856 THE GEOLOGY OF THE NAXDEWAR MOUNTAINS, 



On proceeding higher up the creek, sandstones, shales and 

 interlaminated sills repeat themselves, and the hills on both 

 sides have lava-cappings. 



Boomi Creek. — From Bullawa Creek I made an excursion 

 across the mountains to Boomi Creek. The crest of Boomi Gap 

 has an altitude of about 3,500 feet. South of it lies Kaputar 

 (5,000 feet), north of it Pound Mountain (4,500 feet). In 

 ascending the pass from the west one encounters practically only- 

 trachyte. However, on descending to Boomi Creek one crosses 

 a basaltic dyke, running N. and S., at a height of 3,000 feet; at 

 2,900 feet there is an outcrop of greenish slaty rock; at 2,800 feet 

 an outcrop of coarse conglomerates dipping north. These rocks 

 are probably Permo-Carboniferous, perhaps Carboniferous. 

 Lower down rhyolitic and andesitic tuffs and quartz porphyries 

 are met with as well as conglomerates wdth rhyolite pebbles. 

 These volcanic rocks and conglomerates are lithologically the 

 same as those occurring at Laird's on Maule's Creek, Horse-Arm 

 Creek and Black Mountain south of the group. Below the 3,000- 

 feet level the Boomi Creek country loses the wild ruggedness 

 characteristic of the Bullawa Creek side, consisting of more 

 gently-sloping, wooded and grassy spurs well adapted for grazing. 

 The change in scenery is due to the change in formation, Lower 

 Permo-Carboniferous or Carboniferous rocks being here predomi- 

 nant, and the barren trachytes. Upper Coal Measure and Triassic 

 sandstones being seldom seen. In Boomi Creek there are 

 numerous boulders of coarsely porphyritic basalt with gigantic 

 felspar phenocrysts similar to that which caps the intermediate 

 rocks round Deriah and intrudes them in Kangaroo Gully. This 

 rock is probably derived from Kaputar. 



Eulah Creek. — This creek rises in the Lindesay Tableland and 

 flows parallel to Bullawa Creek into the Namoi. The country 

 in which its two branches head is like that at the head of 

 Bullawa Creek, consisting of sills of intermediate rock intruding 

 Permo-Carboniferous or Trias- Jura sandstones, which rise abruptly 

 on either side to form a tableland capped with lava. The creek, 

 like Bullawa Creek, flows in a gorge-like valley, which at 



