26 



The country througli Tvhicli tlie Murray flows from the 

 "boundary of the colony to Lake Alexandrina is essentially the 

 same ; and as the gorge-like feature persists throughout, it 

 might be thought because of this uniformity that the geological 

 structure is identical from end to end, but it is not so, 



From structural and physiographic features I divide the- 

 course of the South Australian portion of the river into three 

 sections : — 



1. From the boundary to Overland Corner. This length of 

 the river is 143 miles, whilst the distance between the two 

 places is only 60 miles. The width of the gorge varies from 

 six to two and a half miles. 



2. From Overland Corner to Lake Alexandrina. At the 

 former locality the gorge suddenly contracts to a width of 

 about one mile ; and as far as the IN'orth-west Bend the 

 average width is one mile and a quarter, whilst south hence to 

 below Blanchetown, it is three-quarters of a mile. Here and 

 there it opens out to greater width as at Mannum, but is again 

 contracted at its southern end. 



3. The Lacustrine section from "Wellington to its mouth. 

 [Mr. Pollitzer divides the river from Blanchetown according 



to his estimate of the rock nature of the cliffs into four sec- 

 tions : — 1. Blanchetown to the ]S"orth-west Bend, limestone. 

 2. North-west Bend to Overland Corner, partly limestone and 

 partly sandstone. 3. Overland Corner to the Grreat South 

 Bend, and 4. Thence to the Boundary. The third and fourth, 

 *' sandstone in various shapes, and clay."] 



The cliffs of the upper section of the river are composed 

 chiefly of sand, whilst those of the lower section are marine 

 calciferous sandstone, as long since observed by Sturt, who 

 writes " a remarkable change in the geology of the country, as 

 well as to an apparent alteration in the natural productions — 

 the cliffs of sand and clay ceased, and were succeeded by a 

 fossil formation" {loc. cit., IL, p. 139). Indeed, as I shall 

 hereafter explain, we have at Overland Corner — the locality 

 indicated by Sturt in the foregoing quotation — the junction of 

 two sets of unconformable beds. 



Htdeoqeaphical Notes. 

 On the accompanying map (pi. III., fig. 1) the unshaded 

 portion indicates a depressed area within the cliffs sufficiently 

 low to be inundated by high floods ; in the upper section of the 

 river this is relatively of great extent, and is closely reticulated 

 with creeks and lagoons ; in the lower section the river sweeps 

 along a line of cliffs, having on the other side an alluvial bank 

 beyond which is a more depressed area occupied by a lagoon 



