TfiE GEOLOGY OF CENTRAL AUSTRALIA. 
93 
All the above minerals were introduced by the igneous intrusions 
which succeeded the deposition and folding of the Perta-knurra and 
Mosquito Creek sediments. Apart from mica and specially rich deposits 
of tin, tantalite and wolfram, the only profitably worked mineral is gold. 
The Tennants Creek field has come to stay. There is gold at the 
Granites and at Tanami, and there is a vast area of country north-west 
of Barrow Creek towards Tanami which may yet produce many good 
goldfields. Owing to* surface water being absent except in the heaviest 
of rains this area is difficult to prospect, and has not been properly 
prospected. In this area there are many laccolitic granite intrusions 
which belong to the Nullagine, or more likely early Cambrian period. 
These granites introduced copper, silver-lead and gold in places (as they 
have done north-east of Barrow Creek, at Wollogorang, Lawn Hill, etc., 
near the Queensland border) . As disseminated copper and lead ores occur 
in connection with this last igneous period, the evidence of ore deposits 
is obscure unless very careful search is made. 
There is a vast amount of geological work and work on ore deposits 
yet to be done in the Territory. 
IV. THE WATER PROBLEM. 
Central Australia is almost devoid of permanent waters, hence 
there is everywhere need for more bores. Unfortunately there are no 
artesian or even subartesian basins in this part of the Territory. Sub- 
artesian waters are more generally found about 400 miles north of Alice 
Springs in the extensive Cambrian belt of the Barkly Tableland and 
near the South Australian border also in the Cambrian. The vast inter- 
mediate area only carries water in odd small localised pockets, or in 
fault zones. Pegmatite dykes often serve to dam back and hold moderate 
quantities of water in fractures in the Aruntan rocks in the Hart’s 
Range. 
During the last couple of years a vast amount of water drilling 
has been carried out by the army and Allied Works Council, sometimes 
without competent geological advice ; hence failures have been numerous. 
The choice of bore sites is a task which can only be satisfactorily carried 
out by a competent geologist. 
Alice Springs is unique in Central Australia in having a large 
underground water supply. An eroded valley between the Macdonnell 
Range and Birt Upland Plain has been deeply silted up with sand and 
its outlet must have been partly blocked by land-slip. Consequently 
the floods of the Todd River and branches charge these sands with water, 
which can stand being drawn on heavily. 
V. THE SOILS. 
The ranges and hills in Central Australia have little soil and little 
vegetation. Between the ranges there are large plain areas in which 
bores in search of water have often gone 600 to 700 ft. before meeting 
with rock. The soils are windblown mostly, — a kind of loess. They are 
R 
