186 THE BUILDING OF EASTERN AUSTRALIA 



Range, in the Permo-Carboniferous rocks at a depth under 

 the Eastern Darling Downs, and in the Triassic rocks of 

 the East Moreton and Wide Bay districts. 



In New South Wales the Carboniferous and Permo- 

 Carboniferous rocks of the Stroud and Taree districts 

 offer most promise. 



(c.) Opal, being a product of mineral spring action, 

 has no great geological importance, so will be dismissed 

 in this paper. 



[d.) Phosphatic rocks have been discussed in another 

 paper by the writer. (These preceedings, 1909.) 



(e.) Diatomaceous earth of ecohomic value occurs 

 chiefly in direct association with our late Tertiary basalts. 

 It is a deposit formed of the test of organisms which lived 

 in siliceous hot springs. The writer has had excellent 

 opportunities to verify this by his studies in the Warrum- 

 bungle Mountains, where petrified wood and hyalite (water 

 opal) are invariably associated with the rocks surrounding 

 the diatomaceous earths. 



II. Ores. 



The following points are of importance in connectioti 

 with the study of our ore deposits : — 



1. Mineral lodes are mainly confined to folded areas. 

 This is why coal and gold are seldom found together. 



2. The minerals were introduced during a period of 

 igneous intrusion. 



3. The intrusions take place chiefly in the portions 

 of the folded earth segment, which are most remote from 

 the source of the folding force. 



4. Well defined fissure lodes have the same general 

 direction as the trend lines of folding in the same regions. 

 In the immediate proximity of great igneous intrusions, 

 they may take varied directions, diverging from the 

 intrusion. (See L. K. Ward, '' Geology of the Heemskirk 

 Massive," A.A.A.S., 1911, Sydney.) 



5. The direction of the zone of mineralisation is that 

 of the fold lines and major faults. 



Considering Australian ore deposits, it is interesting 

 to note that in the eastern half of the continent, most were 

 formed during the great abyssal injections of the Carbon- 

 iferous and early Mesozoic periods. 



