PROBLEM OF THE GREAT AUSTRALIAN ARTESIAN BASIN. 157 



the presence below the basin of an extensive mass, or 

 more probably a series of masses of igneous matter intro- 

 duced into that position in late Tertiary times (see Sec. X). 



Koenigsberger and Miihlberg 1 in their analysis of the 

 causes of variation in geothermic gradient have shown 

 that values of from 27 to 21 feet per degree Fahrenheit are 

 found only in regions that have been subjected to volcanic 

 action in late geological times. That many thousands of 

 years would have had to elapse before the thermal additions 

 of this nature became dissipated, can indeed be shown by 

 the calculations of Ingersoll and Zobell upon the rate of 

 eooling of laccolitic intrusions. 



(2.) Secular Changes in Temperature. 



In such bores as have been re-examined after the lapse 

 of some years, differences in temperature have not infre- 

 quently been observed; these are about as commonly 

 positive as negative. The values range from 1 to a maxi- 

 mum of 9 degrees, but, when differences of a half or a whole 

 degree appear, such are probably generally due to instru- 

 mental or other errors. 



For the purpose of investigating this problem, the differ- 

 ences between the figures given in the two Interstate 

 Reports, and corresponding to intervals of at least two 

 years, therefore, were plotted for part of the New South 

 Wales basin. 



The analysis shows a fall of from 1° to 4° F. over the 

 greater part of this area, but Oarinda and Warren have 

 rises of 6J° and 9° F. respectively, while a little to the north 

 and to the west of Moree there are areas exhibiting in- 

 creases of from 1° to 8° F. In the same district there 

 appear the following contrasts in bores, the individuals of 

 each pair of which adjoin one another, are down to bed- 



1 Trans. Ins. M.E., Vol. xxxix, pt. 4. especially table 4, 1910. 



