BY H. I. JENSEN. 847 



level but for the numerous volcanic knobs of rhyolite and rhyolitic 

 tuff and breccia which are bestrewn over the plain, and rise 

 above it to a height of 500 feet or more. These rhyolites probably 

 are Tertiary eruptives, but may be older. At all events they 

 intrude the sandstone (Permo-Carboniferous ?). 



West of the Nandewars, around Narrabri, thence westwards 

 in the direction of Walgett and northwards towards Moree, the 

 country is almost perfectly flat, consisting of black soil plains 

 and interspersed sandy patches of the Pilliga type. There are 

 jutting out of the plains a few miles east of Narrabri several 

 small hills composed of porphyritic basalt and basic tuff. To 

 the north-west of the Nandewars, in the Parishes of Mellburra 

 and Myall Hollow, there are a few hills, almost conical in shape, 

 such as The Haystack and The Little Haystack. They, too, are 

 basaltic. In the Counties of Murchison and Darling, east of the 

 Nandewar group, the spurs of the latter are also, according to 

 the Geological Survey Map, capped with basalt; and basaltic 

 intrusions, and extrusions occur at intervals throughout the area 

 lying between the Nandewar Mountains and New England. 

 My own observations, as far as they go, show that volcanic rocks 

 (trachytes and basalts) cap the ridges east of the Nandewar 

 group. 



Dykes of basalt have been noticed cutting the trachytic and 

 phonolitic rocks in the Nandewar Mountains. This shows that 

 here, as in the case of the Warrumbungles, the last eruptions 

 were basic and extended over a wide area. 



Bnllawa Creek, from the petrologist's point of view, is by far 

 the most interesting locality in the Nandewars (Fig.l). On the 

 south side of the creek near Ritter's homestead numerous broken 

 hills intervene between the lava-tableland (Ningadhun, Coryah, 

 etc.) and the creek. In these, sandstone is usuall}^ the dominant 

 formation to a height of about 2,250 feet, and a slight S.E. dip 

 is generally observed. It seldom exceeds 5°. A bed of coarse 

 conglomerate with abundant quartz and cherty pebbles is met 

 with at an altitude of 1,900 feet. To the north of the creek 

 no broken hills intervene; an abrupt razorback range forms the 

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