184 W. R. BROWNE. 
of the outcrop varies; in some places it is upwards of 100 
yards, thinning considerably towards the southern end. 
The gneissic structure is very pronounced, and the basicity 
of the rock increases towards the south, megascopic free 
quartz disappearing and the layers of biotite giving the rock 
the characteristic bluish-black appearance from which the 
whole gneiss has been named. 
The white and pink gneisses.—Along the Sydney Road 
there is a long band of acidic gneisses which has been 
traced from a point 14 miles north of Tillabudgery Trig. 
Station as far as Pearman’s Hill, a total distance of over 
53 miles, and which may extend farther north still. At 
Bunyan the outcrop is not less than 400 yards in width, 
and it forms a strong feature on the west of the Sydney 
road for some distance past the Cooma Creek bridge. Two 
varieties of gneiss are recognised. The white gneiss has in 
many places strongly marked gneissic foliation, the folia 
consisting of quartz and felspar alternately, with subordin- 
ate development of white mica in small flakes. Apparently 
of somewhat later origin, since it intrudes the white gneiss, 
is a pink rock strongly jointed, very compact and seen under 
the microscope to consist mainly of quartz, with very 
subordinate felspar, the whole stained with hematite, and 
possessing marked schistosity. 
The affinities of these two gneisses it has been found 
impossible to determine with complete satisfaction. Not 
far past Cooma Creek bridge the white and blue gneisses 
were found in close association, and what looked like an 
intermediate type of gneiss, very similar to the blue gneiss, 
but with very subordinate mica, was also seen. On the 
whole it seems as if the white and the pink gneisses were 
genetically related to the blue gneiss as later acid differ- 
entiates from thesame magma. A pink rock which intrudes 
the blue gneiss at Pearman’s Hill may bea phase of the pink 
