134 VOLCANIC ROCKS OF SOUTH-EASTERN QUEENSLAND 



Texture. — The crystallinity varies from holocrystal- 

 line to hypocrystalline, the grain-size is always very fine, 

 less than -01 mm., and there is a great variety of fabrics. 

 The fabric may be consertal as in the Chinghee Creek and 

 Glennie's Pulpit rocks {see Plate XIII., fig 1), or micro- 

 graphic as in the Blount Barney rock, but it is generally 

 microspherulitic and often shows an axiolitic nature. In 

 the porphyritic varieties, a strong fluxional arrangement 

 of the groundmass may be noticed, while in the lithoidal 

 varieties there is usually a strong development of banding 

 shown. 



The porphyritic crystals vary in size up to 4 mm. in 

 diameter, but they have an average diameter of 2 mm. 

 In many cases they are rounded and embayed. 



Minerals Present. — The phenocrysts are usually quartz^ 

 and orthoclase or sanidine. though anorthoclase is present 

 in some rocks. It frequently happens in those rhyolites 

 with a hypohyaline groundmass that the quartz pheno- 

 crysts have curved cracks and the cleavage traces in the 

 felspar are stained by a film of iron oxide. There is a 

 marked absence of ferro-magnesian minerals, though in 

 some cases greenish granules which may represent altered 

 ferro-magnesian minerals occur. The groundmass invari- 

 ably consists of quartz and orthoclase, and in some cases the 

 quartz seems to be of a secondary nature. Apatite, zircon 

 and magnetite are seen occasionally, particularly in the 

 Mount Barney rock. 



Specimen 98.--^ — Locality: Portion 58v, parish of 

 Telemon, Upper Chinghee Creek. The rock occurs as a 

 fiow, yellowish white to grey in colour, ard somewhat 

 decomposed. It is porphyritic, being perpatic, and has 

 phenocrysts of quartz, sanidine and anorthoclase averaging 

 2 mm. in diameter set in a very fine consertal groundmass 

 of quartz and orthoclase. Ferro-magnesian minerals are 

 absent although small greenish-brown granules throughout 

 the slide may be altered augite. The specific gravity 

 is 2-37. Name : Bhyolite. 



" These numbers refer to the micro-slide numbers in the University 

 of Queensland collection. 



