190 Correspondence — J. B. Scrivenor. 



COBRESPOlsTDENCE. 



TOPAZ AS A ROCK CONSTITUENT. 



Sir, — When a paper on the Gunong Bakau topaz and cassiterite 1 

 appeared in this Magazine for 1916, it was sufficient for the time 

 being to ask you to publish a report 2 to which the author referred in 

 order to show that before I worked out the structure of Gunong 

 Bakau 3 he held the view that the quartz-topaz rock was " topazised 

 granite", and attempted to explain the horizontality of one of the 

 ore-bodies by faults, thrusts, and a landslip, although in his paper 

 he writes of the "important veins intrusive in the porphyritic 

 granite ", and argues that because certain topaz-bearing rocks in 

 Germany and elsewhere are considered to be altered granitic rocks, 

 the same origin should be accepted for the Gunong Bakau quartz- 

 topaz rock. I do not propose to repeat the evidence on which my 

 opinion that the topaz is a primary mineral was based, but there are 

 two points of general interest that might be mentioned in connexion 

 with topaz as a rock constituent. 



On pp. 300 and 301 of The Natural History of Igneous Hocks 

 Dr. A. Harker writes: "Closely bound up with the greisens are 

 the tinstone veins, the cassiterite probably resulting from reaction 

 between the volatile tin fluoride (SnF 4 ) and water. The destructive 

 action of fluorides is exceedingly energetic. At Geyer, in Saxony, 

 granite is locally converted to a rock containing more than 90 per 

 cent of topaz," and quotes as his authority regarding the Geyer 

 rock Salomon & His' paper in the Zeit. deutsch. geol. Gesellschaft, 

 vol. xl, pp. 570-4, 1888. Dr. Jones follows Dr. Harker in making 

 a similar reference to these authors; but the fact remains that 

 whatever may be the truth about the origin of the topaz they 

 described, Salomon & His did not write anything in that paper 

 that justifies their being quoted as authorities for its formation by 

 the destructive action of fluorides. On the contrary, Salomon and 

 His made it clear that they considered the topaz in the greisen to be 

 the primary topaz that occurs in the granite. They mentioned 

 topaz as being widely distributed as a constituent of the granite 

 stocks, although it seldom becomes a prominent constituent. They 

 said that one must expect the topaz, so characteristic of the granite, 

 in the greisen as well, and on pp. 573 and 574 they described 

 aggregates of topaz with a little felspar and mica which become 

 converted by decomposition into aggregates of 90 per cent topaz 

 with a little kaolin and ferrite. According to these authors the 

 topaz was not formed by pneumatolysis. Never having seen the 

 Geyer or indeed any German greisens in the field, I am not in a 

 position to say whether Salomon & His were correct in their view 

 or the reverse. 



1 Dr. W. R. Jones, "The Origin of Topaz and Cassiterite in Malaya": 

 Geol. Mag., 1916, pp. 255-60. 



2 Loc. cit., pp. 453-6. 



3 Quart. Journ. Geol. Soc, vol. lxx, pp. 363-81, 1914. 



