ARID PORTIONS O:^ SOUTH AUSTRALIA. 41 



ing along some portion of the rift valley. No important rivers empty 

 into Lake Torrens and what water it holds is derived directly from 

 such rains as fall on its surface. It is well without the 10-inch isohyet, 

 the average rainfall of the basin being probably but little over 6 inches. 



Figure 13 (Taylor, I. c.) gives concisely the main points in the geo- 

 logical history of the rift valley of the central portion of the state. 

 As Taylor explains, in "A" is given a hypothetical diagram showing 

 how in former ages there was a well-developed and centrally situated 

 drainage system which led from the Barkly tableland on the north down 

 to Jeffrey's Deep. The general course of the valleys is north and south. 

 The Central Highlands, comprised by the Flinders and adjoining 

 ranges, have not yet been formed. In "B" important alterations are 

 seen to have taken place. The sea has encroached on the land to the 

 south, advancing up the basin of the Murray River. In the mean- 

 time, epeirogenic movements have raised an elongated plateau in the 

 south, and this has affected all of the river courses, in places blocking 

 them and bringing about the formation of lakes. We thus see the 

 origin of all of the leading lakes and basins. In "C" the western por- 

 tion of the uplift is seen to have slipped in, forming Spencer's Gulf and 

 Gulf St. Vincent. The MacDonnell Ranges (Northern Territory) 

 have arisen and the Lake Eyre Basin has sunk away from the earlier 

 grade. The early river system may possibly date back as far as the 

 Cretaceous, when a vast sea covered the western portion of Queensland. 

 It seems certain that in the early geologic times the rainfall was heavy 

 in central and northern South Australia and in central Australia. 

 Taylor suggests that the heavy rainfall in the past may have been 

 due to the presence of great arms of the sea to the east, such as the 

 Tertiary sea at the mouth of the Murray. 



The Murray-Darling lowlands are very extensive. Taylor esti- 

 mates the area to be an approximate square of about 400 miles on 

 each side. Of these only a small portion is included within the state 

 of South Australia, and it is wholly comprised of the ancient Murray 

 estuary or bay, into which, in earlier geologic times, the Darling and the 

 Murrumbdigee, as well as the Murray, emptied by separate mouths. 

 The ancient estuary is for the most part flat, except an area in the 

 southeast, where there are sandhills only a few feet above the sea. 

 According to Howchin, for example, at the point where the River 

 Murray enters the state, its summer level is but 57 feet above sea-level. 

 At Morgan it is only 5 feet 4 inches, which gives a gradient of the river 

 of only 0.5 inch to the mile. The banks of the river at Blanchtown 

 are approximately 150 feet higher than the level cf the stream at 

 low water. As one overlooks the area from the mountains to the 

 west, where the view is very extensive, it is unrelieved by any eminences 

 whatever. This flatness, together with the blueness of the distant mallee 

 forests, gives the impression that one is looking over the sea. 



