PROBLEM OF THE GREAT AUSTRALIAN ARTESIAN BASIN. 155 



One can only conclude that these bores have been sunk 

 very close to the points of location of hot sub-surface, 

 springs. 



In considering now the areal variation in increment — 

 apart altogether from the question of the abnormally steep 

 rate nearly everywhere — the modification introduced by 

 the slope of the water stratum should be analysed, such as 

 would be brought about when water in its upward or down- 

 ward movement crosses the isogeotherms. 



In descending, the water will be absorbing heat from the 

 enclosing walls, and in ascending, the reverse will be true; 

 owing to this fact therefore, and to the extremely low 

 diffusivity coefficient of rock, the temperature of the water 

 would lag slightly behind the normal geothermic value in 

 the first case, and exceed it slightly in the second, the 

 relative values of such amounts being dependent upon the 

 velocity of flow principally. In the Mound Springs the 

 water emerges, probably, without having lost an appreci- 

 able amount of its heat, but with the small rates of move- 

 ment and slopes actually experienced within the basin, the 

 difference between the temperature of the water-saturated 

 horizon and the neighbouring part of the earth's crust, 

 must be of a very small order, even after the lapse of great 

 intervals of time. 



This action may, sometimes, be quite appreciable, how- 

 ever, for upon comparing the Diags. I, II, and IV, the waters 

 descending into the "deeps" about Winton and Bimerah, 

 and again to the west of Bourke, are found to be relatively 

 cooler than elsewhere; this is also conspicuous in the 

 stretch from Barcaldine to near Dubbo, along which the 

 marginal bores show increments as gradual as 1° F. in 70 

 feet, steepening progressively outwards. 



More marked is this influence in the broad region across 

 the basin, from Kynuna almost to Burke, with a gradient 



