Dr. W. R. Jones—Topaz and Cassiterite in Malaya. 259 
felspar”’.! This magma was then invaded by ‘‘a volume of gas. 
as a huge bubble rising from below, where st had been collected 
in a part of the magma on which it could not react”, and attacked 
‘‘eroups of molecules that would have solidified as felspar if they 
had not been disturbed by hydrofluoric acid’’? (italics inserted, 
W.R.J.). The writer will not enter here into the question of the 
miscibility of silicate minerals in a magma which was sufficiently 
mobile to be intruded through long narrow veins, some less than an 
inch thick, nor on the possibility, or otherwise, of the accumulation 
of a huge bubble of vapour in the depths of an igneous mass, but 
will point out that no explanation is given of how a highly fluori- 
ferous vapour can collect in a part of a magma on which it could not 
react. Moreover, a salic magma depends for its fluidity, as Bowen 
states,> ‘‘on its ability to retain volatile constituents,” and the 
presence in such a magma of a free oxide, such as cassiterite, does 
not appear to be possible.* Especially is this the case in a magma 
which later emanated hydrofluoric acid. 
In a footnote to p. 379 of his paper on these rocks the author 
has raised a question of great interest. He points out that ‘‘ without 
segregation one could not expect to have a rock very rich in topaz’’, 
and gives figures to show that with a pure orthoclase magma only 
32°6 per cent of topaz could be formed. 
There is, however, an explanation why the percentage of topaz in 
a greisen does not necessarily depend on the percentage of felspar 
in the original rock. Researches on greisenization have shown that 
where it has been possible to obtain analyses of greisens and their 
neighbouring granites, some of the chemical changes involved appear 
to be as follows :— 
According to Addition of Abstraction of 
Vogt.’ Si Oo, Be Os, F (or HF), Sn On, ; CaO, MgO, NagO, and often 
often also LigO, K2O, and also K20O, Ale Os. 
perhaps Al O3. 
Dalmer.® FeO, F, Sn Oz, and possibly | K,0, Nag O, Si Ox. 
Ale Os. i 
Cotton.” Si O02, FeO, MgO, Sn Oz, — 
Mo Se, Als Os. 
Jones, W. BR. | Ale Oz, Sn Oo, Be Os, HF, | KeO, Nag O. 
: Lig O, and possibly Si Oe. 
? J. B. Serivenor, ‘‘ The Topaz-bearing Rocks of Gunong Bakau’’: Q.J.G.S., 
vol. lxx, p. 379. 
2 J. B. Scrivenor, Mining Magazine, February, 1916, ‘‘ Discussion.” 
° N. L. Bowen, “‘ The Later Stages of the Evolution of the Igneous Rock’? : 
Journal of Geology, supplement to vol. xxiii, No. 8, p. 16, 1915. 
* A. Harker, The Natural History of Igneous Rocks (London, 1909), p. 166. 
° J. H. L. Vogt, ‘‘ Beitrige zur genetische Classification der durch magma- 
tische Differentiationprocesse und der durch Pneumatolyse instanden Erzyo- 
kommen’: Zeit. prak. Geol., 1895, p. 146. 
* K. Dalmer, “‘ Der Alterberger-Graupener Zinnerslagerstittendistrikt ’” : 
Zeit. prak. Geol., 1894, p. 319. 
7L. A. Cotton, ‘‘ Metasomatic Processes in a Fissure Vein from New 
England’’: Proc. Linn. Soc. New South Wales, p. 231. 
