249 



Silurian Rocks. 



The " MacDonnell Ranges South " form a barrier to and rise 

 abruptly from the limestone and shales of pre-Silurian age. I 

 failed to detect any unconformability between them. The escarp- 

 ment is formed by the denudation of the limestones and shales. 

 It is my opinion that when these rocks were in course ef forma- 

 tion, the pre-Silurian rocks were horizontally disposed, otherwise 

 I do not think faults could have occurred and placed such long 

 narrow quartzite ridges in their present positions to the schists 

 had the angle of unconformity been great. 



The area over which this formation extends has not been deter- 

 mined in many directions, but sufficient is known to be able to 

 say that it approaches the Musgrave Ranges on the south ; on 

 the west it extends well on towards Lake McDonald, on the east 

 to beyond the Hale River, and on the north to and inclusive of 

 the MacDonnell Ranges South. There is every probability that 

 beyond these limits the formation stretches to the south-west, to 

 the north-west, north-east, east, and west, over considerable 

 areas, as I am persuaded from the notes of Gosse and other ex- 

 plorers that it extends to and is largely represented in the Mann, 

 Tomkinson, Petermann, and other Ranges ; and much of what 

 has been termed " desert sandstone" must be classed under 

 " Silurian." Similarity of physical structure alone has led to 

 errors. 



Two axes produce the Upper Finke basin. The larger and 

 more important runs east and west through the heart of the 

 MacDonnell Ranges. At the time this upheaval transpired the 

 Silurian quartzites were largely represented over the area now 

 occupied by the basin, as proved by fossil and stratagraphical 

 evidence ; and were by lateral pressure formed into anticlinal 

 and synclinal folds, which run nearly east and west. With one 

 exception, viz., that of an eruptive outcrop, it forms the bed rock 

 of the basin. The second and lesser runs in a north-westerly 

 and south-easterly direction, and junctions with the first near the 

 western end of the MacDonnells, forming an oblique plication of 

 the Silurian foldings, and also the " basin." The latter deter- 

 mines the western watershed, and the former the northern, of the 

 Finke River and its tributaries. To the north and west of these 

 axes small creeks in comparison with the Finke and its ti'ibutaries 

 take their rise, run out into sandhill country, and soon are lost. 

 The Finke may be said to follow the lowest part of the basin, 

 and, like its tributaries, maintains a south-easterly course, occa- 

 sionally breaking through from one synclinal fold to the next 

 farther south. 



To return to the anticlinal and synclinal folds within the basin. 

 How wonderfully even the pressure has been can scarcely be 



