Vol. 70.] THE TOPAZ-BEARING ROCKS OF GUNONG BAKAU. 363 



17. The Topaz-bearing Rocks of Gunoxg Bakau (Federated 

 Malay Spates). By John Brooke Scrivenor, M.A., 

 F.G.S.. G-eologist to the Government of the Federated Malay 

 States. (Read April 8th, 1914.) 



[Plates LI & LIL] 



Contents. 



Page 

 I. Introduction 363 



II. Outline of the Geology of Gunong Bakau 365 



III. Description of the Granite and Associated Kocks . . . 367 



IV. TheFaults 373 



V. Origin of the Quartz-Topaz Vein-Rock 374 



VI. Magmatic Processes that may have led to the 



Formation of the Gunong Bakau Bocks 377 



VII. Summary 380 



I. Introduction. 



Igneous rocks with topaz as a constituent are well known, and 

 it is "unnecessary to give in detail the literature dealing with them. 

 The best known are those in which the topaz is associated with 

 cassiterite, and in such cases it is generally believed that the topaz 

 has been formed by the tin-bearing media acting on felspar, con- 

 verting part of the silicate of alumina into fluoride of alumina. 

 The object of this paper, however, is to describe some clear sections 

 in the Federated Malay States where two topaz-bearing rocks, 

 both carrvincr cassiterite, are shown to have been intruded into 

 porphyritic granite, and where the evidence is conclusive that both 

 topaz and cassiterite are not alteration - products of previously 

 formed minerals, but crystallized from the molten rock as topaz 

 and cassiterite. 



Gunong Bakau ('Gunong' is the Malay equivalent for mountain) 

 is a mountain 1120 feet high, situated in the centre of the Main 

 Range of the Peninsula, on the boundary between the two States, 

 Selangor and Pahang. The name will not be found on any pub- 

 lished map, but it will suffice to say that, in the sketch-map 

 accompanying my paper on the ' Geological History of the Malay 

 Peninsula V its position is near the source of the River Selangor, 

 at the boundary of the State of that name. 



The sketch-map accompanying the present paper (fig. 1, p. 361) 

 gives some idea of the surroundings of the mountain. On the west 

 are the sources of the Rivers Kajang, Bakau, and Luit ; on the east 

 is the source of the River Chinchong. The slopes of the mountain 

 are steep, sometimes as much as 15° ; but, until mining operations 



1 Q. J. G. S. vol. lxix (1913) pp. 343-71 & pi. xxxv. 



