Geologi/ of the Diamond Creek Area. 337 



Section A2;j, Kaiigaruo Grouiids, near the ceiueterv. In this «ec'- 

 tiuii the ophitic texture is absent, and tlie felspar hillis are hirger 

 tlian in section Al"). 



."5. Garden Hill basalt. — This typu dilleis Ifoni tlie second type 

 in the rarity of the phenocrysts of augite, and in its finer grained 

 character. The specific gravity of this type is also markedly dif- 

 ferent from that of tlie former types. The specific gravity of the 

 third type is about 2.86, while that of the fii'st two types is about 

 2.9;J. 



Section A24. soutli of (iarden Hill, is a typical example. Tn 

 hand specimen it is a black, dense, almost aphanitic rock. 



Microscopically it is a huloLrystalline fine gi-ained lock, showing 

 pilotaxitic structuic, and having a tendency toAvards a porphyritic 

 habit. Phenocrysts of olivine occur in a moderately hne grained 

 base of augite, plagioclase. magnetite, ilmenite and apatite. 

 Secondary minerals, as seri:>entine, zeolites and iddingsite are pre- 

 sent. Olivine, colouj'less, is occasonally altered to green serpentine, 

 and I'ed brown iddingsite. Augite occurs very rarely as purple 

 phenocrysts, and is generally 2)resent as microscopic anliedral grains 

 and prisms, having an extinction angle about 45 deg. Long prisms 

 of felspar, Avith maximum extinction angle about 33 deg., indicate 

 labi-adorite of composition near Ab.^ Aug. Microspherulitic zeolites 

 occur distributed throughout the ground mass. 



Section A21, Garden Hill. — Olivine is largely replaced l)y idding- 

 site. Small amount of glass or isotropic zeolite present. 



(c) T)!flce.<<. 



1. Basic dyke. This dyke occurs in a small shaft in section 

 16, allotment D. It was not possible to determine its strike or to 

 trace it on the surface. Macroscopically it is a greenish grey 

 coloured amygdaloidal rock, resendiling a basalt. It weathers to 

 a brown iron-stained material containing numerous unaltei'ed 

 crystals of augite. Microscopically it is a holocrystalline, panidio- 

 morphic, porphyritic textured rock. It consists mineralogically of 

 phenocrysts of olivine and augite in a ground mass of microlites 

 of felspar, granular augite, olivine, magnetite and apatite. 

 Secondary minerals compiise zeolites, talc, calcite and leucoxene. 

 Augite occurs in large idiomorphic phenocrysts, brown or purple 

 in colour, and decidedly pleochroic, indicating a titaniferous 

 variety. Simple twinning im 100 is seen by re-entrant angles and 

 differences in polarisation colours. Multiple tAvinning, tAvin and 



•20 



