IIOKN EXPEDITION — PHYSICAL GEOGRAPHY. 5 



formed by tlic Urdoviciaii ranges, each one on a lower level than its predecessor (in 

 the north, (in to wide and extensi\e jilains occupied )jy Cretaceous rocks and liy 

 stony and loamy plains and sandhills. 



These last features have been formed from the debris derived partly from the 

 Cretaceous strata, which debris has been accunuilating through the Late-Tertiary 

 and Post-Tertiary epochs, and is still accumulating at the present day. Such 

 plains, near their northern limit at least, aie not less than 1000 feet above sea 

 level. Still they slope so gently to the south towards the centre of the basin, /.f., 

 towards Lake Eyre, that their elevation above sea level is no index to their altitude 

 abov(! the surrounding country. Tlie streams which have eroded them ha.ve not 

 gencn'ally excavated their channels deeiier than friim 200 to 250 feet below the 

 original surface of the country. 



(2) The James, Waterhouse, George Gill and Levi Ranges. 



The next division (if the nuiuntain system includes the ranges situated within 

 the Ordovician art'a. Beginning from the north these comprise the ([uai'tzite ridge 

 which forms the southern boundary of the Prc-Candjrian area, and in which arc 

 the Heavitree, Emily, Temple Bar, etc.. Gaps. This ridge is succeeded on the 

 south by the Waterhouse, James, George Gill, Levi and Cliandler Banges. These 

 ranges consist of a series of [lai'allel ridges of (piartzite and sandstone, with a nearly 

 east and west trend, separated by numerous parallel and often \ery persistent and 

 regular longitudinal valleys. These \alleys 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. 



Intersectijig 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 Aalleys, 

 and then suddeidy bursts through the range through a nairow 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 Eyi-e. These ranges 

 extend easterly, as far at least as th(«e of the previous division have been traced, 

 while westerly they practically terminate at 131" 20' (about) east longitude. 



They have a mean coml)ined width, if we include the intervening plains and 

 valleys, of from sixty to seventy miles. 'J'he area occupied by them, therefore, 

 must be moi-e than 15,000 s(juar(; miles. TIk; highest points are situated in the 

 most northern ridge, as in the case of .Mount Gillcn, which must be nearly 3000 



