GEOLOGY OF THE COOMA DISTRICT, N.S.W. 201 
of boss-like appearance. The mass is over two miles long, 
with a maximum width ofa little under two miles; it is 
surrounded on all sides by schists and olivine basalt, and is 
7 miles away from the nearest outcrop of Berridale granite. 
The syenite is in general massive, and weathers into great 
rounded tors like granite, which undergo a kind of spheroidal 
exfoliation. Jointing is developed at times: one reading on 
a joint plane gave its dip as 65° in a direction H. 10° 8. 
Megascopically the rock would be called a syenite, as it is 
seen to consist of felspar (orthoclase) and hornblende, but a 
microscopic examination would place it rather among the 
quartz-monzonites. Towards the eastern periphery of the 
mass there is a considerable development of a porphyritic 
facies, which might be called asyenite-porphyry. Irregular 
basic patches without any definite sharp boundaries are 
also frequent. 
The plutonic rock is intersected by dykes of felspar- 
porphyry or bostonite, with only very small traces of ferro- 
magnesian constituents, and many apophyses radiate into 
the surrounding country. One ofthese forms a conspicuous 
feature among the schists along the Myalla road, and can 
be traced for about 6 miles, ultimately coming to an end 
ina railway cutting a mile north of Cooma. The texture 
of this dyke-rock changes as we get away from the parent 
plutonic mass. The felspar phenocrysts may be upwards 
of half an inch in length and very numerous, the base being 
fine-grained but evidently holocrystalline. Farther away 
the base gets exceedingly fine-grained and phenocrysts are 
very much fewer and smaller, the rock giving the impress- 
ion of a trachytic lava rather than of a hypabyssal rock, 
while the subordinate ferro-magnesian constituents have 
completely disappeared. These variations are evidently 
functions of the distance from the parent magma, the con- 
ditions of consolidation, and especially of heat, as we get 
