SOME GEOLOGICAL FEATURES OF NORTHERN AUSTRALIA. 119 



The tendency for tin to leave the deeper zones of meta- 

 morphism and to deposit in the upper middle zone, where 

 hydration is permitted by the temperature pressure conditions, 

 is thus evidenced by the association of tin with chlorite rock. 

 Obviously reaction SnF4 + SiOg = SnOa + SiF4 is not 

 favoured by high-temperature pressure conditions, in which 

 hydrous minerals are unstable. The silico-fluorides, like topaz, 

 pycnite, are almost universally hydrous. The SiF4, as soon as 

 formed, reacts with silicates to form these minerals. The 

 chlorite rocks are therefore best regarded, not as an indication 

 of a definite geological age but as the typical alteration product 

 of certain tuffs when depressed to the upper " middle " zone of 

 the earth's crust during a period granitic intrusion. 



Apically, Medially, and Basally Truncated Batholiths. — 

 Professor Butler, in " Economic Geology," March, 1915, has 

 shown that for batholiths of the same magma those apically 

 truncated are more favourable to rich mineral occurrences than 

 those medially and basally truncated. In general that holds 

 true for the Territory, where the writer has investigated the 

 matter. It is also the writer's experience that apically tiTin- 

 cated batholiths are characterised by a more aplitic type of 

 granite than usual, often called " sanely granite." 



In areas of general plateau uplift, continued through great 

 periods, there will naturally be extensive areas of granite, 

 constituting mediallj^ and basally truncated batholiths. Those 

 areas are not favourable to the prospector. Alluvial tin, gold, 

 and wolfram occur widely on them in small Cjuantities, but 

 large lodes are few or absent. 



It is on those parts of the massifs over which the schists 

 and metamorphosed sediments are only partly removed, and 

 the underlying granite exposed only here and there in valleys, 

 o ■ as a central boss with schists dipping away from it all 

 round, that one would search with most prospect of success 

 for payable lodes. 



In the Territory the Maranboy and Coronet Hill fields 

 are such areas. Tanami is another in which erosion has not yet 

 exposed the granites anywhere. The heads of the Katherine, 

 Alligator, Limmen, and Mc Arthur Rivers are also in areas 

 where apically truncated batholiths will be found. In North 

 Queensland apically truncated stocks will most frequently 

 occur around the edges of the northern massif, along the belt 

 of erosion of the Silurian festoon moulded on the massif. 



