196 I'ROCEKDINGS OF THE ROYAL SOCIETY OF QUEENSLAND. 



In the " Geology of the Broken Hill District, N.8.W/' by 

 Mr. E. C. Andrews, B.A.,* the lack of consensus of opinion as 

 to the origin of the so-called "' quartzites " is mentioned. 



It has been urged that the chemical nature of the 

 " quartzites " is against their being regarded as originally 

 igneous, but the writer hopes to show on the purely chemical 

 evidence in comparison with the Montville rhyolite that it is 

 not unreasonable to assume an original igneous nature for 

 the quartzites. 



FIELD OCCURRENCE OF THE RHYOLITE. 



The Blackall Range, which has a general north and south 

 trend and rises up to 1,500 feet or more above sea level, is 

 situated 60 to 70 miles north of Brisbane. The range is 

 composed of volcanic rocks of Cainozoic age resting uncon- 

 formably on sandstones belonging to the Bundamba Series of 

 (?) Upper Triassic age. 



Skene's Creek, which is a tributary of the Obi Obi Creek, 

 has its head waters on the western fall of the Blackall Range 

 between MontviUe and Flaxton. After cutting its way through 

 the basaJt and andesite forming the upper portions of the 

 Range, the creek flows in a general westerly direction into the 

 Obi Obi Creek. 



Following down the creek-bed a change from basalt to 

 underlying rhyolite takes place in Portion 93v, Parish of 

 Maleny, and after passing through Portion 73 into Reserve 

 546, there is, after two preliminary drops t©talling about 80 

 feet, an almost sheer drop of 230 feet at the Bon Accord Falls. 

 These falls are composed of the rhyolite under consideration. 



Its field occurrence with its spherulites and fluxion 

 structure is similar to many rhyolites of Middle Cainozoic ag" 

 in Southern Queensland. 



There is a wide distribution of the rhyolite as it is found 

 in the Obi Obi Creek bed more than 2 miles south of the Bon 

 Accord Falls, and it can be traced continuously between these 

 points. 



A thickness of at least 1,000 feet of rhyolite occurs, and 

 may be determined by passing down into the Obi Obi Creek 

 from Portion 132v through Portions 187 and 186. 



* Mem. Geol. Survey N.S.W., Geology No. 8, 1922, p. 107. 



