256 Dr. W. R. Jones—Topaz and Cassiterite 1n Malaya. 
first time, to their importance as a source of tin-ore. The object 
of the present paper is to adduce evidence in support of the generally 
accepted theory for the origin of this topaz and cassiterite, and 
against that of their primary origin. It is not suggested that topaz 
and cassiterite never occur as primary minerals, for cases are known 
where the evidence appears conclusive that they can so occur on 
a small scale, but for the present attention will be confined to this 
particular locality, where the occurrence is on a large scale and of 
economic importance. 
The veins of ‘ quartz-topaz’ vary in thickness from about 15 feet 
to less than an inch, and in some places have now been extensively 
worked for tin-ore, notably on the mines of Messrs. Bibby and 
Ruxton and adjoining mines. 
The difficulties against accepting the ordinary theory for the | 
genesis of the topaz in these veins are clearly set forth by the author 
of the paper referred to, and may be summarized as follows: the 
absence of alteration in the country rock ; the dissimilarity between 
the vein rock and rock in the same locality that is clearly an alteration 
rock ; the presence in some of the rock of an iron-rich zinnwaldite 
which is different from the neighbouring secondary mica; the marked 
difference between the tin-ore bodies known to be of secondary origin 
and the bodies of ore in these veins. 
The absence of alteration in the country rock in the locality under 
consideration does not appear to be more pronounced than in other 
tin-fields where topaz and cassiterite have clearly been shown to 
be of secondary origin. The dark borders to these veins are stated 
to be ‘‘the result of reaction between media that came off from the 
vein rock and the porphyritic granite, and a portion of the original 
vein rock’’,! and to contain a little topaz and an abundance of 
tourmaline. Also the granite country rock ‘‘is altered for a few 
inches by emanations from the vein rock to form familiar pneumato- 
lytic modifications’’.? This alteration is certainly as extensive as 
is the case in places at Geyer and Khrenfriedersdorf, areas in the 
Erzgebirge tin-field, for there the alteration of the country rock 
is for a distance of only 2 to 6 inches,* whereas in the Graupen 
area the country rock of the Luxer cassiterite-bearing vein has not 
been altered at all into greisen.t That the topaz and cassiterite 
of the Erzgebirge tin-field is of secondary origin has been very con- 
clusively established, for the mine exposures in Altenberg Zwitterstock 
show ‘‘that the impregnation has been confined to the upper portion 
of the granite”’, and that ‘‘at a depth of 700 feet normal granite is 
encountered, in which zwitter bands are completely lacking or only 
sparingly present ”’.® 
The writer’s observations of various tin-lodes have led him to the 
conclusion that the amount of alteration of the country rock is 
1 J. B. Scrivenor, op. cit-, p. 370. 
2 Tbid., p. 375. 
3 J. T. Singewald, jun., ‘‘ The Erzgebirge Tin Deposits ’’ : Hconomic Geology, 
vol. v, p. 267, 1910. 
4 Tbid., p. 177. 
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