74 
which have disappeared, having been separated by small intersecting veins of a 
harder material, perhaps due to segregation along certain lines, which now stand 
out as low reticulate ridges. 
A noteworthy feature of the rock is the manner in which it peels off. Firstly, 
on a small scale, thin flat flakes an inch or two in diameter and from one-eighth 
to a quarter of an inch in thickness are seen to be separating from the rock in 
all directions, and along no definite plane or set of planes. Secondly, on a 
large scale there may been seen, sometimes leaning against the base of the rock, 
having slipped from a higher level, or lying round its base, immense blocks of 
rock which have peeled off from the mountain, the phenomenon resembling on 
a large scale the concentric weathering of many eruptive rocks. In one or two 
places large blocks can be seen detached sufficiently to allow the sunlight to 
pass between them and the main mass, but still not entirely separated. One of 
these measures 8ff. x 5ft. x 200ft., while others which are leaning against the 
base of the rock measure 6ft. x 5ft. x 20ft. and 12ft. x 6ft. x 60ft. Caves, 
usually of a small size, occur both near the base and on the sides and slopes of 
of the rock. Lastly, a ridging is observable, which probably indicates the 
direction of foliation planes, trending in a N.W. and 8.H. direction. 
Fifteen miles west of Ayers Rock is another remarkable mountain mass, the 
most prominent and elevated portion of which is called Mount Olga. ‘this, 
with numerous other peaks which rise from a common base, forms an isolated 
mass surrounded by red sandhills. Mount Olga rises to an elevation of about 
1,500ft. above the plain, and over 3,000tt. above sea level. It appears to be 
composed of a coarse conglomerate from top to bottom, which consists for the 
most part of pebbles of granite and other eruptive rocks. The southern face of 
this mass is about five miles in length, and its western extremity rises perpen- 
dicularly for nearly 1,500ft. 
Mount Olga from a distance presents a most remarkable outline, the many 
rounded dome-like elevations reminding one of the features usually presented by 
granite ranges. 
B.-- RIVERS. 
The country traversed by the Expedition between Oodnadatta and the 
Macdonnell Ranges lies wholly within a region or internal drainage, divisible 
into two basins, the centre of the largest and most important being Lake Eyre, 
and that of the other being Lake Amadeus. ‘The basins of Lakes Eyre and 
Amadeus will be described in detail later. We will in the meantime confine 
our attention to that portion of the Lake Eyre Basin which is drained by the 
Finke, one of the most important rivers of Australia. 
The Macdonnell Ranges and the eastern extension of the Hart Range may be 
said to divide the drainage of the interior into two areas, of which the southern, 
which forms part of Lake Eyre Basin, contains the most important rivers. The 
Finke, with its large tributary, the Hugh, takes its rise on the southern slope of 
these ranges, and flows toward Lake Eyre ina general southerly to south-easterly 
direction. The majority of the small creeks, which issue from the northern 
slope, do not discharge their waters into any comm n basin, but each maintains. 
an independent course for a longer or shorter distance from the ranges, to become 
sooner or later absorbed by the sandhills and loamy plains. These latter creeks, 
of which the Darwent, Dashwood, Charley, Six Mile, Muller, &., are ex- 
amples, leave the ranges and flow northwards over the surface of the elevated 
plain, which is known to the north of Alice Springs as Burt Plain. 
