Vol. 70.] TOPAZ-BEARIXG ROCKS OF GTONONG BAIKAL". 377 



name greisen is inapplicable. The topaz, which is far more 

 abundant than, and differs in habit from, the topaz in any greisen 

 that I have examined, suggests the name ' topazfels,' but here again 

 the name is associated with the alteration of a pre-existing rock. 

 The best-known example of such an alteration is the altered quartz- 

 porphyry of Mount Bischoif, in Tasmania ; and, after reading 

 W. von Firck's paper, 1 and comparing the plates of microphoto- 

 graphs accompanying the paper with the quartz-topaz rock, I can 

 say that the latter is a very different thing.' 2 For example. I have 

 not seen in any of the quartz-topaz veins even a suggestion of a 

 pseudomorph of topaz after felspar. 3 The only reference that I 

 can find to a rock like the quartz-topaz rock is in Bosenbusch s 

 ' Mikroskopische Physiographic der Massigen Gesteine ' 3rd ed. 

 (1896) p. 84, where, in describing the Altenberg ' Zwitter,' he 

 says : — 



' In diesem normalen Zwitter tritt in geringer Yerbreitung eine grane 

 Varietat oline griinen Glimmer auf. Sie besteht aus 71.36 " Quarz, 

 27.21 % Topaz, xind 1.43 % Zinnstein.' 4 



The rock in Quarry M and the mica-patches elsewhere might 

 then be called greisen : but, as the name is not applicable to the 

 greater number of the veins, and as its use in the case of Quarry M 

 might give rise to a misconception regarding the origin of the rock, 

 I name the vein-rock generally by its two constant constituents. 

 In the case of the extraordinary tourmaline-corundum rocks of 

 Kinta, I resisted the temptation to inflict a new name on petro- 

 logy, and have less difficulty in doing so now. Descriptive names 

 embodying the main constituents, although sometimes clumsy, are 

 greatly preferable to names based on localities which convey no 

 further information about the rock ; and, as regards the G-unong 

 Bakau rocks, the main point is to avoid giving a false impression 

 of their mode of origin by the misuse of existing terms. 



There is no question about the topaz-aplite having consolidated 

 direct from the molten material. 



VI. Magaiatic Processes that may have led to the 

 Formation of the G-uxoxg Bakau Bocks. 



Xot the least interesting problem raised by the study of these 

 rocks concerns the processes in the granitic magma to which their 



1 ' Die Zinnerzlagerstiitten des Mount Bisckoff in Tasmanien ' Zeitschr. 

 Deutsch. Geol. GeseUsch. vol. li (1899) pp. 431-64. 



- I am indebted to Mr. W. H. Twelvetrees, Government Geologist of Tas- 

 mania, for information regarding this rock. 



3 See op. cit. pi. xxvii, fig. 2. 



4 Zirkel mentions the same rock, ' Petrographie ' 2nd ed. (1894) p. 124; 

 Prof. A. Lacroix describes a granulite in the Haute- Vienne with quartz, 

 lepidobte, albite, and topaz (' Mineralogie de la France & de ses Colonies ' 

 pt. i, pp. 62-64). A granulite with cassiterite, but no topaz, is described by 

 Dr. L. L. Fermor (Eec. Geol. Surv. India, vol. xxxiii, 1906, pp. 235, 236). 

 The double Lise of the word granulite in petrology would make its employ- 

 ment for the Gunong Bakau rocks objectionable, even if the quartz-topaz rock 

 contained felspar. 



