Vol. 70.] TOPAZ-BEARING ROCKS OF GUNONG BAKU". 381 



the magma attacked to boil up and consolidate in the granite as 

 an intrusive vein-rock. Segregation of the first-formed minerals 

 during cooling led to the irregular distribution of the constituent 

 minerals in the veins. 



I am indebted to Dr. H. H. Thomas, Sec.G.S., for assistance in 

 preparing the photomicrographs reproduced in PI. LII. 



EXPLANATION OF PLATES LI & LII. 



Plate LI. 



Pig. 1. A quartz-topaz vein in Quarry C : note the flat top. (See p. 368.) 



2. Section in Quarry C, showing the reaction -border of schorl-rock. 



Plate LII. 



^Microscope-sections of the Gunong Bakau rocks. The numerals refer 

 to the numbers of the slides in the Author's collection.] 



Pig. 1. Quartz-topaz rock. Gunong Bakau. Slide Xo. 2312. Ordinary 

 light. X 19 diams. The figure shows topaz with well-defined 

 cleavage, and quartz, also a few scattered grains of cassiterite. 



2. Quartz-topaz rock. Quarry C, Gunong Bakau. Slide Xo. 2331. 



Ordinary light. X 9 diams. 



3. Quartz-topaz rock with zinnwaldite. Quarry M, Gunong Bakau. 



Slide Xo. 2335. Ordinary light. X 13 diams. 



4. Border of quartz-topaz rock with zinnwaldite. Gunong Bakau. 



Slide Xo. 2404 b. Ordinary light. X 15 diams. 



5. Topaz-aplite. Gunong Bakau. Slide Xo. 2397. Ordinary light. 



x 13 diams. 



DlSCUSSIOX. 



Dr. J. W. Evaxs remarked that it was a matter for congratu- 

 lation that the head of the Geological Survey of the Federated 

 Malay States had, as a member of the British Survey, worked for 

 some years in Cornwall, and so was well acquainted with the granites 

 of that area and the pneumatolvtic deposits associated with them. 

 He was thus well equipped for dealing with the similar rocks 

 of the Malay Peninsula. Xo intrusive igneous rock, composed 

 mainly of quartz and primary topaz, appeared to have been pre- 

 viously recorded ; but the specimens shown, and the mode of 

 occurrence described by the Author, certainly made out a case that 

 demanded very careful consideration. 



The Author claimed that the rocks described by him formed the 

 summit of the granitic intrusion, and in that respect differed from 

 the granitic exposures of Devon and Cornwall, which he said had 

 been planed down. As a matter of fact, the officers of the Geo- 

 logical Survey had expressed the view that comparatively little 

 had been removed from the surface of the Dartmoor Granite, and 

 that opinion was widely held. 



The speaker thought that it was unwise to extend the term 

 reaction -border, which had a very definite meaning, to the 

 marginal phenomena described by the Author. 



Q~ J. G. S. No. 280. i 



\ y 2e 1 



V - V> **MI « «*** 



