BY H. C. RICHARDS. 189 



of the ridge, having been exposed by the ■v\eathering of the 

 covering material. 



The microscopic section {see Plate xii, Fig. 4) shows the 

 rock to be a finely scoriaceons black glass through which 

 there are minute microlites of felspar. The cavities are 

 lined with very fine roimded grannies which have a refractive 

 index lower than Canada balsam and are probably tridy- 

 mite. 



Middle Vokanic Bocks. 



These consist of trachytic tuffs and flows. 



Trachytic Tuff. This occurs at the base of the series 

 and is several feet thick. It is made up for the most part 

 of trachytic material but in the lower parts it contains 

 abundant fragments of the underlying scoriaceous basalt. 

 These particles are usiially about the size of filberts. The 

 best development of tuff occurs along the Tambo road 

 ■and on the sides of the ridges in the neighbourhood of the 

 opal patch. Its extension in a south-westerly direction 

 could be traced for some distance, also in a northerly 

 direction around the flanks of Gill's Hill. On the southern 

 slope of the Mountain, at the base of the steep cliffs of 

 trachtye, a trachytic agglomerate \\as met with at one 

 point. It is much Aveathered and occurs on top of the 

 "basic representatives of the lower division of volcanic 

 rocks. Certain bands in the tuff contain abimdant rounded 

 grains of sand. 



Trachyte. This occurs outcropping on the surface of 

 the Opal Ridge, Gill's Hill and the Mountain, Avhile on the 

 flanks of Mt. Boorambool it is found outcropping between 

 the lower basaltic and upper basaltic series. It is a very 

 fine-grained, pink rock, showing a very definite fluxion 

 structure. It rests on top of the tuff and in some cases, 

 a-s on the upper ridge, near Gill's Hill, it has cooled so 

 quickly as to form pitchstone. The pitchstone, in some 

 cases, occui's as a dense black glass and bears a marked 

 resemblance to the Mount Lindsay pitchstone* ; it differs, 

 however, in being almost entirely free from felspar pheno- 

 crysts. In other cases it shows a very good fluxion banding 

 and some blocks of this have been brought to the surface 



*Richards H. C, Proc. Roy. Soc. Q'ld., xxvii (2), 1916, p, 138. 



