THE GEOLOGY OF CENTRAL AUSTRALIA. 
87 
Right in the township of Alice Springs, between the railway 
station and the hospital, there is a notable hill of orthogneiss (gneissic 
granite) called Billy-Goat Hill. Along the northern road for 55 miles 
north of Alice Springs all the outcrops are gneissic and mostly 
gneissed igneous rocks, such as gneissic porphyries, gneissic porphyrites, 
gneiss-amphibolite, etc. These gneissic igneous rocks appear to be the 
oldest group of rocks in the massif. 
The Macdonnell, Hart’s and Strangway Ranges, which form the 
massif, are extremely rough in nature. High sharp ridges, often with 
cliff faces on one or on both sides, steep cones and pyramids are a 
constant feature. Alice Springs lies at an elevation of 1,902 ft. but the 
nearby Macdonnell Ranges reach to 3,000 and even 4,000 ft. The plain 
north of the Hart’s Range is also 1,900 ft. in elevation near the foothills, 
yet large sections of the range average over 3,000 ft. in elevation and 
peaks reach up to 3,600 and 3,800 ft. Nevertheless these high ranges 
are traversed here and there by passes of much the same elevation as 
the nearby plain. Such natural breaks are of structural origin, water 
erosion having cut gorges along fault planes or dykes. 
The great Central- Australian mountain uplift was already very 
dissected before the earliest Cambrian strata were laid down, although, 
as Madigan ’s papers show, further uplifts took place in the Proterozoic 
and Cambrian in the same area. 
There are two peneplains in evidence in this region, both of 
Tertiary age, one about 600 ft. higher than the other. The lower and 
more recent one is that of Alice Springs and the plains around the 
ranges. It is probably post- Tertiary. The older peneplain (Birt Plain 
peneplain) is probably Miocene or Pliocene. Both these peneplains drop 
in elevation in all directions from the Cental Massif. A slow uplift 
has been taking place in the Massif area in post-Miocene times. 
The geological formations represented in the Hart’s and Strangway 
Ranges are much the same as those represented in the Macdonnell Range. 
The oldest rocks in Central Australia are known as the Arunta 
Complex. Rocks of this series only occur in the Hart’s and Strangway 
Range proper, though south of the main Hart’s Range (towards the 
White Range) where it merges into the Macdonnell Range, formations 
belonging to the Perta-knurra and Pertatataka enter into the mountain 
building. 
The oldest formations of the Arunta complex are strongly 
metamorphosed and gneissic granites, porphyries and porphyrites, and 
amphibolites are represented in them. The next oldest is a series 
composed of metamorphosed and gneissic sedimentary rocks with inter- 
calations of crystalline limestone. There are basic biotite-amphibole 
gneisses formed from basic tuffs, biotite gneisses formed from calcareous 
sandstones, and felspathic and quartzy gneisses formed from sandstone 
and grit ; and apparently interbedded with these are zones of amphibolite 
derived from basic volcanic flows, or perhaps from sill-like intrusions, 
