go ROCKS USED IN THE MANUFACTURE OF TRONATTAS. 



inch in length, having a weight of 3j^lb. It is safe to 

 assume that this specimen was struck off a block of 

 much larger size, and, what is still more remarkable, 

 the plane face (pollical face), which has a siiperficial 

 area of about 17 square inches, cuts the lines of strati- 

 fication at a right angle. If the cleaving had been done 

 along the line of bedding it might perhaps be easier to 

 understand how such a large plane could be the result 

 of a single blow; but as it cuts it at a right angle, the 

 blow which struck this specimen off must have been 

 administered with tremendous force. It is probably the 

 fine flaking qualities that made the hornstone the 

 favourite material used for the tronatta, and there can 

 also be no doubt that the valued " mora trona," the 

 black flint of the vocabularies, specially refers to horn- 

 stone. 



While the true cretaceous flint of Europe is a sedi- 

 mentary siliceous rock, the hornstone of Tasmania is of" 

 different origin. Primarily it is a sedimentary rock.but 

 this sedimentary rock has subsequently been altered by 

 the eruption of volcanic rocks such as diabas or basalt. 



According to the nature of the original rock, a large 

 number of varieties were produced, but foremost of all 

 are dark blue and grey varieties, showing an extremely 

 fine banded texture. For the present I am unable to 

 say anything about the original rock, because none of 

 the quarries shows the gradual passage from the unal- 

 tered to the highly metamorphosed rock ; but there is 

 every probability that the original rock was a finely- 

 grained, thinly-bedded s'hale, such as occur in the coal 

 measures of Permian age. 



I have taken the specific gravity of 19 specimens of 

 hornstone collected at different localities, which are 

 given in the subjoined table : — 



1. Grey Plornstone, from Mona YslIq 2.500 



2. Blue Hornstone, from Mona Vale 2.506 



3. Blue Hornstone, Syndal Quarry 2.610 



4. Reddish Brown Hornstone. Mona Vale .... 2.616 



5. Blue Hornstone, Syndal Quarry 2.631 



6. Blue Hornstone, Mona Vale 2.644 



7. Dark Grey Hornstone, Mona A^ale 2.645 



8. Dark Blue Hornstone, Kempton 2.666 



9. Blue Hornstone, Kempton 2.679 



