446 HE. S. Willbourn—Alteration of Granitic Rock. 
essentially interstitial. In the quartz-tourmaline veins which are 
described above the tourmaline is idiomorphic and the quartz is 
interstitial, but, as has been pointed out before, when one of the long 
prisms of tourmaline penetrates into the country rock, although the 
crystalline continuity is preserved, in that the crystal extinguishes as 
a whole, yet that portion of it in the quartz-mica rock now encloses 
the quartz grains, so behaving like the tourmaline in the quartz- 
tourmaline bodies. The arrangement of the bodies along parallel 
planes could be explained as a flow-structure, but none of the pheno- 
crysts show signs of similar orientation: so it seems to be reasonable 
to take it as further evidence pointing to the secondary nature of the 
bodies, and that where this arrangement along parallel planes exists 
it is due to former planes of weakness in the rock which afforded 
easy access to mineralizing vapours. The single grain of tourmaline 
observed in the coarser quartz-muscovite rock exposed on the beacon 
hill lay in a crack and appeared to have had a secondary origin. 
Conclusion. 
There is little doubt but that the rock is the result of the 
greisenization of a very fine-grained granitic rock, some of which has 
been less altered and can still be recognized as a granite porphyry. 
The rock, however, was not all porphyritic, and the greater part of 
it might have been an aplite. Perhaps the alteration of the felspar 
to tourmaline and the alteration to secondary mica took place in 
distinct operations. Some specimens of the felspathic rock appear to 
contain no secondary muscovite at all, yet a considerable quantity of 
the felspar has been altered to tourmaline. Again, as described 
above, some of the rock contains practically no tourmaline, and the 
felspar seems to be completely altered to muscovite. There is an 
indication that the process of tourmalinization was the later of the 
two. It has been noted that the quartz-tonrmaline bodies in the 
quartz-felspar rock are much less definite in outline than those in 
the matrix of quartz and muscovite. The only difference in the two 
rocks is that one contains much felspar and little muscovite, while 
the other contains much muscovite and little felspar. Presumably 
this difference was responsible for the difference in the quartz- 
tourmaline bodies, and so it must have existed before the tourmalini- 
zation occurred. 
The origin of the Ayer Kuning rocks and that of the Russian 
beresites seems to have been very similar. Beresite is a quartz 
porphyry in which the felspars have been altered to secondary mica 
and quartz. The Ayer Kuning rock is the result of a similar 
alteration of a very fine-grained granite porphyry which passes into 
an aplite without phenocrysts. ‘The Chindras rock contains little or 
no tourmaline, but in Ayer Kuning rocks the blue tourmaline is a 
characteristic feature. A point of difference between these rocks and 
beresite is that the latter is rather poor in quartz owing to the small 
amount in the original quartz porphyry. ‘The Ayer Kuning granite 
porphyry appears to have been of normal composition. 
