101 
Orthoclase is clouded and strongly illuriiinatved under 
crossed nicols as a result of its alteration. 
Plagioclase is subordinately represented, the fragments 
being comparatively small. A dark mica (biotite), in aggre- 
gates of curved flakes, is plentiful. It is very probably secon- 
dary,'^ and occurs interstitially. Its decomposition has pro- 
duced hematite. Few tiny flakes, apparently white mica, are 
also present. 
Ores of iron are plentiful. Ilmenite occurs as asymmetri- 
cal masses, opaque, and (by incident light) shows the imper- 
fect system of striation and characteristic brownish tinge. More- 
over, this form is replaced in jDarts by a semi-transparent 
variety, "with a clove-brown colour," f suggestive of mica- 
ceous titanic iron. Magnetite is also represented as irregular 
patches. 
With regard to the constituents of this rock being essen- 
tially of grains of quartz, orthoclase, and microcline, it is 
interesting to note that Dr. C. Chewings has described! a 
granite from Mount Olga (a sister outcrop to Ayers Rock), 
which is composed almost essentially of quartz, orthoclase, and 
microcline. The constituent grains of the rock from Ayers 
Rock, therefore, have in all probability been derived from the 
same granite as the specimen from Mount Olga. Mount 
Olga consists for the most part of a metamorphic conglomerate. 
EXPLANATIONS OF PLATES. 
Plate XIII. 
r)fi. 1. — Mount Conner : an outlier of Ordovician quartzite, sur- 
rounded by saltbush flats. 
Fig. 2. — ^Fiolian erosion ; mushroom-shaped outcrops of gneiss in 
the desert north-west of the Mann Ranges. 
Plate XIV. 
Fig. 1. — Intrusion of poiTDhyritic granite within hornblendic 
garnetiferous schist, south of Mount Cockburn. ^lann 
Ranges. 
Fig. 2. — Ordovician quartzite, shattered by insolation, Mount 
Conner. 
* Compare Tate and Watt: op. cit., page 8 — "Although once 
a sedimentary rock, it has been to some extent altered by meta- 
moi-phic agencies, a small amount of mica, perhaps of secondary 
origin, having been formed." 
t Rosenbusch : Microsc. Phvs. of Rockmaking Minerals i Idd- 
ings), 1900, page 167. 
I Chas. Chewings : Beitrage zur Kenntnis der Geologie Stid- 
und Central Anstraliens. Heidelberg, 1894 — "Ein Granit von 
Mount Olga (Central-Australien) besteht fast ganz aus Quarz, 
Orthoklas, nnd Mikroklin.' 
