52 



maintain the same position in regard to the range and the 

 alluvium for four miles in an easterly direction, and one mile 

 westerly, from Acacia Well. All the fossils seen belonged 

 to the same genus. Owing to the well having fallen in no 

 water was obtainable within ten miles, hence the search was 

 a rather hurried one. Eroded specimens occur on the slope. 

 On bare patches of rock the fossils are plainly visible, and 

 where the strata are broken across they are seen to penetrate 

 the rock at right angles to the bedding planes, in much the 

 same way that annelid burrows frequently do. The range 

 is composed of hard blue and grey dolomitic crystalline lime- 

 stone, in which the fossils occur, and the strata are less 

 disturbed and less altered than elsewhere. 



The same dolomitic limestone persists to the westward 

 for over 150 miles; it also runs in an easterly direction for 

 many miles. The east and west quartzite range, that forms 

 the northern boundary of this dolomitic limestone, also forms 

 the southern boundary of the granitic plateau and, as before 

 stated, is the most striking physical feature in the MacDonnell 

 Ranges. It supplies a number of the highest peaks, and 

 many of the romantic-looking gorges, and, as a wall of rock, 

 is probably unique. Mount Benstead is one of the peaks. 

 The view from Acacia Well is one of steep-sided and rugged 

 mountains in all directions. The ranges to the south are 

 probably Ordovician. 



Pre-Cambrian. 



The central plateau is, for the most part, composed of 

 gneiss, schist, and granitic rocks, and lies north of Alice 

 Springs. It is a "central boss" of wholly crystalline rocks, 

 both igneous and sedimentary — principally the former. A 

 feature is that, in places, the rocks have a definite and deter- 

 minable dip, but the planes may be either those of 

 stratification, cleavage, or foliation. 



