Speight. — Qenlofiical Fcofitrfis of Chihfchurr]i Artesian Area. 435 



the sea, and that there is a possibility of mixture oecurring between the 

 salt water aud fresh under these circumstances. It will generally be thought 

 that an outlet under the sea would at once destroy the possibility of getting 

 artesian water near the shore, but I think it can be shown that even with an 

 outlet the presence of a counterpoise of sea-water may be almost as effective 

 as an impermeable bed blocking the outflow. The conditions will resemble 

 to a certain extent those of the well-known laboratory experiment of the 

 U tube, with its branches filled with unequal lengthened columns of liquids 

 of unequal density which balance one another. It is perfectly possible 

 to draw off a supply of the lighter liquid from a level above that of the 

 heavier liquid. If now we suppose that salt and fresh water are the two 

 liquids, then if we have a constant supply of fresh water carefully intro- 

 duced it will be possible to draw off a continuous supply of fresh water at a 

 higher level. Let us now apply this experiment to the circumstances on the 

 shore of the artesian area — say, at New Brighton. In this case the rise of 

 the tide is about 6 ft., and the depth of the first-stratum wells near the shore 

 about 144 ft. We may suppose, therefore, that the water-bearing stratum 

 outcrops at an approximate dejDth of 144 ft. below low water. Talcing the 

 specific gravity of sea-water as 1-025, the length of a column of fresh water 

 which would exactly balance this would be 144 x 1-025 — that is, 147-6 ft. 

 So that a well sunk at the level of low water and supplied with a continuous 

 amount of fresh water from inland would flow at about .3 ft. above the 

 surface. If now the tide rises, the well will rise with it. Theoretically, 

 a rise in the tide of 6 ft. with no admixture of the liquids would cause the 

 well to flow 6 ft. higher. The New Brighton wells do not vary as much as 

 this, a discrepancy due to a certain extent to the friction which obstructs 

 the rapid flow of water through the beds, thus diminishing the effective 

 pressure, and also to the admixture of the salt water with the fresh at the 

 bottom of the well, which also reduces the difl'erence betAveen the level of 

 the sea and that of the wells. This mixture must take place owing to the 

 disturbed conditions at the bottom of the well, due principally to the move- 

 ment which must take place as the water is drawn off. This explanation 

 accounts for the mixture of salt water with fresh in the case of wells near 

 the shore, and also the falling-off in saltness as well as in the tidal effect 

 on increasing the distance from the shore. 



Conclusion. 



It is hoped that the diagrams given with this paper may be of some 

 interest and use to the general public, as they afford a certain amount of 

 information as to the depths at which the water-bearing beds are to be 

 found, and seeing that these records are put into such a form as to be readily 

 understood. 



The main results of this preliminary investigation of the area are to 

 demonstrate, — ■^ 



(1.) That the geological arrangement of the beds is not so irregular as 

 Avas anticipated at first, but that certain probable marine beds are persistent 

 over considerable areas. 



(2.) That the water-bearing beds, being gravel, and in all probability 

 laid down on a land-surface or by the agency of strong and varying currents, 

 are liable to great variation in thickness and also to be split up by inter- 

 calated sandy or clay beds. This increases the number of water-bearing 



