8 HORN EXPEDITION — PHYSICAL GEOGRAPHY. 



friable sandstono slightly tinged yellowish-red externally by hydrated ferric oxide. 

 The pre.senoe of this layer of indurated rock expl.iins the fact that the isolated 

 Cretaceous table-topped hills have usually the form of truncated cones, the topmost 

 stratum generally presenting a vertical 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 and Mount Olga. 



In addition to the foregoing mountain ranges there are some isolated 

 mountains within the area examined that require a brief description. Rising like 

 an enormous water-worn 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 S.S.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 inter- 

 esting relic of an ancient geological formation has puzzled explorers in no small 

 degree. It rises to an elevation of about 1100 feet above the surrounding plains, 

 and about 2500 feet above sea level. The sides of the rock, which has a circum- 

 ference at its base of nearly five miles, are very steep, almost vertical in places and 

 practically inaccessible, although Mr. Gosse succeeded after great trouble in 

 ascending it. The rock is quite bare, with the exception of a few tig-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 unmistakeable, 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, 

 liaving been formed. The rock is a very indurated, and to some extent altered, 

 arkose sandstone, decidedly gritty in parts. 



The sides of this rock ascend in places quite vertically for a distance of 500 

 to GOO feet, while some of the more sloping faces are marked by a .series 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, which have disap- 

 peared, having been separated by small intersecting veins of a harder material, 



