394. 
Quartz-Porphyries at Mt. Bischoff must be considered the 
“ carriers” of tin ores. 
The specimen, I have been presented with, and which I 
have described as closely resembling Quartz-Porphyry, had 
therefore to be recognised as representing the occurrence of 
tin ore at that mountain, and therefore as occupying a very 
important position. 
If, however, it has now been unmistakably proved that that 
specimen is actually not Quartz-Porphyry, but, a porphyritic 
Topaz Rock, then the question arises, whether any such Quartz- 
Porphyry .occurs at. Mt. Bischoff at all, and whether the 
whole formation, assumed to be such rock, is very probably a 
Topaz Rock, and in what manner, and under what conditions 
the latter occurs contiguously to the real Quartz-Porphyry. 
The Topaz Rock, as described, is certainly a very remarkable 
formation in its character, and it is to my knowledge 
exceptional in its kind. I have named it Topaz Rock, because 
of its composition—like the well-known so-called Topaz 
Rock, near Auerbach, in Saxony—of Quartz and Topaz, 
though there it exhibits not the slightest resemblance in its 
mode of occurrence to that of Mt. Bischoff. 
A probably not unimportant fact, which tends to connect 
both of these occurrences, otherwise so widely apart, should 
not, at this stage, be omitted to be mentioned. M. Breith- 
aujet,* it should be borne in mind, has already proved that 
the Topaz Rock of Auerbach forms, at the surface, a huge 
well-like mass, which in itself contains or forms an immense 
lode of Tin Ore. 
Topaz, it is well known, is a very frequently and widely 
disseminated mineral in close conjunction with Tin Ore 
deposits; but, to our knowledge, such an extraordinarily 
developed formation as that at Mt. Bischoff in Tasmania is 
not known to exist elsewhere. The occurrence of Tourmaline 
there is also very different from any other known im con- 
- nection with Tin Ore deposits. These two minerals—viz., 
Topaz and Tourmaline (which are, however, not even men- 
tioned in Messrs. Wintle and Ulrich’s descriptions)—in their 
dense forms appear to play a very important and prominent 
part in the celebrated Tasmanian Tin Ore deposit. It might 
prove, in all probability, very interesting and valuable to 
ascertain whether or not such dense—and therefore easily 
passed over-—masses of Tourmaline occur also with other Tin 
Ore deposits. 
A. Von GRoDDECK. 
* New Year Book for Mineralogy, 1885, page 788. 
