190 VOLCANIC ROCKS OF SPRINGSURE. 



from the shaft sunk in the opal patch. This material 

 bears a remarkable similarit.y to the pitchstone on the 

 Springbrook Plateau*, near MacPhorson's Range, in south- 

 east Queensland, and occurs in a similar manner as a glassy 

 selvage at the base of the flow of trachyte. On Gill's 

 Hill and Mt. Boorambool, the trachyte flows are about 

 100 feet thick while on the Mountain the accumulp.ted 

 thicknesses are at- least GOO feet. 



On the Opal Ridge the trachyte is rather more weathered 

 than elsewhere and is spherulitic for the most part. Sections 

 of the more compact material show this very clearly. The 

 material, however, has been altered into kaolin and chalce- 

 dony which occurs abundantly through it. 



The precious opal Avas found in this weathered trachytic 

 material and, as noted by Jacksonf, its occurrence in this 

 respect differs from that of the opal deposits in south- 

 western Queensland, north-western New South Wales 

 and Central Australia, which occur in the Cretaceous sedi- 

 menta.ry rock. Jackson speaks of opal occurring in a 

 similar manner to this deposit near Rocky Bridge Creek 

 in New South Wales, while the writer has seen a specimen of 

 precious opal obtained from the O'Connell Town quarry 

 in the Brisbane Tuff. 



The main mass of trachyte on the Mountain is rather 

 uniform in character and while the prevailing colour is pink, 

 in places it is a greyish-white. As great gorges, in places 

 300 feet deep, have been carved through the trachyte and 

 as it is almost devoid of soil an excellent opportunity of 

 examining the material is afforded. The rock is very 

 platy as a result of the fluxion structure and thin flakes of 

 the material when knocked together give forth a pronounced 

 metallic clinking sound. The stone well merits the term 

 " Clinkstone," and it is very interesting to find that it is 

 reaUy a phonolite in the strict petrological sense. 



Specimen 277, obtained from the Virgin Rock on the 

 Mountain, is representative of the main mass. It is a very 

 fine-grained pink rock with occasional phenocrysts of 

 anorthoclase which range up to 1mm. in length. Under the 



*Richards, H. C, Proc. Roy. Soc. Q'ld., xxvii (2), 1916, p. 137. 

 tJackson, C. F. V., Geol. Surv. Q'ld., Pub. 177, 1902, p. 33. 



