56 
ECONOMIC GEOLOGY 
Ballarat plateau to determine the position and depth of its 
main lead, of whose meandering course there was no indi- 
cation on the surface, 
These bores initiated deep-lead mining, which depends 
largely on geology both for the tracing and working of the 
leads. Western Victoria includes a great plateau across which 
in Middle Kainozoic times rivers flowed southward to the 
Southern Ocean, and northward to the Murray River. 
Numerous volcanoes discharged sheets of basalt, which filled 
the valleys on the plateau. Subsequent denudation has 
removed the softer rocks which formed the banks of these 
8 
D 
F16. 18.—A SECTION ACROSS A DEEP LEap. 
BB, basalt plateau overlying a former river bed reached by No. 2 of the 
three bores, 1, 2,3. The country at the right end of the section is 
granodiorite sending up tongues which are associated with auriferous 
quartz-lodes in the slates and quartzites of the country, AB, the 
former outline of the valley, on the floor of which the basalt lava 
flow was discharged. CD, the present surface with river valleys 
formed on either side of the basalt plateau. 
valleys, and the lava flows have been left upstanding as 
plateaus (Fig. 18). The ancient river beds lie under the 
basalt and as they flowed over rocks containing many gold- 
quartz lodes, the river gravels contain alluvial gold. The 
profitable mining of these gravels required accurate know- 
ledge of their position and depth. The Geological Survey 
of Victoria therefore made lines of bores across the basalt 
plateaus to determine the course of the ancient river system, 
the tracing of which was an interesting problem in physical 
geology and yielded many surprises. The bore records are 
used to construct a section showing the varying level of the 
bedrock, and the height and position of the river bed. Its