119 



ON THE TOPAZ QUARTZ PORPHYEY OE STAN- 



NIFEEOUS ELVAN DYKES OF MOUNT BISCHOFF. 



ByW. H. Twelvetrees, F.G.S. and W. F. Petterd, O.M.Z.S. 



Read October 11, 1897. 



(PLATE.) 



This rock was first described by S. N. Wintle in 1875* as 

 eurite porphyry, which it had been termed by Professor G. 

 H. F. Ulrich, who, however, did not publish his description 

 until 1877f. In 1875 also the late Chas. Gould wrote as 

 follows : — 



" Mount Bischoff is a conical eminence rising to about 

 2,500ft. above the level of the sea. ... It consists of 

 a small protrusion of a porphyritic rock having a felsitic 

 base, with granules aud crystals of quartz and felspar ; it 

 weathers white, and is honeycombed or vesicular on the 

 surface, most probably from the decomposition and removal 

 of pyrites, which is freely disseminated throughout in places!." 



Profeesor G. von Eath, of Bonn, first determined the 

 existence of topaz in specimens sent to him by Professor 

 Ulrich. His description was published in 1879§. The rock 

 was further submitted to methodical investigation in 1884, 

 when A. von Groddeck microscopically examined specimens at 

 Clausthal, received from Tasmania. Von Groddeck's two 

 papers on the subject disclose a thorough treatment of the 

 material available*. He definitely negatived the idea of his 

 sample being quartz-porphyry at all, aud called it a por- 

 phyritic topaz rock. In 1892 Mr. H. W. Ferd. "Kayser in 

 his paper on Mount Bischoff, read before Section C, 

 Australasian Association for the Advancement of 

 Science at Hobart, 1892, refers to dykes of Eurite or 

 Quartz-porphyry and Topaz-porphyry, and in a subsequent 

 paper by Messrs. Kayser and Provis (1895-6)f , the dykes are 

 called Topaz-porphyry and Quartz-porphyry. Mr. A. W. 

 Clarke, in 1892, wrote a short note on the microscopical ap- 

 pearance of a specimen of this rock in the collection of Mr. 

 E. L. Jack, Government Geologist for Queensland, but while 



*" Stanniferous Deposits of Tasmania." Trans. Roy. Soc. N.S. Wales, vol. ix 



1875, p 87. 



t " New York Year Book of Mineralogy," 1877, p. 494. 



t " Q. Journal Geolog. Soc," vol. xxxi., 1875, p. 109. 



§ " Berichter der Niederrheinischen Gesellschaft," Bonn, 1S79. 



Proc. Roy. Soc. Tasmania, 1885 p. 38S, and 1886, p. 189. 

 t Proc. Inst. Civil Engineers, vol. cxxiii., 1895-6, p. 4. 



