66 
J. R. H. McWhae. 
Granitic gneiss which lias no hydrothermal minerals and contains 
greenish brown biotite, is considered to be a less altered form of A formed 
in the border zone of the cooling magma by more rapid cooling and greater 
frictional forces. 
A cataclastie form of the gi'anitic gneiss, greenish in colour, with 
strained and cracked plagioclase grains, undulose extinction in the quartz, 
and with much introduced epidote, is freiinently found near epidote-filled 
veinlets genetically related to the muidi younger dolerite dykes. 
The great bulk of the xenoliths are amphibolites Avhich suggest, by their 
chemical and mineralogical composition (23, p. 105) that they are derived 
from the hornblende schists with little or no addition of material from the 
gneisses. The presmice of quartz in these basic rocks is probalily due to 
the conversion of pyroxenes to hornblende liberating silica (13, p. 311), The 
high refractive iiulices of the hornblende ( jS = 1.672 and /S —1.675) is in- 
dicative of recrystallisalion of a basic ig’iieous rock in the sillimanite zone 
(30, p. 107). According to Harker (13, p. 281), at the highest gi’ade of 
regional metamorphisin of basic igneous rocks, in bands “richer in lime, the 
place of honiblende is partly or wholly taken by colourless diopside” and 
the felspar is commonly a medium andesine. These features were noted 
in the petrological examination of the amphibolites. 
The hybridised granitic gneiss (iv) is considered to be the result of 
partial assimilation and granitization of these xenoliths by the granitic 
gneiss magma, and small scale lit-par-lit injection of acid material into the 
xenoliths from the magma. 
The basic epidote chlorite xenolith (ii) is considered to be a xenolith 
of basic igneous rock similar to the hornblende schists xenoliths in the gneiss 
at Toodyay except that it has been extensively ehloritized. 
The sillimanite-mica schist xenolith (iii) is a fragment from the mica 
schists of the met-asediments. 
n. THE YOUXGER IGXEOUS IXTRUSIVEb. 
1. Younger Granites. 
Porphyritic granite occurs in tho south-east corner of the area and in 
a small intrusion into the granitic gneiss, about an acre in area, near the 
eastern metasediments. In the south-central portion of the area a uniform- 
grained gi*anite takes the place of the porphyritic type. About 30 chains- 
north of this a small dyke of gneissie granite outcrops. 
(a) Porphyritic granite arid xenoliths. 
A characteristic feature of this rock wherever seen in the field, is the 
presence of xenoliths up to four feet in diameter of finei’-grained slightly 
darker-coloured material (text fig. 5). 
The granite itself is a coarse-grained, porphyritic rock light grey in 
colour with frequent small, rounded gi’ey xenoliths up to four feet in 
diametei*. The “phenocrysts^^ are euhedral microclines averaging an inch 
in length which are dotted with random orientation throughout the granite 
and also in the xenoliths (text fig. 5). The groundmass is composed of 
