374 ME. J. B. SCEIVENOE OjS" THE [Dec. I914, 



10 feet. Generally the throws are about 1 to 3 feet. In Quarry 

 H there are some well-marked slickensides, but the effect of the 

 faults on the ore-bodies worked there was not appreciable. In 

 Quarry C there is some evidence of movement along the top of 

 the quartz-topaz vein, between the schorl-rock and the unaltered 

 granite. 



Faults such as these are common throughout the granite-masses 

 of the Malay Peninsula. They probably mark the final adjust- 

 ments of the cooled magma, and perhaps also faults due to earth- 

 movements during the Tertiary Period. In one place, about 15 

 miles from Grunong Bakau, I have seen evidence of a big fault ; 

 but, on the whole, the faults cutting the granite and associated 

 rocks are of no structural importance. This is certainly the 

 case on Grunong Bakau. 



V. Oeigijst of the Quaetz-Topaz Yeii\ t -Rock. 



The foregoing description of the rocks exposed on G-unong 

 Bakau, although condensed, will, it is hoped, suffice to show that 

 we are dealing with a very interesting piece of petrography. That 

 the quartz-topaz vein-rock consolidated from a molten state, and 

 was not formed by the alteration in situ of a pre-existing rock, 

 is in my opinion beyond doubt ; but, before we discuss the possible 

 changes in the granite magma that led to its intrusion into the 

 porphyritic granite, it will be as well to adduce in detail the 

 evidence on which this opinion is based. One reason why this is 

 advisable is, that any one who had seen a hand-specimen only of 

 the rock, would, in view of the literature on rocks with topaz and 

 cassiterite, expect to find that it had been formed b}^ the alteration 

 of another rock ; and, in fact, in a previous publication I gave 

 expression to such an expectation myself, fortunately with a certain 

 amount of reserve. 1 



The study of the quartz-topaz veins has been particularly in- 

 teresting to me, because some years ago I had an opportunity of 

 studying in equal detail a t}^pical case of the formation of greisen 

 with topaz in Cornwall, 3 and the points of difference between the 

 two rocks have proved to be very instructive. 



The clearest way of giving the evidence which militates against 

 the quartz-topaz rock being an alteration-product such as the 

 Cligga-Head greisen, is to employ the device of redact io ad 

 ahsurdum, and to assume for the sake of argument that the veins 

 were formed in the same manner. "We are then confronted by the 

 following difficulties : — 



(1) In typical greisen-veins there is the clearest evidence of 

 alteration extending from a fissure and gradually dying away at a 

 distance from the fissure. In the quartz-topaz veins no such 



1 ' The Geology & Mining- Industries of Ulu Pahang ' Kuala Lumpur, 1911, 

 p. 24. 



2 ' The Granite & Greisen of Cligga Head (Western Cornwall) ' Q. J. G. S. 

 vol. lix (1903) pp. 142 59. 



