47 



seen in the banks of the Hugh l^etween Owen Springs Station 

 .and the Undertree Well. 



The sandy covering of the plains between the ranges conceals 

 the rock formation below, and the flat-topped hills which occur 

 in some portions of them were away from the track and therefore 

 I was unable, through the exigencies of travel, to do more than 

 .surmise their probable structure. 



III. The Cextral Plateau. 



Seen first from the Waterhouse Range at Orraminna Pass, 

 thirty miles away, the aspect of the Macdonnell Range forming 

 the south edge of the plateau is anything but imposing. The 

 narrow gaps in the range show no sign of other ridges beyond, 

 and this one ridge can be plainly discerned by its overhanging 

 scarped edge facing the north, and its foot sloping into the plain, 

 to be an immense layer of rock which has been tilted up on edge. 



This ridge is composed of an intensely hard red quartzite in 

 layers of about six inches thick, but the layers have little or no 

 cross jointing. It strikes nearly east and west, and rests directly 

 on a very felspathic mica schist in wdiich are l^ands of granite of 

 •coarse texture. Above the quartzite is a thick stratum of lime- 

 stone, very impure and variable in composition. It may be best 

 described as an immense number of thin bands of argillaceous 

 and siliceous limestones with partings of argillaceous sandy 

 matter often intermixed with seams of brown ironstone, and tlie 

 whole mass pervaded by manganese dendrites. 



At the gaps, which are about two chains wide, the ditterent 

 creeks which rise further to the north flow out on to the plain, 

 .and during rains form an extensive area of flooded ground, whicli 

 is eventually drained by the Todd Creek. The gaps, commencing 

 with the western one through which the telegraph line passes, 

 iire Temple Bar, Heavitree, Emily, Jessie, and the Slip-panel — 

 the latter, as its name suggests, being so narrow as to be closed 

 to cattle by a panel or so of fencing. Where the gaps open on 

 to the plain the limestone is worn back for some considerable 

 distance on either side, and the plain is strewn with stones of 

 quartzite, porcelain and chalcedony. Between the gaps, however, 

 the limestone foots the quartzite in the form of black-looking 

 knolls, whose colour is due on its weathered faces to a superticial 

 •coating of manganese. The knolls of limestone reach, perhaps, 

 100 feet up the slope of the quartzite, and are covered with 

 mulga. Above this the quartzite stands up three, four, and 500 

 feet red and naked, except where here and there a cycad main- 

 tains a footing among narrow crevices in tlie rocks. The summit 

 is a narrow ridge, and the north side is a sheer precipice or steep 

 •escarpment caused by the breaking away of the undermined rock 



