BY FRITZ NOETLING, M.A., PH.D., ETC. 97 



most crude and unsatisfactory one. If determined in 

 this way, all the rocks and minerals here enumerated 

 have exactly the same hardness, namely, that of quartz ; 

 that is to say about seven, though this would probably 

 represent the maximum. The hardness of the rocks 

 here mentioned ranges between 6 and 7, though most 

 will be about 6.5 tO' 6.75, but it would be absolutely im- 

 possible to distinguish the finer grades, which are cer- 

 tain to exist. If determination of hardness were made,, 

 it should be carried out according to the more im- 

 proved methods suggested by Rossival, but this is a 

 very tedious operation, which requires a lot of me- 

 chanical appHances not at my disposal. It is therefore 

 unnecessary to make a special reference to the hardness,. 

 because it. must be a matter of course that rocks con- 

 sisting mostly of silica must have a hardness closely 

 approaching to that of quartz. I fail to understand why 

 under these circumstances Herr Klaatsch could have 

 stated that the rocks used in the manufacture of tro- 

 nattas were rather soft. This is by no means the case 

 — in fact some of the grey hornstones appear to be 

 harder than flint, the ordinary material of the European 

 rocks. Herr Klaatsch's statement is one of those super- 

 ficial observations by which this author has gained rather 

 a notoriety, and in this particular case his erroneous 

 opinion is either due to insufficient mineralogical know- 

 ledge or insufficient material, or both. 



When we compare the specific gravity of the rocks 

 here mentioned we obtain some rather interesting re- 

 sults. Taken as a whole we have : — 



1. Hornstone. — Range, 2.500 — 2.847; difTerencCj^ 

 0.347 ; average, 2.687. 



2. Porcellanite. — Range, 2.308 — 2.700; difTerence, 

 0.392 ; average, 2.498. 



3. Breccia. — Range, 2.540 — 2.782; difference, 0.242; 

 average, 2.636. 



4. Others. — Range, 1.940 — 2.680; difference, 0.740; 

 average, 2.472. 



It will be seen that the breccia shows the smallest 

 difference between extremes, while that of the fourth 

 group has, as might be expected, the largest. Horn- 

 stone and porcellanite show a fairly wide range, which 



