152 VOLCANIC ROCKS OF SOUTH-EASTERN QUEENSLAND 



Trachytic Tuffs, Breccias &c. 

 These are of limited development and the chief occur- 

 rences are on the Main Range near Spieer's Gap and 

 Mount Matheson, also near ]\Iount Flinders. The intense 

 explosive action producing these accumulations seems to 

 have taken place after the trachytic rocks had been ex- 

 truded, and on the Main Range after the fissure had been 

 almost plugged up so that one got eruptions of the central 

 type. Jensen^^ has described this material and the author 

 has no new developments to add. 



There is a very characteristic breccia developed near 

 Mount Matheson. and it may pass into a coarse agglomerate 

 with trachyte boulders several tons in weight, or into 

 trachytic tuff with a beautiful fragmentary glass as a base. 

 The breccia has been considerably altered and chalcedony 

 is abundant all through it; also a ferruginous cement is 

 common. 



The tuff is well developed just below the elbow on 

 the Warwick coach road through Spieer's Gap. It is 

 seen to be a typical tuff, and has a fragmental glass ground- 

 mass which shows the curious drawn-out fragments with 

 concave surfaces. The groundmass has not undergone very 

 much alteration and is much better preserved than in the 

 Brisbane tuff. All through the rock are crystals of sanidine, 

 anorthoclase and plagioclase, also abundant inclusions of 

 rock-fragments. These rock-fragments are trachyte, ande- 

 site, basalt and shale. 



At Ivory's Knob near ]\lount Flinders there is also 

 a very good trachyte tuff. 



Chemical Composition of Trachytes. 



Most of these rocks are seen to be rather acid in nature, 

 and they are either sodi-potassic or dosodic, with soda in 



^"Proc. Linn. Soc, X.S.W., for 1909, vol. xxxiv. 



