200 PROCEEDINGS OF THE ROYAL SOCIETY OF QUEENSLAND. 



appear to have been originally hollow and the spaces have been 

 filled ^xith secondary quartz. As the original groundmass area 

 has a pinkish staining through the kaolin and the secondary 

 quartz is clear, the juxtaposition of these materials gives an 

 appearance in the hand specimen somewhat resembling micro- 

 graphic structure. The size of the granules of " secondary " 

 quartz range up to 2 mm. long and is more than one might 

 expect in such a fine-grained rock. The size is no doubt due in 

 part to the size of the openings available in the hollow 

 spherulites. 



Specimen 264. 



This comes from the south side of Skene's Creek, in 

 Portion 143v, Parish of Maleny, and about | mile west of the 

 Bon Accord Falls. It differs in appearance from most of 

 the rhyolite and yields a chocolate-coloured soil. 



Megascopic Characters. — The rock is a deep lavender 

 colour and very fine grained. Small phenocrysts of a light- 

 coloured felspar occur sparingly through the rock. 



Microscopic Characters. — Phenocrysts of orthoclase, 

 slightly cloudy, lath-shaped, and ranging up to OSo mm. in 

 length and of quartz granules generally allotriomorphic, clear 

 and ranging up to 0"3 mm. in diameter, occur set in a 

 groundmass of a rather unusual type. (See Plate VI., fig. 2). 



The groundmass is largely glassy, while through it are 

 abundant acicular crystals of felspar, and both granules and 

 rods of a reddish-brown iron oxide, which from its colour by 

 reflected light appears to be haematite. As mentioned above, 

 many of the rod-like crystallites which have been formed in 

 the cooling glass appear to have been stained or replaced by 

 this material in the same manner as those recorded by G. A. J. 

 Cole for a rhyohte from Beaver Lake, Yellowstone Park. 



The abundance of this iron oxide is rather surprising, 

 and one finds difficulty in accounting for its concentration in 

 this particular portion of the magma. In a basalt from Bun- 

 damba, near Ipswich, one finds the glassy base which occurs 

 to a small amount thickly studded with black granules and 

 rods of magnetite and perhaps ilmenite. This rhyolite, how- 

 ever, is almost entirely glassy, and the rods and granules, while 

 similar in occurrence, are more like haematite. There appears 

 to be a good deal of " secondary " quartz throughout the 

 groundmass. 



