UNUSUAL. RHYOLITE PROM THE BLACKALL RANGE. 201 



Specimen 475. 



This comes from Portion 183, Parish of Maleny, and is 

 about 1| miles south of the Bon Accord Falls and on the other 

 side of a basalt-capped ridge. There is little doubt that the 

 rhyolite is continuous beneath the basalt capping, as it can be 

 traced continuously around the end of the ridge from one point 

 to the other. 



Megascopic Characters. — It is a compact rhyolite much 

 like 261a except that it does not show fluxion and spherulitic 

 structures. 



Microscopic Characters. — The rock has small phenocrysts 

 of orthoclase and quartz set in a cryptocrystalline groundmass 

 through which there is arranged much " secondary " quartz 

 in the form of allotriomorphic granules. 



Specimen 265. 



This was obtained from the bed of the Obi Obi Creek, at 

 " The Narrows," where the bed of the stream has cut a narrow 

 canyon through the massive rhyohte flows. " The Narrows " 

 occur at the south end of Portion 183, Parish of Maleny, and of 

 Reserve 594, and are distant nearly 2 miles south of the Bon 

 Accord Falls. 



Megascopic Characters. — The rock is composed of 

 spherulites, most of which are hollow and merit the term of 

 " hthophysse." Successive concentric rings around the hollows 

 are visible in many of the spherulites. In size they average 

 approximately 2 5 mm. The colour is pink to grey, and through 

 the rock one sees occasional pink orthoclase phenocrysts. 



Microscopic Characters. — The rock appears to be very 

 much altered and to be composed of kaolin and secondary 

 siHca. The spherulites have been filled up with secondary 

 silica, and the rods forming the concentric layers have been 

 altered into kaolin. This rock is probably much of the same 

 type as Specimen 473, except that the hoUow spherulites were 

 much smaller in the latter rock. 



CHEMICAL CHARACTERS. 



An examination of the complete chemical analysis reveals 

 the somewhat remarkable character of the rock, and perhaps 

 the very low alumina percentage (5-43) is even more surprising 

 than the very high silica. The lime, magnesia, and total iron 

 oxide percentages are very similar to those in the analyses of 

 the average rhyolite and of the typical Southern Queensland 



