98 ROCKS USED IN THE MANUFACTURE OF TRONATTAS. 



seems to indicate a rather varying composition of 

 tlie original rock from Which they are derived. Horn- 

 stone is the heaviest of all, and it is closely followed by 

 the breccia, while there is a considerable difference be- 

 tween these two and porcellanite, as will be seen from 

 the following figures : — 



Hornstone, 2.687; 



Difference, 0.051, 

 Breccia, 2.636 ; 



Difference, 0.138. 

 Porcellanite, 2.498 ; 



Difference, 0.022. 

 Others, 2.472. 



According to these figures there is less difference 

 in the specific gravity of the more pure siliceous mine- 

 rals and the porcellanite than between the porcellanite 

 on the one side and the breccia and hornstone on the 

 other. If a conclusion can be drawn from this, it seems 

 to indicate that the porcellanites are a fairly pure sili- 

 ceous rocks with a comparatively small admixture of 

 other substances, while in the breccia, but particularly 

 the hornstone, the percentage of non-siliceous matter 

 must be fairly high. 



If we compare these rocks to other substances, the 

 following table of specific gravity will be of interest: — 



Obsidian buttons, average 2.388, maximum 2.500, 

 minimum 2.312. 



Fragment of white glass, 2.424. 

 Fragment of green bottle glass, 2.674. 

 Flint Salzinnes (Belgium), 2.565. 

 Flint St. Symphorien (Belgium), 2.535. 



The heavy weight of the green bottle glass is most 

 conspicuous, and, as we know that this is chiefly due to 

 ■ the high percentage of iron, the conclusion I have drawn 

 with reference to the hornstone and breccia is well sup- 

 ported. 



Now, if we take the specific gravity of the cretaceous 

 flint to be 2.500 — 2.600, we can arrange the rocks here 

 described in a verv illustrative table. Tliev are : — 



