HORN EXPEDITION — PHYSICAL GEOflRAPHY. 9 



perhaps due to segregation along certain lines, whicii now stand out as low 

 reticulate ridges. 



A noteworthy feature of the i-oek is the manner' in which it peels fitV. 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 ai-e seen to be separating fr(im the rock in all 

 directions, and along no definite plane or set of planes. Secondly, on a large scale 

 there may be seen, sometimes leaning against the base of the rock, having slipped 

 from a higher level, or lying round its base, innnense blocks of lock which have 

 peeled off from the iiiountain, the phenomenon resend)ling on a large scale the 

 concentric weathering of many eruptive rocks. In one or two places large blocks 

 can be seen detached sulliciently to /dlow the sunlight to pass between them and 

 the main mass, but still not entirely separated. One of these measures S' x H' x 

 200', while others which are leaning against the base of the rock measure G' x 5' 

 X 20' and 12' x C x GO'. Caves, usually of a small size, occur both near the 

 base and on the sides and slojics of the rock. Lastly, a ridging is observable, 

 which probaljly indicates the direction of foliation planes, trending in a N.W. and 

 S.E. 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 isolatetl mass 

 surrounded by red sandhills. Mount Olga rises to an elevation of about 1500 feet 

 al)Ove the plain, and over 3000 feet above sea level. It appears to be composed of 

 a coarse conglomerate from top t(j bottom, which consists for the most part of 

 pebbles of granite and other eruptive rocks. The southern fac(^ of this mass is 

 about five miles in length, and its western extremity rises perpendicularly for 

 nearly 1500 feet. 



Mount Olga from a distance presents a most I'cmarkable outline, the many 

 rounded dome-like ele\ations reminding one of the features usually pi-csented by 

 granite ranges. 



13. — RIVERS. 



The country traversed by the ICxpedition between Oodnadatta and the 

 McDonnell Ranges lies wholly within a region (»f internal drainage, divisihie into 

 two basins, the centre of the largest and most important being Lake Eyre, and 

 that of the other being Lake Anuideus. Tin; basins of Lakes Eyre and Amadeus 

 will he desci-ibed in detail later. We will in the meantime confine our attention 



