examination of the several features observable in the respective 
localities. To initiate and induce such examinations the fol- 
lowing may deserve attention :— 
389 
1. Porpuyritic Topaz Rock rrom Motunt BiscHorr. 
This resembles, to a considerable degree, the Quartz-Por- 
phyries, whether such is examined by the naked or unarmed 
eye or by means of powerful lenses. It consists of a light 
grey-coloured, dense, hornstone-like base, in which numerous 
transparent Quartz-crystals can be observed up to 3 mm. in 
size. One imagines, likewise, of heing able to distinguish, in 
the white coloured cross fractures of this rock, crystals of 
felspar. It carries also Iron Pyrites in both large and small 
crystal or in crystalline aggregations. 
A preliminary chemical examination showed that, after the 
removal of the metalliferous ores, the pulverised rock would 
completely fuse with soda, but that hydro-chloric acid had 
no effect except the deposition of a white precipitate, composed 
of Silicious Acid and Alumina. Distinct Fluor reactions were 
observed with traces of Lime and Magnesia, but alkalis were 
not present. 
The special chemical analysis of this sample of rock, freed 
from iron pyrites through nitric acid, as carried out by Dr. 
Sommerlad in the Royal Academy’s Laboratories, gave the 
following results :-— 
Silicious Acid oe Be a: he 76°68 
Lime ve ae ae ii a 4 tpi) 
Fluor wae 5; i pe A 6°48 
Alumina... spe a te: Hae 19°99 
Magnesia... si ae ad ey 42 PAGES 
Phosphoric Acid... ip a rife € 2 EAGER 
Total ... ae 10434 
Specific Gravity 3014 
Two separate tests were made, according to Rose’s method, 
for ascertaining the percentage of Fluor. The high percentage 
of Fluor, and the total absence of alkalis, are therefore con- 
vincing proofs of this rock not belonging to the Quartz- 
Porphyries, and that it can now be stated that it consists of, 
in round numbers, 35 per cent. of Topaz and 65 per cent. of 
Quartz. To what form the 1:19 per cent. of Lime in this 
rock can be assigned is not quite clear; but it is quite pos- 
sible that some very minute crystals of “ Titanite’’—to be 
described lower down—contain this Lime. In the same 
manner, it is doubtful also how the Phosphoric Acid occurs, 
except with “ Apatite.’ The solution which was obtained by 
treating the ores with Nitric Acid in order to secure their 
extraction was found to contain a considerable percentage of 
G 
