“1 
means of a series of terraces formed by the Ordovician ranges, each one on a 
lower level than its predecessor on the north, on to wide and extensive plains 
occupied by Cretaceous rocks and by stony and loamy plains and sandhills. 
These last features have been formed from the désris derived partly from 
the Cretac-ous strata, which dédris has been accumulating through the Late- 
Tertiary and Post-Tertiary epochs, and is still accumulating at the present 
day. Such plains, near their northern limit at least, are not less than 1,000ft. 
above sea level. ‘Still they slope so gently to the south towards the centre of 
the basin, 7.e., towards Lake Eyre, that their elevation above sea level is no index 
to their altitude above the surrounding country. The streams which have 
eroded them have not generally excavated their channels deeper than from 
200ft. to 250ft. below the original surface of the country. 
(2) Tue James, Warernousr, GrorGE Git, anD Levi Ranczs. 
The next division of the mountain system includes the ranges situated within 
the Ordovician area. Beginning from the north these comprise the quartzite 
ridge which forms the southern boundary of the Pre-Cambrian area, and in 
which are the Heavitree, Emily, Temple Bar, &c., Gaps. This ridge is suc- 
ceeded on the south by the Waterhouse, James, George Gill, Levi, and Chandler 
Ranges. These ranges consist of a series of parallel ridges of quartzite and 
sandstone. with a nearly east and west trend, separated by numerous parallel and 
often very persistent and regular longitudinal valleys. These valleys are 
generally very narrow, often less than a mile wide, but sometimes open out, as 
in the case of the Missionary Plain, to a width of from twelve to fifteen miles. 
Intersecting these ranges are numerous short transverse valleys, frequently 
entirely occupied by the channels of the creeks, which have eroded them. One 
of these creeks often extends for long distances along one of the longitudinal 
valleys, and then suddenly bursts through the range through a narrow gorge, to 
resume its course along a second longitudinal valley at a lower level than the 
first, and perhaps to pass through a second gorge in its passage towards Lake 
Eyre. These ranges extend easterly, as far at least as those of the previous 
division have been traced, while westerly they practically terminate at 131° 20’ 
(about) east longitude. 
They have a mean combined width, if we include the intervening plains and 
valleys, of from sixty to seventy miles. The area occupied by them, there- 
fore, must be more than 15,000 square miles. ‘The highest points are situated 
in the most northern ridge, as in the case of Mount Gillen, which must be 
nearly 3,000ft. above sea level. The mean elevation of this ridge is abcut 
2,500ft. above sea level, that of the Waterhouse Range about 2,200ft., that of 
the James Range about 2,000ft., while, lastly, the mean elevation of Chandler 
Range is only about 1,500ft. to 1,600ft. From these figures it can be clearly 
seen that there is a gradual decrease in elevation in the ranges from north to 
south, each range to the south constituting, as it were, a step in the descent from 
the Macdonnell Ranges to the plains. 
The chief factors, in addition to the position of the longitudinal valleys 
occupying the original troughs of the folds, that have influenced the direction 
of the iines of denudation are (1) the lines of weakness on the crowns of the 
anticlinal arches, and (2) the position of the bands of limestone. An example 
of the influence of (1) is furnished by the valley of Petermann Creek, which 
