248 



level expanse of plain and drifting sandhills, with here and there 

 low bluff hills and salt marshes dotted over it, was at a subse- 

 quent period to the Silurian upheaval the highest tract of land 

 in Central Australia. 



When the quartzite capping, which will shortly be noticed, be- 

 came denuded, the pre-Silurian rocks weathered away much faster 

 than the rocks to the south, which are partially in places and 

 wholly in others covered by Silurian quartzites ; thus we have 

 high land still where quartzite exists, and lower land where it 

 has been denuded. 



The southern edge of the Burt Plain is bounded by an escarp- 

 ment of crystalline rocks consisting principally of chlorite schists 

 and granite ; the escarpment rises abruptly to an average height 

 of 800 to 1,000 feet. These remarks refer to that portion of the 

 range from Mount Zeil to near the Alice Springs. The drainage 

 to the north is short, but the southern drainage starts imme- 

 diately from the top of the range. This ridge is really the 

 watershed of the MacDonnell Ranges, though higher peaks and 

 ranges exist some few miles farther to the south. 



Micaceous schists and metamorphic granite follow on in regular 

 sequence to the south, with occasional outcrops of coarse eruptive 

 granite and other eruptive rocks. Long ridges of hard white 

 quartzite nearly vertically disposed, and dipping comformably 

 with the schistose and stratified granite rocks run through the 

 country east and west, and form absolutely impassable barriers 

 to the traveller, saving along the creek-channels, which intersect 

 them. These ridges form the highest ranges in the MacDonnell, 

 and mark the lines of faults, which are frequent. Mounts Zeil 

 and Liebig are exceptions, being formed of metamorphic granite. 

 Then follow metamorphosed clays and shales interstratified with 

 yellow and blue crystalline limestone, which conclude the series. 

 In the neighbourhood of these rocks there are marks of great 

 erosion and denudation. Prior to the great Silurian upheaval, 

 hereafter to be mentioned, these rocks were overlain by more 

 than a mile in thickness of Silurian quartzites, limestones and 

 shales ; the only remnants of which are the ridges and patches of 

 quartzite and limestone. In the neighbourhood of the granitic 

 dykes the Silurian remnants are much scattered. 



Mica is abundant in both eruptive and metamorphic granite, 

 and I feel sure marketable sizes will be found much farther to the 

 west than that discovered at Hartz Range. Quartz reefs are of 

 frequent occurrence throughout the granite and schistose rocks, 

 but I regard the western end of the MacDonnell as a splendid 

 field for the prospector. Topographical surveys and a critical 

 examination of rock specimens can alone determine accurately 

 moot questions on the pre-Silurian rocks of Central Australia, 

 and particularly of the MacDonnell Ranges. 



