308 C. #. Tilley—Para-Gneisses in South Australia. 
An analysis of a less quartzose variety of the type 6 gneisses has 
been made. 
The constituents are perthitic orthoclase quartz, plagioclase 
garnet, biotite, magnetite, and a few flakes of graphite can also be 
detected. 
The analysis of this rock is given on p. 807, and with it the analysis 
of a slate from Vermont. 
An inspection of this analysis shows that the available criteria 
for a sedimentary origin are satisfied. 
Magnesia and potash are in excess of lime and soda respectively. 
The presence of corundum in the norm to the extent of 5-4 per cent 
is also a partial confirmation. 
The analysis is close to that of a slate from Vermont, described 
by T. N. Dale,! and the correspondence would be still closer Sf the 
slate analysis were recalculated after removal of excess water.. 
A comparison with the analyses listed by Grubenmann indicates 
its grouping with Class [1—the aluminium silicate gneisses. If the 
procedure of Grubenmann be adopted in representing graphically the 
composition of the gneisses by an adaptation of the Osann projection, 
we have the following triangular diagram (Fig. 3). 
Fic. 3.—(Osann.) 1. Garnet gneiss, Sleaford Bay. 2. Mean position of the 
aluminium silicate gneisses. 
1 Clarke, Bull. U.S.G.S., No. 419, p. 211. 
