96 
ing 03 mm., with a high refractive index and double refrac- 
tion (zircon). 
Felspars essentially orthoclase and microcline ; plagio- 
cJase very subordinate, irregular, and finely twinned ; crystal 
outlines generally corroded, and the mineral clouded by par- 
tial decomposition : the cleavage cracks tinted by subse- 
quently deposited iron ores. Twinning after the Karlsbad 
law is observed in the orthoclase. Microline subordinate. A 
micrographic (granophyric) intergrowth between quartz and 
felspar on a small scale is visible in parts of the section. 
Biotite strongly pleochroic, from light greenish-brown to 
almost black. 
Magnetite as small, angular individuals. 
Garnet Gneiss. 
Plate xvii., fig. 2. 
Locality. — Mount Davis (two miles north), Tomkinson 
Ranges. 
Maci'oscojyically. — A fine-grained quarzitic gneiss, with a 
rich, red-garnet development; foliated, the biotite in regular 
planes, the quartz and felspar foliations often wedging out. 
Portions of the rock appear very quartzose, compact, with 
largish fragments of smoky quartz. 
Microscoioiccdly . — A quartz orthoclase mosaic. The folia- 
ated character, though clearly visible in hand specimens, is 
not apparent under the microscope. 
The quartz contains minute liquid inclusions, and aggre- 
gates of black particles disseminated through its mass, which 
appear to be carbonaceous, the former not infrequently 
grouped centrally. A fair amount of isotropic mineral is 
also present 
Felspar : large clouded crystals of orthoclase and smaller 
subordinat3 plagioclase. 
Biotite strongly pbochroic, in shades of brown to almost 
black, when the rays vibrate parallel to the cleavage ; elon- 
gated or irregular, and is in parts decomposed, the resulting 
iron oxides staining the adjoining minerals reddish-brown ; 
often enclosing magnetite and felspar. 
Magentite as fine dust and larger individuals, sometimes 
filling fissures between the felspar. 
Shattered crystals of red garnet, the i.irgest of which are 
a millimetre in diameter, are plentiful. They behave com- 
pletely isotropically under crossed nicols, though the quartz 
and felspar exhibit undulose extinction rather markedly. 
This rock appears to be a "clastic gneiss." 
