858 THE GEOLOGY OF THE NANDEWAR MOUNTAINS, 



Crags the hills are commonly capped with a coarse andesitic or 

 trachytic breccia. 



It is probable that on the tableland between Eulah and 

 Bullawa Creeks, basalt caps the alkaline intermediate rocks. 

 The curious, long dykes surmounting the plateau are evidence of 

 fissure-eruptions which first gave rise to tuffs, and later on to 

 lavas, the flows of which have been undermined by the subsequent 

 weathering away of the underlying tuff-beds. 



^*^^V<o *^ LEFT EC/LA K 



rig.8.— Plan of Country round Mt. Deriah. 



Bobbiwaa Creek is separated from Spring Creek by a razorback 

 spur similar to that which separates Spring Creek and Bullawa 

 Creek; but, unlike the other creeks dealt with, it flows in a broad 

 plain-like valley, three or four miles wide as far up as Dripping 

 Rock, and a few miles lower down it leaves the mountain 

 altogether. The hills facing Bobbiwaa Creek in its upper 

 reaches are not cliffs, hence the valley is nowhere canon-like. 

 In fact, this creek forms a natural gap between the volcanic 

 masses north and south of it. The country between Bobbiwaa 



