BY W. F. PETTERD, C.M.Z.S. 185 



A detailed paper upon the subject, entitled " On the 

 Topaz-Quartz-Porphyry or Stanniferous Elvan Dykes of 

 Mt. Bischoff," by Mr. W. H. Twelvetrees and the writer, 

 has appeared in the Pro. Roy. S"oc. Tas., 1897. from 

 which the following is taken : — 



'' That dykes of an acidic porphyritic rock traverse the 

 Palaeozoic slates and sandstones at Mt. Bischoff is welJ 

 known. This rock carries top«.z both crystalline a^d 

 amorphous, and that mineral at Mt. Bischofl appears 

 always to be associated with cassiterite. Professor Krause, 

 alluding to these dykes, says : ' The white porphyry com- 

 posing the summit of Mt. Bischoff contains in a felsitic 

 base crystals of quartz and an abundance of fine-grained 

 amorphous topaz, with here and there a cavity lined 

 with groups of radiating acicular crystals of topaz. 

 Pseudomorphs of topaz after quartz are also not uncom- 

 mon.' This, perhaps, is the latest description of the rock 

 in question, but it applies to only one variety of a very, 

 variable rock. In prosecuting our investigations our aim 

 has been to obtain samples as little altered as possible, in 

 the hope of being able to detect the presence of minerals 

 of the parent rock. We have succeeded in finding speci- 

 mens showing constituents which have not succumbed to 

 the obliterating process of topazisation. When sliced 

 they reveal quartz felspai and mica as porphyritic con- 

 stituents. The felspar outlines are mostly filled in with 

 talc and radiating crystals of topaz (pycnite). This 

 explains the rarity of felspar in the altered rock. Topaz 

 crystals settle in the interior of a crystal of felspar, replace 

 its substance, and finally its outline is lost in the ground- 

 mass of the rock. In this way many phenocrysts are now 

 indeterminable. This topazisation is what Rosenbusch 

 calls a pneumatolytic phenomenon, viz., the development 

 of topaz and tourmaline in rocks proceeding from granite. 

 Fluoric and boracic vapours, given off at the time of 

 intrusion and consolidation of the vein-matter, are recog- 

 nised as agents competent to effect the observed results. 

 These solfataric vapours under hydroplutonic conditions 

 act upon a magma protruded from a deep-seated rock- 

 mass containing the elements of a granite. The protruded 

 rock-vein thus becomes topazised and tourmalinised. It is 

 hardly possible to separate physically the moments of 

 topazisation and final consolidation, for we must conceive 

 of this process being at work while the veinmass was, as 

 a whole, still viscous. The phenocrysts of felspar were 

 probably attacked and digested during their passage from 



