34 
Hex T. Prider. 
the graiiitie gneisses. The petrological evidence regarding the relative age 
‘of the gneisses and sediments and the jiossibility of granitisation of the 
'Cardnj) sediments is discussed in the next section of this jmper. 
III. PETROLOGY. 
(A) The older ht/hrid gneisses and the associated xenoUths. 
These rocks are best ex]iosed in the Roads Board ([uarry and most of 
the material described btdow came from this locality. Since the material 
forming tlie xtmoliths has largely controlled the character of the hybridised 
granitic gneiss it will be described first. 
Amongst the xenoliths three distinct types of material can be seen^ 
thus : — 
(i) Quartz xenoliths. 
These are fragments up to four or five inches in diameter, mostly 
smaller. The qiuu'tz is clear and coarsely crystalline and under the 
microscoin* is se(m to [)e an allotriomorphic mosaic of irregular shaped 
grains with ci'enulate boundaries. No signs of original clastic struc- 
ture are visible and the larg(! grains show only very slight strain shadows. 
Minut(» gas-lifjuid inclusions are very abundant. This material may have 
originated either from a (juartzite or from quartz veins. It has contj’ibuted 
a considerable amount of what would at first sight appear to be primary 
(magmatic) quartz of the gneiss, for the size of the xenolithic (uiaitz l)odies 
varies down to the order of o mm. diameter. 
(ii) K pi dot e-muscovite xenoliths. 
These xenoliths occur in fragments up to four inches diameter. The 
rock is massive, v(‘ry tin(‘ grained and consists of a fine mesh of small mus- 
covite' (lakes with idioblastic tabh'ts of highly birefi’ingent epidote uniformly 
and abundantly dispersed throughout. The epidote is present to the extent 
of abend dO j)er cent, of the re)ck. In ]>laces remnants of untwinned ( ?) 
plagioe'lase (with the I'efringence ap])roximately the same as tliat of canada 
balsam) ('rowded with sericitic inclusions are present. This rock appears to 
have resulted from the alteration of a basic plagioclase rock which must have 
been closely associated with the rocks from which tiie next grouj) of xenoliths 
were derived. 
(iii) JTornhlcnde-epidote-hiotiic hornfels .coiolitlis. 
XeTioliths of this type, which are dark grtHUiish to l)lack in colour, are 
the most abundaid typ(‘’and liave exerted (considerable control on the char- 
acter of the hybrid gm>iss, baving conlril)uted most of the f(*rromagnesian 
content of that rock. Rocks of this ty{)e a]>i)('ar to be ratluu' constant in 
cliaracter wherever noted and are best developed in the Roads Board quaiTv. 
They are fiJie even grained, Avilh no trace of any dii'ccted structure, the 
structure being coarse hornfelsic. Tuider the mi('roscope the rock is seen to 
consist of a (h'cussate aggregat(‘ of liiotite and hornblende associated with 
granular aggregates of epidote. Irregular grains of magnetitf' ar(‘ scattert'd 
uniformly throughout the rock, and a]mtit(‘, in stout euhedra to 0,.“i mm. 
diameter, is abundant. 
The biotite and hornblende are closely associated. The biotite is a 
bi*ownish-green, ])ractically uniaxial variety with jS = 1.637 and in all of 
the slices examined is considerably in excess of the bhiish-green amphibole 
