BY H. I. JENSEN. 127 



with trachyte and re-exposed by denudation. In either case it is 

 ■older than the trachyte. 



Microscopic examination. — (1) Texture : holocrystalline, vari- 

 able and, on the average, fine-grained in grain-size, and porphyritic 

 granular in fabric. The phenocrysts were less corroded than in 

 the Pt. Arkwright rocks. 



(2) Constituents the same as in Sp. Nos. i. and ii. 



(3) The orthoclase may, in reality, be a microperthite. It 

 •occurs in phenocrysts which show optical zoning, probably due to 

 alternate layers of albite and orthoclase. Plagioclase is the most 

 abundant constituent, occurring in beautifully zoned phenocrysts 

 containing hornblende and glass intrusions. The}' vary in 

 •composition from that of labradorite at the centre to oligoclase- 

 albite at the exterior. 



(4) Name : Porphyrite. 



Sp. No.89. Quartz-Diorite. Loc: Noosa Heads. (Plate xiii., figs. 

 14 and 15). 



i. In handspecimen or massive it is of a bluish colour. The 

 grain-size is variable between medium and coarse. A few specks 

 of pyrites occur in it. This rock has intruded Trias-Jura sand- 

 stones at ISJoosa, and has effected gr^eat contact metamorphism. 



ii. Microscopic structure. — (1) Texture (a) crystallinity, holo- 

 crystalline; (b) grain-size, moderately even in handspecimen; 

 medium-grained; the average diameter of grains appears in hand- 

 specimen to be 2-5 mm., but under the microscope all the grains 

 ■excepting those of amphibole are composite; hence the average 

 grain-size is finer — about l - 5 mm.; (c) fabric, hypidiomorphic 

 granular. 



(2) Constituents (in order of abundance; — (a) plagioclase, (b) 

 •orthoclase, and (c) hornblende are essential minerals. In notable 

 quantity we have (d) quartz, and in minute amount (e) chlorite, 

 if) biotite, (y) magnetite, (h) apatite. 



(3) Remarks on minerals. — (a) The plagioclase occurs in idio- 

 morphic and hypidiomorphic crystals showing albite, pericline 

 and Carlsbad twinning. These crystals are beautifully zoned, 

 due to diminishing basicity from the centre outwards, as shown 



