87 
hills are almost invariably capped by an exceedingly hard, 
silicified layer of rock, the base being of a more friable and 
softer character.'^" 
At Indulkana the top layer is composed of a compact, 
chalcedonic grit, with irregular, sub-angular fragments of 
colourless and blackish quartz scattered through the mass, 
with a secondary interstitial cement of a form of quartz. 
Though the rock may be a coarse grit, the surfaces of frac- 
ture, which are in parts sub-conchoidal, are remarkably 
smooth ; the compactness of the rock causing the planes of 
fracture to pass through the included particles. Professor 
Tate described the desert sandstone as being composed of 
/'sharp grains of glassy quartz, varying much in size, cement- 
ed by opaque, white siliceous matter, and more or less stain- 
ed red by oxide of iron." This description would apply 
equally well to the Indulkana outcrops. In places the for- 
mation becomes very fine-grained, showing a laminated 
character or a distinct fissility, and a fairly regular system of 
vertical jointing, in a north-easterly direction. 
A second variety of desert sandstone has been produced 
by an opalisation of ihe mass. Examples of this character 
were seen at Hector's Pass (Mann Ranges), south-east of 
Giles West Camp (Musgrave Ranges), and south of Avers 
Ranges (Northern Territory). The rock is an impure form 
of common opal; in colour white to bluish-white; containing 
cellular cavities and small black inclusions of carbonaceous 
matter. It breaks with a true conchoidal fracture. vSurface 
outcrops only were found of this variety. 
At North Creek the formation consists of a very fine- 
grained, splintery, chalcedonic quartzite, the individual 
grains being hardly distinguishable with the naked eye. The 
rock is traversed by small veinlets of oxide of iron, subse- 
quently precipitated. The colour varies considerably : white, 
yellow, reddish, blue, and purple. The rock is brittle 
and rings when subjected to the blows of the hammer. 
At Yarrabollinna Waterhole the character of the rock 
again changes entirely. Large, bluff-shaped masses are com- 
posed of an excessively fine-grained form of silica, so fine that 
it shows no sign of a gritty feel when rubbed between the 
fingers, resembling somewhat the touch produced with kaolin, 
which mineral is present in small measure only. The pure 
forms are snow-white, others are variouslv tinted. Within 
this deposit nodular masses of a cherty form of silica occur, 
which are bounded by an outer concentric growth of whit-e 
chert. (See Plate xv., fig. 1.) 
* See Tate and Watt : Rep. Horn Exped. Cent. Aus. Phys. 
Geog., page 8; General (xeology, page 68. 
