258 Dr. W. R. Jones—Topaz and Cassiterite in Malaya. 
a little fluorite; (2) the ‘quartz-topaz’ rock carrying tin-ore and 
(3) the ‘topaz-aplite’ at Messrs. Bibby & Ruxton’s mine; (4) 
pegmatite veins, composed mainly of milky quartz, in mines near by. 
The differences in the gangue of these lodes do not appear to be 
more striking than in numerous other areas in Malaya and in other 
tin-fields, and not even so marked as in some. It might even be 
said that variations equally great have been observed elsewhere in 
different parts of even the same lode. ‘The Luxer! vein at Graupen 
in the Erzgebirge tin-field, for example, is characterized by extremely 
variable filling. The prevailing gangue is milk-white quartz which 
locally gives place to coarsely crystallized orthoclase intergrown with 
albite, and to fluorite; also a dark-green lithia mica and a compact 
variety of kaolinite (‘steinmark’) occur as subordinate gangue 
minerals ; and the cassiterite is usually evenly distributed, as in the 
Gunong Bakau veins. At Zinnwald, in the same tin-field, sulphides 
are confined solely to the middle of the veins, and the tin-lodes contain 
as essential constituents quartz and a greenish or brownish-green 
mica, whereas in the neighbouring area of Altenberg topaz is the 
predominant constituent. In this tin-field at Sadisdorf a lode?” 
formerly worked for tin-ore is now worked solely for wolfram and 
molybdenite. Near the footwall occur large nests of pure wolfram, 
which together with the enclosing quartz are cut by secondary 
horizontal stringers of quartz carrying tin-ore and gilberite. These 
stringers are again cut by still younger vertical fluorspar stringers, 
and towards the hanging wall a great deal of molybdenite and also 
a little zinnwaldite are encountered, and bismuth and bismuthine 
also occur along both walls. . 
All the types of rocks found at Gunong Bakau in Malaya are also 
found in some other tin-fields, notably the Erzgebirge tin-fields, 
where ‘‘all stages of transformation from granite to greisen are 
encountered and frequently the greisen areas include unaltered 
masses of granite’’.* At Geyer the granite is locally converted to 
a rock containing more than 90 per cent of topaz.‘ 
It is stated by Mr. Scrivenor ‘‘ that the sequence of events in the 
mass forming Gunong Bakau is clear. First the porphyritic granite 
consolidated; then the quartz-topaz veins were intruded; and then 
the topaz-aplite arrived’’.> If the topaz in the quartz-topaz veins 
is a primary mineral the differentiation of the original magma must 
have taken place under extraordinary conditions, for a rock, free 
from felspar, is supposed to have been intruded after the first, 
containing porphyritic crystals of felspar, and before the third, which 
is described as being ‘‘rich in felspar’”’. The absence of felspar in 
the second intruded rock is explained by supposing that ‘‘in the 
depths of the igneous mass there was a magma which, if undisturbed, 
would have crystallized out as a rock composed chiefly of potash 
1 J. T. Singewald, jun., ‘‘ The Erzgebirge Tin Deposits ’’: Hconomic Geology, 
vol. v, pp. 176-7, 1910. 
2 Thid., p. 169. 
3 Ibid., p. 174. 
4 Solomon & His, Zeit. deutsch. geol. Ges., vol. xl, p. 250, 1888. 
5 J.B. Serivenor, ‘‘ The Topaz-bearing Rocks of Gunong Bakau’’: Q.J.G.S., 
vol. lxx, p. 378. 
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