Spkioht. — Geological Features of ('JniafchurcJi Artesian Area. 433 



that such anomalies are frequent. The quantity falls oft' naturally if the 

 well be tapped at a height above the ground, but in certain cases a very 

 considerable flow is obtained even at a high level. Observations have not 

 been made to see if the diminution of flow follows the law stated by 

 .Slichter in the paper cited previously, but it is hoped that they may be 

 carried out. 



It is evident that not only the depth but the permeability of the strata 

 and the amount of supply affect the yield from any particular well. The 

 well-sinkers say that the best flows are obtained a little above the lower 

 boundary of the water-bearing stratimi, and, further, that where the 

 water is distributed throughout a water-bearing bed the flow is always 

 poor. These facts tend in the direction of the idea that there are well- 

 defined layers of water percolating, probably at a somewhat rapid rate, 

 through the porous gravel-beds of the area. When one considers the number 

 of wells, the large amount which flows from some of them, and the great 

 waste of water that is going on continuously, it will be readily realized 

 that they must derive the supply originally from some very prolific and 

 constant source, and one which is not very materially diminished by the 

 enormous tax on it. 



Source of Water. 



The water which supplies the wells appears to come from two sources — - 

 viz., the rainfall on the plains and the underground percolation through 

 the shingle from such rivers as the Waimakariri. That the first source 

 is undoubted is proved by the falling-ofi in the amount of water supplied 

 by the wells during a period of dry weather, and their immediate recovery 

 after rain. On one occasion the level of the water in the well at the Canter- 

 bury Museum jumped up 6 in. after several days' rain. The same eft'ect was 

 noted by Captain Hutton. The response is so marked that there is no doubt 

 of the source of the supply, nor that the permeable beds outcrop but a 

 short distance — probably a few miles — from where the wells are sunk. 

 However, as the rainfall over the Canterbury Plains in the neighbourhood 

 of the artesian area is comparatively low, and has an average of about 

 25 in. per annum, with a minimum of 14 in., recorded to date, it is evident 

 that this amount could not furnish a supply sufficient to satisfy the great 

 drain on it and keep fairly uniform, were there not some more reliable and 

 constant source. The only satisfactory explanation for this main supply 

 is that there is a very large amount of percolation through the shingle of 

 the plains from leaks in the river-beds, which finds its way across the out- 

 crops of the permeable beds and thus contributes towards the artesian 

 supply Such leaks do undoubtedly occur, and dm'ing dry seasons a very 

 large proportion, if not all, of the water in certain of the shingle river-beds 

 is flowing imderground. At a small depth there is usually an abimdant 

 supply, even when they are perfectly dry on the surface, and it takes 

 but a slight shower of rain to make such a river again flow on the sur- 

 face. In some of the rivers, like the Selwyn, the water disappears com- 

 pletely from sight even during normal seasons, to reappear miles lower down 

 at the surface where the underlying beds are not so permeable. Many 

 springs also occur along a belt which runs approximately through the place 

 where the Selwyn water comes to the surface parallel to the coast-line and 

 near the upward hmit of the known artesian area. It is probable that the 

 containing impermeable beds of clay and hard sand reach their furthest 



