BY H. I. JENSEN. 



93 



Glass and the Maroochy district, and the basalts of Buderim and 

 Blackall Range. 



The most important hypabyssal rocks are, however, the por- 

 phyrites of Point Arkwright, and allied quartz diorites of Noosa 

 Head and Mt. Cooroy. 



The Point Arkwright mass is probably a sill or laccolite thrust 

 into the Trias-Jura strata which it has altered to quartzite. The 

 main mass is a bluish diorite like porphyry. At High Cliff, to 



Fig. 14. -Plan of Point 

 Arkwright. 



Po.NT 



'/\B.i<.wk.ichT 



V 



SdadsTo 



the W.N.W. of Pt. Arkwright proper, the rock has differentiated 

 into a dark porphyrite with numerous, red, aplitic dykes and 

 veins of granophyre. The blue porphyrite contains numerous 

 inclusions, some angular, some oval or round, composed of the 

 more basic minerals of the main rock. These segregations are 

 fine-grained. Other outcrops of the blue porphyrite occur at the 

 base of Mt. Coolum, and at the junction of Eudlo Creek and the 

 Maroochy near Buderim (Text fig. 1). 



A spur on Wardrop's land, N.N.E. of the Toolburra Hills, is 

 composed of a granophyre exactly like that of the High Cliff 

 aplitic veins. It is a dyke-like mass intruding sandstone. 



At Noosa Head a quartz-diorite occurs intruding Trias-Jura 

 sandstone which it has metamorphosed. It is probably a laccolitic 

 mass. Mt. Cooroy is composed of a reddish rock resembling 

 syenite. In constitution it is a monzonitic quartz-diorite with a 

 fine-grained micrographic base of quartz and felspar. The Cooroy 

 mass is seen, by the existence of the base, to be either a hypa- 

 byssal mass or even a volcanic plug. In the railway cuttings 



