274 aREAT SERPHNTINE-BELT OP NEW SOUTH WALES, Vl., 



iii. Features due to Differential Erosion. — The most resistant 

 rocks in the region are the jaspers, which form a marked series 

 of peaks and ridges, and have been reduced to the general level 

 only in regions of very long-continued erosion, with local differ- 

 ential crust-movement. Second only to the jaspers in their 

 resistance are the Baldwin Agglomerates, which form rough 

 ridges in the Black Mountain, but are reduced to a peneplain- 

 level on the Bingara Range. Somewhat less resistant are the 

 Rocky Creek conglomerates, and still less resistant than these 

 are the tuffs in the Devonian claystones of Barraba or Tamworth 

 age, which, however, give rise to marked ridges {e.g., the Aber- 

 deen Range) which cross the Manilla River obliquely. The 

 phyllites, etc., of the Eastern Series vary in their resistance 

 according to their degree of silicification. They could never be 

 classed as readily yielding to erosion. The same holds with 

 regard to the more siliceous radiolarian cherts of the Tamworth 

 Series. 



Of the igneous rocks, we find always, among the resistant 

 masses, the dolerites of the Blue Knob type, the Tertiary trachytic 

 rocks, and basalts. The granites vary greatly in resistance ac- 

 cording to their composition, as Mr. Andrews has emphasised (in 

 33 and other papers). The high regions south of the junction of 

 Cope's Creek and the Gwydir River, or that near Bendemeer, 

 are highlands of resistant acid granite; the granites of the lower 

 region near Cope's Creek and Bundara are more basic. The 

 gabbros associated with the serpentine vary in their resistance 

 to erosion, according to the nature and degree of their altera- 

 tion. They are generally less resistant than the serpentine. 

 The case of the last-mentioned rock is peculiar. As is usually 

 the case, the serpentine, though soft, is very resistant to erosion. 

 It forms, along the eastern side of Hall's Creek, a mountain-wall 

 almost as steep and unbroken as that of the Baldwin Agglomer 

 ates of Bingara Range, on the opposite side of the valley. This 

 is, doubtless, due to the chemical stability of serpentine, which, 

 being fully hyd rated, decomposes with extreme slowness under 

 atmospheric conditions, so that disintegration is very slow; it is 

 much less so, however, in the case of schistose than of massive 



