BY W. N. BENSON. 



m 



serpentine, owing, possibly, to the readier attack of frost-action 

 on the former, or the easier access of atmospheric agencies to the 

 inner parts of the rock. A serpentine-boulder, however, is rapidly 

 broken up in a stream, owing to its very small resistance to 

 mechanical attrition. Hence pebbles of serpentine are seldom 

 carried very far from their source. 



The widespread Barraba mudstones and the least siliceous of 

 the Tamworth claystones are, in contrast to all the above ro<'ks, 

 very easily eroded; and consequently give little sign of relief due 

 to differential erosion, but are almost always reduced to gently 

 rolling, mature surfaces broken by low ridges of the more 

 resistant interstratified tuffs. Where the Baldwin Agglomerates 

 and Barraba mudstones are faulted together, differential erosion 

 brings such dislocations into clear relief. 



2%e Evolution of the Manilla River. 



The manner in which the several factors cited above have 

 co-operated in producing the present topography, seems to be 

 most clearly illustrated by a consideration of the river-systems 

 immediately adjacent to the Nandewar Range, namel}^, those of 

 the Horton and Manilla Rivers. Dr. Taylor (46) has called 

 attention to the curious courses of these streams, but since his 

 study of them was based upon an inaccurate geological map, his 

 explanation is not in accordance with that suggested by later 

 investigations, which, however, are still incomplete. 



Ancient valleys marked by river-drift break across the Ser- 

 pentine Line at Paling Yard, Woods' Reef, and Crow Mountain, 

 converging to a point east of Barraba. The elevation of the 

 Gap at Paling Yard is approximately 2,320 feet (official map);* 

 where the old valleys met, east of Barraba, the elevation of the 

 upper surface of the drift must have been not more than 2,040 

 feet, and on this lies a thickness of 100 feet of basalt, the upper 

 surface of which is about 2,140 feet above sea-level. North of 

 Barraba, the upper surface of the basalt is 2,036 feet above sea- 

 level, and below the basalt, which is about 60 feet thick, further 



* The elevations cited are all. based upon aneroid observations, and are 

 thus only rough approximations. 



