254 C. HE. Tilley—Para-Gneisses in South Australia. 
limonitic weathering product, and its fusion before the blowpipe to 
a black magnetic mass. 
The colour of some of these garnets as seen in hand-specimens is’ 
often the characteristic purplish red of almandine. 
Biotite and quartz are often wrapped around the periphery of 
these grains. 
Graphite is invariably present in these rocks, and in some sections 
may rise to an important constituent. The orientation of the 
graphite flakes is like that of biotite, parallel to the banding, and in 
sections’ cut perpendicular to the foliation the flakes appear as 
elongated rods. In reflected light the flakes appear with a steel-- 
grey lustre, like that of magnetite. The rod-like sections, however, 
have the characteristic frayed and serrated edges and a striated 
appearance on the ground surface, these features arising as a result 
of the softness of the mineral during grinding. 
In these rocks these features, together with the rod-like sections, 
afford a ready means of distinction from magnetite. The graphite 
is usually associated with biotite, and is often enclosed by that 
mineral. 
The order of abundance of the chief minerals in the rock is quartz, 
orthoclase, biotite, garnet, and plagioclase. 
(ii) Type B.—The garnet gneisses described under this heading 
are interbanded with metamorphosed impure dolomites, now repre- 
sented by rocks in which diopside is an important constituent. 
These gneisses are very similar to type a already described, but the 
banding and texture are here of a coarser nature. Rose-pink to 
purplish-red garnets are abundant often with good crystal outline, 
and average size of tin. Dark mica is more abundant, and the 
general colour of fractured surfaces is dark grey. Steel-grey flakes 
of graphite are readily made out with a low-power lens. Weathered 
surfaces may show a prominent augen structure, the augen being of 
garnet, quartz, and felspar. These gneisses have in places been 
intruded by pegmatite veins, and composite types may thus be 
developed, in which part of the felspathic constituent is original and 
part introduced. 
Under the microscope the minerals seen to be present are quartz, 
orthoclase, garnet, biotite, plagioclase, graphite, leucoxene, sphene, 
zircon, and a little muscovite and magnetite. The features of these 
rocks are so closely allied to those described for type a, that a 
detailed description is unnecessary. The potassic felspar is the 
typical fibrous perthitic variety. Plagioclase is more abundant than 
in type a, but is of the same composition—oligoclase andesine. 
A number of grains as calcic as andesine were, however, determined. 
Almandine garnet occurs in grains of somewhat larger dimensions, 
and isinvariably isotropic. The presence of the spessartite molecule 
is indicated by the presence of manganese. The remaining con- 
stituents of the rock swing round in a bulge when the almandine 
porphyroblasts are met. The elongated lenses of quartz parallel to 
