LE. 8. Willbourn—Alteration of Granitic Rock. 445 
As mentioned above, the quartz-muscovite rock of Chindras is 
traversed by gold-bearing quartz veins, and this is another point of 
resemblance with beresite. The Malay headman at Ayer Kuning 
informed the writer that his father had got a small quantity of gold 
many years ago by washing the soil of the Ayer Kuning hill range, 
but not sufficient to encourage him to go on with the work. 
The elvans of Cornwall resemble these rocks to a certain extent, 
but differ in that they frequently contain brown primary tourmaline 
and also they are coarser in grain. Regarding the alteration of 
elvans, Dr. Flett' says: ‘‘ abundance of fine muscovite in the ground 
mass of an elvan is probably a certain indication of pneumatolytic 
action, as muscovite is very rarely a primary ingredient of these 
rocks.” The formation of greisen in the elvans of Cornwall has 
often been accompanied by the formation of tinstone. The nearest 
locality to Ayer Kuning in which cassiterite is found in any quantity 
is at the contact of granite with sandy shales, four or five miles to 
the west. 
The Ayer Kuning and Chindras rocks differ from ordinary greisens 
in two respects. Greisens are rocks consisting of quartz and white 
mica, usually with tourmaline and topaz as accessories. The Ayer 
Kuning rock contains no topaz, and the Chindras rock in addition 
contains no tourmaline. Also the Ayer Kuning rock differs from 
other described greisens in its very fine grain. The felspathic 
portions of it may be described as a yery fine-grained granite 
porphyry.” 
The Grainsgill greisen* is another rock formed probably in a 
similar way by the alteration of felspar to white mica, and it also is 
like the Ayer Kuning rocks in that it is deficient in topaz and tin- 
stone, but the latter differ in their very much finer grain. 
It seems clear from the specimens examined that all of the Ayer 
Kuning and Chindras rock was felspathic in the first place, and that 
in the greater part of it the felspar has subsequently been altered to 
white mica. This is suggested by the similar texture of the fel- 
spathic and non-felspathic rocks, although there is one difference, 
namely, the lack of phenocrysts in the fine-grained quartz-muscovite 
rock. However, the coarser quartz-muscovite rock of the beacon hill 
and of Bukit Chindras contains phenocrysts. The field evidence 
leaves no doubt that the two rocks are part of the same intrusive 
mass, for specimens of both the felspathic rock and the quartz- 
muscovite rock were collected from the same exposure. 
There is good reason to believe that the quartz-tourmaline bodies 
are secondary structures, and not products of consolidation. Their 
grain is uniformly like that of the surrounding white rock, and the 
tourmaline, which is always the blue variety, is in the same relation 
to the quartz as is the secondary mica-quartz aggregate in that it is 
} Mem. Geol. Survey, Land’s End District, p. 65, 1907. 
* As the ground-mass is not eryptocrystalline but a fine-textured crystalline 
aggregate of quartz and felspar, the best term is either granite porphyry or 
microgranite, and granite porphyry is the better of these, because of the pheno- 
erysts. 
Onde Giss, vols lit py 142) 1895: 
