142 ALEX. L. DU TOIT. 



the geological structure in this quarter will be referred to 

 later. 



The main portion B of the basin deepens rapidly to the 

 south-west of Longreach, and the floor goes down to nearly 

 4,500 feet below sea-level around Bimerah. The region 

 westwards is as yet unprobed, and the floor may in points 

 descend to depths of well over a mile ; in South Australia 

 the deep bores at Mount Gason and Goyder's Lagoon have 

 not struck bed rock, although the bottom of the last named 

 is over 4,700 feet below sea-level. Along the margin here- 

 abouts, the now buried land surface was one with consider- 

 able topographical irregularity, and this unevenness seems 

 to have been aggravated by Tertiary faulting, as at Warrina. 



Owing to the scarcity of bores within this huge western 

 region, the shape of the floor is almost unknown, but to 

 the north of Broken Hill there are long partially exposed 

 ridges of palaeozoics, forming the Stokes and Grey Ranges, 

 stretching away into Queensland to the neighbourhood of 

 Thargomindah, and only by means of boring can it be proved 

 whether perhaps this sub-surface feature continues north- 

 westwards. It may well be that the western portion D is 

 only imperfectly connected with B, in the same way that 

 B is linked to E. Some further remarks concerning these 

 ridges will be made in treating of the Tertiary History of 

 the Basin (Section IX). 



The widespread covering of superficial deposits conceals 

 to a large extent the boundary between the lower-lying 

 Mesozoic infilling and the marginal rim of older rocks, so 

 that the delimitation of the basin has presented many 

 difficulties, more particularly along its southern side. 



In South Australia, Ward has concluded (I. §§47-51, 93, 

 166-204) that the west-south-western boundary of the 

 basin follows more or less closely the line of railway, with 



