207 



In July, 1921, the writer, while on an expedition into 

 Central Australia, had a further opportunity of pursuing 

 these investigations. Limitations as to time and rapidity of 

 travelling prevented extended observations of this kind, so that 

 the locality tested for the purpose was mainly that in the 

 vicinity of the Macumba Head Station, situated about 34 

 miles to the northward of Oodnadatta. 



Here, as at Stuart Creek, the gibber slopes of the table- 

 land yielded examples of the large, roughly-chipped, and 

 ferruginously-coated implements. Some of the smaller worked 

 implements were found mixed with the larger, but whilst the 

 former are found generally distributed over the country, 

 irrespective of the nature of the ground, the heavier 

 implements appear to be restricted to the gibber or upland 

 regions. 



The country south of the MacDonnell Ranges is strongly 

 differentiated by two well-marked physiographical features. 

 The lowlands and wide river valleys are covered by rolling 

 hills of sand, temporarily fixed by growing vegetation, or in 

 a condition of drift. At a higher level, not usually exceeding 

 100 ft. above the normal level, are flat-topped hills that go 

 under the name of the "tableland." These flat-topped hills 

 represent an ancient land surface, which, by differential 

 denudation has been dissected, and the remnants of the same 

 are left at a higher level than the intermediate ground. The 

 larger fragments of the tableland have the features of a 

 "mesa," from which extend spurs and buttes with abrupt sides 

 and terminals, like gigantic tips front smelting works. 



These peculiar and picturesque features can be explained 

 from the fact that the older land surface, represented by the 

 tableland, consists of a capping of hard rock, underlying 

 which is a softer layer of rock, whitish in colour, and of an 

 argillaceous kind. The hard capping of these flat-topped hills 

 is sometimes a true sandstone, but, more commonly, the 

 Original fragmental. material, whether of sand, fine gravel, 

 or clay, has become consolidated by the infiltration of colloid 

 silica. The silica has penetrated the interstices of the sand- 

 stone, converting the mass into a chalcedonized rock, which, 

 in the case of the more finely -textured varieties, possesses r 

 more or less, a conchoidal fracture and vitreous lustre. The 

 origin of this extensive silicification of the surface rocks in 

 the interior of Australia can be explained from the peculiar 

 climatic conditions of the country (see Howchin's Geology of 

 South Australia, p. 450). Similar effects are also produced 

 in arid regions in other parts of the world. 



The great heat of the summer months and the extremes 

 of temperature which may occur in Central Australia at any 



