578 



THE GEOLOGY OF THE WARRUMBUNGLE MOUNTAINS, 



this rock represent old valleys which were filled with it in the- 

 volcanic period. The old ridges, consisting of softer sandstone^ 

 have now become valleys. 



'. Phonolitic columnar trach . 2^0 

 1. Vesic jtihoMoliTic trac/jy^e 



/^.Trachyfte, Cuff llOO 



5. ^eslc.arfyed trach 



6. Trac\^. Breccia 



25O0, 



Fig. 7. — Section across Uargon Creek from Black Mountain to Naman 



Ledges. 



The arfvedsonite trachytes have in most cases been necks 

 injected into tuff cones. It is easy to understand that later 

 erosion would find the soft tuff beds more subject to attack than 

 the hard segirine trachytes around. Consequently many plugs 

 (or mamelons of originally viscous rock) stand now in valleys, 

 their inaccessible walls forming a striking feature. 



This view is borne out by the present structure of Tonduron 

 (The Spire), which consists of a central plug of massive trachy- 

 dolerite, and a number of high hills surrounding it, two of which 

 (on the east and west respectively) attain mountain-like dimen- 

 sions. These hills are remnants of the old crater- ring, and tuff 

 and lava cone. The original mountain w^as built up of alternate 

 layers of tuff and lava, with a plug of lava in the core. By the 

 action of meteoric waters working along the soft tuff and breccia 



