92 ROCKS USED IN THE MANUFACTURE OF TRONATTAS. 



2. PORCELLANITE. — This is a much coarser 

 grained rock than hornstone, ahvays showing a shiny 

 waxy lustre, thereby strongly differing from the dull 

 hornstone. Its colour varies from a pure white to 

 almost black ; most frequent are red, yellow, and brown 

 tinges ; grey is not infrequent, the dark tones being the 

 rarest ; sometimes several colours, for instance red and 

 yellow, alternate, and the rock becomes streaky. 



Tronattas made of porcellanite are much less fre- 

 quent, and so far I know of only two localities where 

 porcellanite is fomid in situ. One is near Pontville sta- 

 tion, and this Avas used as a quarry (Weston's Quarry) ; 

 the other is an outcrop near Maryvale, which, however, 

 was probably never worked. In AVeston's Quarry the 

 colour of the rock varies from white through grey and 

 red. 



Porcellanite never shows a patina, and this proves 

 that its chemical composition must considerably differ 

 from that of hornstone. 



The fracture is conchoidal, and some of the porcel- 

 lanites flake as well as hornstone ; yet it was a much less 

 favourite rock than the latter. I believe the reason for 

 this is the coarser grain, because tronattas made of por- 

 cellanite are generally never so elaborately worked as 

 those of hornstone. 



Blown sand does not affect the porcellanite as much 

 as the hornstone. Of course it also gets the peculiar 

 coating due to this cause, but inasmuch as it has a shiny 

 lustre of its own, the blown sand does not affect it much. 



The heat of fire acts quite differently on porcellanite. 

 Instead of a multitude of superficial cracks there are 

 only a few, which divide the rock in small polygonal 

 pieces of rather peculiar appearance, such as are found 

 in Weston's Quarry, near Pontville station. If exposed 

 to a very strong heat porcellanite apparently loses its 

 waxy lustre, and becomes dull ; at the same time the 

 surface gets covered with a 'glassy coat. 



Porcellanite is, like hornstone, a metamorphosed 

 sedimentary rock, and at Pontville it can be distinctly 

 seen that it is an altered sandstone. Opposite the rail- 

 wa}'' station the western hills are formed of sandstone, 



