2 MARKS OK PERCUSSION ON SILICEOUS KOCKS, 



It is certain that these ciTects must be a function of 

 the composition of the matter, supposing the energy being 

 the same. The effects of a blow striking with the energy 

 of 100 foot pounds, must be quite different if the substance 

 be lead or antimony, a hornblende rock, jadite, nephrite, 

 or flint. It would be outside the scope of this paper 

 to investigate into the effects of blows on Horablende 

 rocks or Nephrite and other allied substances largely used 

 in the manufacture of stone implements. I propose to 

 deal here only with the effects of blows on that substance 

 of which most of the stone implements are produced, viz., 

 siliceous rocks. 



In its purest form the siliceous minerals are repre- 

 sented by crystallised quartz, having 46.67 per cent, of 

 silicon. Through the admixture of other substances, a 

 large variety of minerals are produced, but in all of them 

 the percentage of silicon is considerably smaller than in 

 crystallised quartz. The mineral most commonly used in 

 the manufacture of stone implements is flint in Europe, 

 the various siliceous rocks resulting from the metamorphism 

 of permian rocks, which ars called chert or hornstone and 

 porcellanite in Tasmania. (1) 



However different in composition these minerals and 

 rocks may be, they have one feature in common, viz.. an 

 exceedingly fine conchoidal fracture. 



But even the casual obsei-ver cannot fail to notice that 

 the nature of the fracture greatly varies in the different 

 kinds of siliceous rocks. I have no data fcr determining 

 what causes produce the most suitable fracture for the manu- 

 facture of implements. Pure rock-crystal and its nearest 

 relation, chalcedony, have not, particularlv not the latter, 

 the same fine fracture, as, for instance, the impurcr flint, 

 and it seems certain that the quality of fracture does not 

 depend on the pui'eness of the silica. In fact, the in- 

 stance here quoted proves that the less purer mineral has a 

 better fracture than the purer one. Natural and artificial 

 glasses have an exceedingly good fracture, and it almost 

 seems as if the quality of fracture were dependent on 

 the presence of iron. I advance this theorv with all re- 

 serve, as long and tedious chemical and physical examina- 

 tions would, be necessary to establish it. 



In manuals and text-books of Mineralogy the frac- 

 ture of siliceous rocks is described as "conchoidal," the sur- 

 face produced bv fracture having elevations and depressions 

 in form like one-half of a bivalve shell. 



1) Noetli"(j, I'rfliminnry Note on the Rocks used in the manufacture of the 

 Tionattii. — Pap. and Proceed. Roy. Soc, Tas., 19I»9, page ».i. 



