132 VOLCANIC AREA OF EAST MORETOV, ETC., DI8TRICTS, Q., 



composition, bring out their close affinity. These rocks probably 

 represent the magma which augmented in different proportions 

 that of the Noosa type to give the other rocks mentioned. 

 Probably both types of magma have been derived by a process of 

 differentiation from a single magma at a considerable depth. 

 The fact that the more acid portions were extruded last, and are 

 largely eutectic mixtures of quartz and felspar (e.g., Sp.No.121), 

 favours this hypothesis, for in the deep-seated reservoir the last 

 portion to consolidate would be of such a composition with an 

 acid magma. The micrographic granophyres are therefore the 

 "mother liquor" of the original magma which has been squeezed 

 out in the last stages of intrusion. At High Cliff portions of 

 this have been forced into the already cooled porphyrite in the 

 form of aplitic veins and dykes, and at Wardrop's a mass of it 

 appears at the surface, while the more basic products were not 

 met with, though they probably exist at a depth, the aplitic 

 material having burst through not only the porphyrite, but also 

 the superjacent sandstone. 



The cryptographic and micrographic base in the Cooroy rock,, 

 and the cryptographic base in the Pt.Arkwright porphyrite 

 point to these rocks being made up of a mixture of a dioritic 

 magma with the aplitic differentiation product already discussed. 

 Eutectic conditions prevailed in the final stages of consolidation,, 

 proving that the cooling was sudden, and most of the felspar had 

 already fallen out of solution. The granophyre groups and 

 micrographic phenocrysts of the Pt.Arkwright rock are an 

 interesting effect of consolidation under eutectic conditions. 



Sp. No. 83. Porphyry. Loc: Branch of Delaney's Ck., below 

 Black's Hill. 



A reddish-brown rock in which, with the naked eye, one may 

 distinguish a few hornblende phenocrysts. The rock is porphy- 

 ritic in felspar. 



Microscopic examination. — {1) Texture: (a) crystallinity, holo- 

 crystalline; (6) grain-size uneven; phenocrysts reaching 7-8 mm., 

 lying in an aphanitic base; (c) fabric, porphyritic with a pilotaxitic 

 base. 



