BY H. I. JKNSEN. 131 



General Remarks on the foregoing Rocks. 



The very close relationship of the porphyrites of Pt. Arkwright 

 and Eudlo Ck. with the Noosa Head quartz-diorite and the 

 Cooroy Monzonite is borne out by field occurrence, microscopic 

 examination, and chemical analysis alike. In the textures there 

 are differences depending on rate of cooling. The Noosa rock 

 evidently crystallised slowly under pressure, while that of Mt. 

 Cooroy had almost finished crystallising when extruded, consisting 

 then of crystals imbedded in a plastic base consisting of a eutectic 

 mixture of quartz and orthoclase. The final consolidation may 

 have taken place under volcanic conditions, as the form of the 

 mountain suggests. 



The felspars of all these rocks are very similar. They vary 

 from labradorite of composition Ab x An l5 to albite and orthoclase. 

 Zoning, incipient decomposition commencing in the centre and in 

 the outermost orthoclase- albite zone, inclusions of a similar 

 nature and arrangement, corrosion and shadowy extinction are 

 features common to all. The Pt. Arkwright porphyrite cooled 

 most rapidly as shown by its much more fine-grained base. 



The hornblendes are, in these rocks, of essentially the same 

 character, being a Pargasite somewhat rich in lime, calcite and 

 chlorite being decomposition products. They occur in beautifully 

 idiomorphic crystals, sometimes more or less corroded, on which 

 the faces m, b and c are well developed, while a is usually absent. 

 The colour is olive-green; in very thin sections light green. 



The Noosa type seems to represent the original magma which 

 gave rise to these rocks. Various stops occurred in consolidation 

 during which alteration in the magma took place, perhaps by the 

 injection of new materials of a more acid and alkaline nature 

 There was continual motion during the formation of crystals in 

 this magma, which probably went on in a deep-seated magmatic 

 reservoir. The Cooroy rock consists of this magma together with 

 a considerable alkaline addition. 



The beautiful micrographic intergrowths found in the grano- 

 phyres from Wardrop's (Sp.No.36) and from High Cliff, Pt. Ark- 

 wright (Sp.No.121) (Plate xiii., fig. 13), and their similar mineral 



