Kai!quhat?son. — The Platinum Graveh of Orepnki. 455 



case, however, no mention, heretofore has been made; of a granite. We 

 shall therefore describe each in detail. 



The melaphyre has the appearance in hand-specimens above outlined. 

 It occurs very prominently round the coast towards Colac Bay from River- 

 ton, and extends back into the Longwood Range for some distance, until 

 it appears to thin or pinch out, as in the map. The actual boundaries of 

 the mass, or the junction of it with the main rock of the range, have not 

 been made out with any accuracy, owing to the great difficulty of follow- 

 ing the rock through the interminable bush. From what could be gathered 

 from miners, however, as well as from a consideration of the rough direction 

 at Riverton, it is thought that the position in the map is fairly accurate. 



The rock is very tough, and splinters with more than usual difficulty, 

 much like some close-grained phonolites. It does not seem to be acted 

 on by denuding agencies with any great rapidity, and is fairly hard. 



In section, the most noticeable feature on looking at the slide is the 

 large amomit of decomposed material. In this decomposition not only is it 

 the feldspars that take part, but the olivine too is almost all serpentinized. 

 The mnierals seen in the section are — 



Plagioclase. — This forms several phenocrysts in the section examined, 

 and, judging from the extinction-angle on — i.e., at right angles to — the 

 brachypinacoid, which varies between 25° and 30°, the mineral is mostly 

 u basic variety of labradorite gradually passing into bytownite. The 

 crystals show the albite lamellar twinning very well, for some of the pheno- 

 crysts are not so decomposed as others. Combined with the albite the 

 pericline and Karlsbad twinning occurs, but rather rarely. Besides the 

 phenocrysts is an abundance of very fine lath-shaped feldspars forming 

 a great portion of the groundmass. These are all semi-decomposed, form- 

 ing, like the phenocrysts themselves, patches of calcite. 



Olivine. — Very few patches of this mineral have been identified with 

 certainty. Its place is almost always taken by yellowish-green masses 

 of serpentine and other products due to the weathering of the mineral. 

 In those which have been identified the mineral is quite without crystal 

 outline, exhibits the usual cracks, and is usually surrounded with the 

 serpentine border due to decomposition. The form appears to have been 

 rounded, or in some cases oval, and the mineral is usually quite colour- 

 less. Here and there occur patches of what appears to have been olivine, 

 but nothing now is left except a mass of material like calcite, showing 

 peculiar radial extinction. 



Augite. — This is remarkable as being the only essential mineral which 

 has resisted the weathering-agents. In hardly a single case does the augite — 

 at any rate, in phenocrysts — appear at all decomposed. It occurs in roughly 

 rectangular forms, is usually pale in colour, and here and there shows 

 a lamellar twinning. The forms represented are the orthopinacoid, the 

 base, and rarely a dome. The extinction-angle is high, being usually over 

 30° in the prismatic zone. Where decomposition has set in the products 

 are mostly chlorite, yellowish-green in co our. The mineral occurs as it 

 does in most basalts, in two generations. The groundmass appears to 

 have been studded with small grains of the mineral having the characters 

 of the phenocrysts, but in a more advanced stage of decay. 



Besides these essential minerals there occur small grains of magnetite 

 and rather numerous crystals of apatite in long acicular crystals. Their 

 further characters cannot, however, be made out. 



