96 ROCKS USED IN THE MANUFACTURE OF TRONATTAS. 



it appears that it is too soft for rough use. Anyhow,. 

 none of these minerals played an important role in the 

 economic life of the Aborigines. 



The heat affects these minerals in a way quite dif- 

 ferent from hornstone or porcellanite. In wood opal the 

 fire produces a few cracks, wiiich appear to penetrate 

 deeply into the interior. The cracks are rather irregular, 

 fairly wide apart; in somewhat this comes closest in ap- 

 pearance to hornstone, but the cracks are more ir- 

 regular, further distant, and deeper. 



Very different is the appearance of the white, milky 

 chalcedony. This is intersected by a large number of 

 irregular cracks running in all directions, and pene- 

 trating through the whole mass of the specimen. A 

 more brownish rather transparent chalcedony shows the 

 same features, part of the surface is broken off, and 

 instead of the smooth surface produced by flaking, it is 

 rough and jag'ged. 



These pieces of chalcedony resemble very closely to 

 the famous cracked flints from the Oligocene of Thenay 

 in France ; in fact, the darker specimen can hardly be 

 distinguished from the E,uropean ones. 



The examination of 11 specimens gave the following' 

 results for specific gravity : — 



1. Opaque Wood Opal, Maryvale . . . . ■. . . . 1.940 



2. White Chalcedony, Maryvale 2.289 



3. White Chalcedony, Baskerville 2.433 



4. Milky Opaque Chalcedony, Mona Vale .... 2.436 



5. Fossil Wood, Mona Vale 2.465 



6. Wood Opal, Mona Vale 2.533 



7. Chalcedony, Melton Mowbray 2.583 



8. Do., Melton Mowbray 2.592 



9. Do., Melton Mowbray 2.606 



10. Fossil Wood, Droughty Point 2.666 



11. Milky Quartz, Mona Vale 2.680 



The average specific gravity, if it be permissible to 

 take it of such a heterogenous group, would be 2.472. 



Before proceeding any further, I must explain why 

 I never mentioned the hardness. The old method of 

 determining the hardness of a mineral or a rock is a 



