136 VOLCANIC ROCKS OP SOUTH-EASTERN QUEENSLAND 



black pitchstone, with which it is associated, and the more 

 crystalline rhyolite which has been poured out from the 

 vent M^hich has been plugged up by this material. All 

 through the south-eastern area of acid pyroclastic material 

 this rock is found, and is, together with the black pitch- 

 stone, the most constant inclusion. Name : Rhyolite. 



Specimen 229. — Locality: 78v, parish of Palen, Tyler- 

 ville. This material represents lava which has been poured 

 out from Mount Lindsay. It is a light grey in colour, is. 

 perpatic, and has phenocrysts of quartz and sanidine set 

 in a hypohyaline groundmass which contains quartz and 

 orthoclase. Micro-spherulitic structure is common, and an 

 axiolitic arrangement is frequent. Name : Rhyolite. 



Specimen 220. — Locality: Mount Barne3^ This forms 

 the main plug of Mount Barney and it is somewhat different 

 from the other rhyolites, as it is much coarser. It is 

 greyish white in colour and in patches is stained brown 

 by the limonite resulting from the altered magnetite 

 in the rock. This rock contains phenocrysts of felspar, a 

 good deal of which is possibly anorthoclase, and quartz, 

 the felspar being the more abundant. In many cases the 

 felspars are subidiomorphic, whereas the quartz is allo- 

 triomorphic. The groundmass consists of a micrographic 

 intergrowth of quartz and orthoclase, which is developed 

 to a great perfection. {See Plate XII., fig. 5.) The 

 intergrowth forms a framework around the phenocryst 

 which in some cases has been embayed. Frequently one 

 sees the intergrowth radiating off from a particular point 

 or line and there is apparently no nucleus of felspar or 

 quartz. Distributed through the rock are abundant small 

 granules of magnetite which are frequently being altered 

 into limonite. Minute grains of apatite and zircon occur 

 as inclusions in the phenocrysts. Specific gravity 2-50. 

 Name : MicrograpJiic Rhyolite. 



Specimen 102. — This light-grey rock occurs as a dyke 

 through the Walloon measures on Johnston's Creek near 

 Cunningham's Gap, portion 62v, parish of Clumber. It 

 has a fine development of black pitchstone, containing 

 porphyritic felspars, on its margin. The rock is holo- 

 erystalline and porphyritic. The rock is perpatic, and 



