•70 
J. R. H. McWhae. 
quency in tin; granitic gneiss. Possibly some of these veins are genetically 
related to the Younger Granite. A small graphic granite outcrop occurs 
in the western band of metasediments in the southern part of the Area. This 
is a very coarse-grainedj white graidiic p(*gmatite (22575), composed of 
microcline { 80 ])er cent.) up to two inches in grain-size and quartz (20 per 
cent.), frtHpiently occurring at the etlge of mici'ocline crystals. Its genetic 
relationshij) is unknown. 
A very coarse-grained magnetite-microcline pegmatite (22522) occurs 
in the granitic gneiss and is thought to ha\o been derived from it in the end 
stages of crystallisatioji. iMhrocline (HO per cent.) grains arc one to two 
inches in size and frecpumtly have a graphic inte)'gir)wth of quartz witliln 
the grains. Irregular grapliic (juartz (8 per cent.), saussuritised plagioclase 
(0 per cent.) and magnetite (2 ])er cent.) make up the rest of the rock. 
The coarse pegmatites, aplograniles, and aplites intrusive into the 
younger granites are all com])Osed of quartz, saussuritised oligoclase, clear 
microcline (frequently microperthitic) and accessory epidote and green mica. 
The average conij)osition is microcline 40 ])er cent., oligot-last* ‘Jo per cent., 
quartz 25 ]‘)er (*ent. 
No rare ininerals have been found in any of the pegmatites. 
Origin of the lounger Grmiitcf ^. — All the younger granites have the 
same niinei'al assemblagt* which is almost identical witli the minerals of the 
granitic gneiss; there appear to be reli(‘ minerals (some oligoclase, horn- 
blende, and (|uartz) of the grajiitic gneiss in the Younger Granite, and the 
rounded xenolith.s in the porphyritic granite are very similar miuernlogically 
to the .veuoliths in the granitic gneiss, (the bluish green hoi-nbiende, one to 
two per cent, titanite and other minei-als being common to both). So the 
Younger Granites ai)]>('ar to be the restdt of the palingenesis of the g 7 *anitic 
gneiss and the xenoliths in the i)or])hyntic granite are relics of the xenoliths 
in the granitic gneiss. 
Th{‘ ^'idieuocrysts'' of microcline with inclusions of all the other minerals 
w<we evidently of very late crystallisation. They occur as isolated crystals 
(fig. 5.) similar to the rapakivi ovoids Avhieh deselop in the country rocks 
surrounding the rapakivi granite (25, )>. 7fi) and they have evidently grown 
from potash emanations from the granite. Granitization, rather than' crystal- 
lisation from a magma, is thought to hav(‘ f{)rmed these poikiloblasis. 
Tlu' reason for difference in texture between the iJorphyritic and tho 
uniform-grained granite is uot understood. 
The granite dyke and many of Ihe i)(‘ginatites are thought to be 
geuetically related to the Younger Granites. 
The retrograde nudamorjihism of siUimanite and eordierite of the 
nn'lasedimeiits to sericite and possibly the addition of silica in the forma- 
tion of the, cordierile-anthophvllite rock are thought to be the result of 
contact nietamorphism by the Younger Granite. 
2. Quarts Dolerites. 
p -Tliese arc uniformly hard, dark , grt'v, nielaiiocratie rocks,' holn- 
crystafliiie, -witli fine to medium grain and a uniform texture. 
