(3) 
Chambers Pillar, for instance, a well-known feature situated ten miles north 
of the old Idracowra cattle station on the Finke, might be likened to an earth- 
pillar, the indurated ferruginous sandstone of its summit taking the place of the 
boulder of the earth-pillar, and protecting the sandstone of the pillar from 
removal by denudation. The isolation of Chambers Pillar, for it is surrounded 
on ail sides by red sandhills, is probably due to the purely local character of the 
indurating process, or rather perhaps to the induration having been more intense 
in the locality of the Pillar than in the once surrounding rock, now entirely 
denuded. The Pillar, if we include in this term the whole structure from base 
to summit, is divisible into two parts—a basal portion or pedestal 500yds. in 
circumference at its base and 100ft. high, and a column surmounting it, 67ft. 
high and 80yds. in circumference at its base. The whole, with the exception of 
the few feet of more indurated rock on its summit, consists of a yellow and 
white friable sandstone slightly tinged yellowish-red externally by hydrated 
ferric oxide. The presence of this layer of indurated rock explains the fact that 
the isolated Cretaceous table-topped hills have usually the form of truncated 
cones, the topmost stratum generally presenting a verticai edge for its whole 
thickness, while the slope of the portion of the hill occupied by the underlying 
softer strata varies from 32° to 40° from the horizontal. 
(4) Ayers Rock anp Mount OteGa. 
In addition to the foregoing mountain ranges, there are some isolated moun- 
tains within the area examined that require a brief description. Rising like an 
enormous waterworn boulder, half buried in the surrounding sea of sandhills, 
is that remarkable isolated monolith known as Ayers Rock. It is situated about 
thirty-two miles 8.8.W. of Lake Amadeus. The summit of this monarch of 
the desert can be seen from a distance of more than forty miles. At a nearer 
view its smooth, bold, flattened, dome-like outline stands out clear and distinct. 
‘This interesting relic of an ancient geological formation has puzzled explorers © 
in no small degree. It rises to an elevation of about 1,100ft. above the sur- 
rounding plains, and about 2,500ft. above sea level. The sides of the rock, 
which has a circumference at its base of nearly five miles, are very steep, almost 
vertical in places and practically accessible, although Mr. Gosse succeeded after 
great trouble in ascending it. The rock is quite bare, with the exception of a 
few fig-trees, which maintain a precarious footing in the few crevices on its 
bare sides. 
The rock has been often mistaken for granite, to which it bears some super- 
ficial resemblance, both in its lithological aspect and in its mode of weathering. 
The original sedimentary character of it, however, is unmistakable, numerous 
very small rounded pebbles of quartz and felspar being distinctly visible in hand 
specimens. Although once a sedimentary rock, it has been to some extent 
altered by metamorphic agencies, a small amount of mica, perhaps of secondary 
origin, having been formed. The rock is very indurated, and to some extent 
altered, arkose sandstone, decidedly gritty in parts. 
The sides of this rock asvend in places quite verticallv for a distance of 500ft. 
to 600it., while some of the more sloping faces are marked by a senies of 
terraced waterfalls rising one above the other. <A peculiar netted appearance is 
to be seen on some of the faces, a good example being visible on the northern 
face. ‘This is due to the irregular weathering of the rock, the softer spaces, 
