

 consists of felspar-laths (labradorite), augiteof the second genera- 

 tion in idiomorphic grains, olivine grains, glass and haematite. 

 The lastmentioned is an original constituent. 



Name : Hyalopylitic Porphyritic Olivine Basalt. 



The foregoing descriptions are of rocks which have no very 

 marked relationship with the lavas of the alkaline series. All 

 the other rocks about to be described from this area are closely 

 interrelated and definitely fall into the alkaline division. 



N.27. Loc: Thor's Creek, branch of Bullawa Creek. 



Handspecimen of a reddish colour not unlike N.28 already 

 described, but the microscope reveals important differences. 



Macroscopicalty it appears coarse-grained, but in reality it has 

 a fine-grained base. 



Texture holocrystalline, uneven-grained, porphyritic, with 

 hypidiomorphic granular fabric. 



Constituents : the phenocrysts consist chiefly of cloudy micro- 

 cline and anorthoclase microperthite containing an abundance of 

 inclusions of apatite and zircon. The microperthite felspar is 

 commonly enveloped by a zone of pure orthoclase with straight 

 extinction. Smaller phenocrysts of magnetite and haematite are 

 also present. 



The granulitic groundmass consists of orthoclase, nepheline, 

 apatite, magnetite, haematite and other iron ores, yellowish-green 

 pleochroic acmite, and rutile. 



The apatite occurs in greenish-grey and bluish-grey idiomorphic 

 hexagonal prisms capped with a pyramid at either end and 

 longitudinally striated. 



Rutile is abundantly represented in long microscopic needles. 

 The nepheline is interstitial. A little wohlerite (?) is also 

 73 



