IV.—The Origin of Topaz and Cassiterite at Gunong Bakau, Malaya

Topaz is commonly supposed to have been formed by the action of fluorine-bearing vapours on felspar, but evidence has recently been advanced with the object of showing that some important veins intrusive in the porphyritic granite of Gunong Bakau, a mountain 4,426 feet high, situated in the centre of the Main Granite Range of the Malay Peninsula, were formed of a rock in which “the topaz and cassiterite are not alteration products of previously formed minerals”. The author of this theory gives the rock the descriptive name of ‘quartz-topaz’, and adds as his reasons for not calling it ‘greisen’ the fact that in places it contains very little mica and that, unlike the majority of greisens, it is not an alteration product.