198 Proceedings of the Royal Society of Victoria. 



showing- marked undulose extinction and exti'emely fine twinning. 

 Some green glass, a little nosean, and abundant ilmenite also 

 occur. The fluxion structure is well developed and there are a 

 few vesicles filled with green-stained silica. 



c. The Solvsbergites. 



The third member of the Macedon I'ock series forms the 

 Camel's Hump and two outliers in the plain to the north. 



The rock of the Camel's Hump is the best known geologically, 

 as it was analysed by Newbery, and is the type of the " felspar 

 porphyry" or "syenite porphyry" of the Geological Survey. 



The rock is greyish brown in colour, and consists, in the main, 

 of large phenocrysts of anorthoclase, set in a fluidally arranged 

 series of lath-shaped felspars, which may be either soda-sanidine 

 or anorthoclase. Some of the phenocrysts have been fractured, 

 and the edges are quite shai'p and uncorroded ; hence the rock 

 probably became thick and semi-viscous before consolidation, and 

 has not flowed far from its vent. In the interspaces between 

 the felspars are mossy patches and groups of small crystals of 

 aegerine and riebeckite. The aegerine is more abundant and 

 occurs in larger crystals than the riebeckite ; the latter is, 

 howevei", conspicuous from its fine blue colour and strong pleo- 

 chroism. 



Similar in form to the riebeckite, but in smaller and rarer 

 patches, is a dark brown to opaque mineral, which Mr. Prior has 

 kindly identified as cossyrite. It is strongly pleochroic and the 

 grains are angular or subangular. 



The rock on the southern side of the Camel's Hump is some- 

 what fresher. The aegerine, riebeckite and cossyrite, though 

 minute, are often idiomorphic ; but in places these minerals are 

 moulded on the felspars. Ilmenite, including small zircons, 

 occurs. The biotite is sometimes intensely corroded, and some 

 zones have been changed to chlorite. 



The Hanging Rock, which rises from the plain at the northern 

 foot of the Macedon ridge is closely allied to the rock of the 

 Camel's Hump ; but the fluidal structure is less defined. The 

 felspar laths of the ground mass tend, moreover, to occur in 

 short broad prisms rather than in long laths. The interspace 

 between the felspars is occupied by a granular felspar mosaic. 



