BY H. I. JENSEN. 129 



The Noosa rock consolidated under fine-grained tuffaceous sand- 

 stone, while that of Point Arkwright cooled under coarse 

 conglomerates. The heated vapours accompanying the intrusion 

 may therefore have made their escape more easily in the latter 

 place, and the rock cooling quickly would take on a porphyritic, 

 more typically hypabyssal, facies. 



Sp. No.150. Porphyrite. Loc: Boulders embraced in felspathic 

 sand in railway cutting between Eumundi and Cooroy, 

 about 77 |m. from Brisbane, 

 i. (1) Texture: (a) noncrystalline, (b) uneven, porphyritic, fine- 

 grained base, (c) porphyritic with hypidiomorphic granular base. 



(2) Constituents. — -These are the same as in the Noosa x'ock. 

 The hornblendes are somewhat corroded and partly changed to 

 chlorite. Two generations of felspar are present. The base is 

 not so fine-grained as in the Point Arkwright rock. 



(3) Name : Porphyrite. 



Sp. No.183. Porphyrite. Loc: Eudlo Ck.,near Buderirn Mountain. 



i. This rock closely resembles those described from Noosa Hd. 

 and Pt. Arkwright. 



ii. Microscopic structure. — (1) Texture : (a) noncrystalline, (b) 

 uneven; fine-grained base in part microcrystalline; (c) porphyritic, 

 base hypidiomorphic granular. 



(2) Constituents. — Both in nature and relative proportions the 

 same as in the Noosa rock. 



Name : Quartz-orthoclase-porphyrite. 



Sp. No.142. Monzonite, Loc. : Summit of Mt. Cooroy. (Plate xiii., 

 tigs.16-17). 



i. Handspecimen a grey to flesh-coloured, medium-grained rock, 

 not unlike Gib Rock Syenite (Bowral) in general appearance. 



ii. Microscopic examination. — (1) Texture : (a) crystallinity, 

 noncrystalline; (b) grain-size, uneven; phenocrysts imbedded in 

 microcrystalline partly micrographic base; (c) fabric, porphyritic; 

 phenocrysts, hypidiomorphic; base, partly allotriomorphic gran- 

 ular, partly micrographic. 

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