GEOLOGY OF THE COOMA DISTRICT, N.S.W. 179 
three varieties of gneiss are seen to intrude the schists, 
fragments of which they frequently include. 
The limits of these three formations, and particularly of 
the Oooma gneiss and the mottled gneiss, cannot be 
definitely laid down, owing to obscuration of their textural 
peculiarities by metamorphic and other influences, and also 
to the fact that a great deal of the gneiss has been intruded 
as tongues along the planes of schistosity of the invaded 
formations. The mapped-in occurrences, therefore, must 
not be taken to represent the total extent of these rocks. 
The mottled gneiss.—The mottled gneiss is the oldest of 
the three: it is a very fine-grained rock, with a charac- 
teristic mottled appearance due to the alternation of 
patches of biotite in wavy bands with patches of the light- 
coloured constituents. This mottled gneiss has a fairly 
wide distribution throughout the metamorphic area, in 
relatively small patches as a rule. Mount Gladstone 
(Cooma Hill), about three miles to the S.W. of Cooma, is 
composed principally of this rock, which is also found 
intruding the schists in various places, principally to the 
west of the town. So much alteration of the schist has 
taken place, through metamorphism due to intrusion and 
the injection of countless little pegmatite veins, and so 
much change in the mottled gneiss from similar causes, 
that it is at times impossible to separate them. No doubt 
a certain amount of contamination of the igneous material 
has taken place during the process of injection: indeed 
this is proved by the fact that microscopically sillimanite 
is seen to be a constant constituent of the mottled gneiss. 
The appearance of the gneiss in hand specimen would 
never suggest its igneous origin, its fine grain and mottled 
schistose appearance being characteristic rather of an 
altered sedimentary rock. Indeed fora good while I was 
inclined to class it as such, until the discovery of schistose 
