120 



noticing " a radially arranged mineral highly colored between 

 crossed nicols," failed to recognise it as Topaz*. Finally, the 

 topaz and quartz-porphyries of Mount Bischoff are recorded 

 in the work mentioned at foot§. No systematic microscopical 

 examination seems to have been published since von 

 Groddeck's time. His material was plainly limited, and the 

 essential nature and derivation of the rock were not dealt 

 with in his painstaking inquiry. 



After renewed examination in the field, and with the aid 

 of an extensive series of microscopical slides, it may be per- 

 mitted to us to re-state the problem, and, as we believe, 

 advance a step in the process of its solution. 



That dykes of an acidic porphyritic rock traverse the 

 Palseozoic slates and sandstones at Mount Bischoff is well 

 known. This rock carries topaz both crystalline and 

 amorphous, and that mineral at Mount Bischoff appears 

 always to bo associated with Cassiterite. Professor Krause, 

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

 posing the summit of Mount Bischoff contaius 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 uncommon."* 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 specimens showing constituents 

 which have not succumbed to the obliterating processes of 

 tojDazisation. When sliced they reveal quartz, felspar, and 

 mica as porphyritic constituents. The felspar outlines are 

 mostly filled in with talc and radiating crystals of topaz 

 (pycnite). This explains the rarity of felspars in the altered 

 rock. Topaz crystals settle in the interior of a crystal of 

 felspar, replace its substance, and filially its outline is lost in 

 the ground-mass of the rock. In this way mauy phenocrysts 

 are now indeterminable. 



This topazisation is what Rosenbusch calls a pneumatolytic 

 phenomenon, viz., the development of topaz and tourmaline 

 rocks in veins proceeding from granite. Fluoric and boracic 

 acid vapours, given off at the time of intrusion aud consoli- 

 dation of the vein matter, are recognised as agents competent 

 to effect the observed results. These solfataric vapours 



t " Geology and Paleontology of Queensland." R. L. Jack and R. Etheridge, 

 jr., 1892, vol. 1, p. 729. 



§ Catalogue of the minerals sf Tasmania. W. F. Petterd, p. 90, 1896. 



* An introduction to the study of mineralogy. F. M. Krause, 1896, p. 220 



