182 W. R. BROWNE. 
bucca Road these are found of lenticular shape about 
three feet long, and with a maximum breadth of about 
four inches. They occasionally consist of a hard outer 
shell of an inch or one and. a half inches thick, with a core 
of softer rotten schist. This may bea kind of case-harden- 
ing, due to the heat etc. of the eruptive rock. Partial 
digestion of the inclusions seems to have occurred, or their 
recrystallization, as exhibited in the cuttings just outside 
Cooma on the Berridale road. 
In general appearance the Cooma gneiss is very variable. 
Typically it has quite a granitic fabric and massive struc- 
ture, but at times it assumes a schistose appearance and 
develops a rude cleavage, particularly near its boundaries. 
Again, a typical gneissic structure may be exhibited, bands 
of light and dark minerals alternating, due in some cases to 
lit par lit injections of pegmatite, but often, I believe, to 
recrystallization at the time of metamorphism. Here and 
there are to be found dykes or bands of a more acid gneiss, 
consisting of quartz, felspar, and white mica, of the same 
general texture as the ordinary gneiss, but differing in the 
absence of black mica, 
The normal gneiss itself consists, megascopically viewed, 
of quartz, felspar and biotite in varying proportions, 
generally with a subordinate amount of white mica. Locally 
the acidity of the rock may be increased, while again, 
especially along the borders of some large pegmatite vein, 
the biotite content increases very largely, giving the rock 
a very basic appearance. Preliminary microscopic examin- 
ation shows the constant occurrence of topaz, and of little 
zircons with pleochroic haloes in the biotite. . 
The grain of the gneiss is asarule very even, but felspars 
up to 14 inches long and quartz grains up to 4 inches long 
occur, giving the rock in places a porphyritic appearance. 
These are evidently fragments which have resisted crusb- 
