426 Transact lo II ■•<. 



catchment-area and well-head is sufficient a slight flow may be obtained 

 almost anywhere ; but it is essential to abundant flow, and, indeed, to the 

 slightest flow, unless other conditions are exceptionally favourable. The 

 most favourable condition of supply is found when the porous formation 

 rises from beneath less porous strata and forms the surface over a broad area." 

 After this general statement of the conditions governing artesian wells 

 we must pass on to consider those which affect the particular area under 

 consideration. 



Extent. 



The artesian area referred to in this paper consists of the belt of country 

 which fringes the coast from the mouth of the Ashley River southward 

 through Kaiapoi, Christchurch, Tai Tapu, Ellesmere, to the mouth of the 

 Rakaia. Its length is nearly fifty miles, but its breadth varies consider- 

 ably, being very narrow at the north and south, and reaching its greatest 

 width in the neighbourhood of Christchurch, where it is about ten miles 

 wide. Its inland boundary is roughly marked by the ra Iway-line which 

 runs from Christchurch north to Leithfield and south to Soutlibridge. In 

 one or two places the area extends slightly over this approximate boundary, 

 the artesian well farthest from the sea being at Islington, on the Main South 

 Line, about eight miles from Christchurch. 



Structure of the Area. 



The structure of the artesian area has been arrived at by considering 

 the records of well-sinkers and plotting the depths and character of the 

 beds encountered on sinking. In order to show this, the records of a large 

 number of wells hav,' been drawn in a series of vertical sections. Of course, 

 it is impossible to give the records of all the wells that have been examined, 

 but a large number of typical ones has been selected on lines run in various 

 directions through the area, so that a fairly accurate conspectus of the 

 structure can be obtained. In drawing up these records no note has been 

 taken of the height of the surface of the ground above sea-level in each case, 

 since this would have necessitated careful levelling at the time of sinking. 

 It was impossible to do this, as many of the wells were sunk over fifteen 

 years ago ; also, owing to excavations and filling in, the level of the ground 

 in their vicinity has changed in the meantime in many cases, though by no 

 great amount. In any case, the error arising from this neglect can be very 

 sUght, as the height of the surface of the ground over by far the largest 

 part of the artesian area does not exceed 15 ft. above the sea, and its level 

 is remarkably uniform. This omission does not affect to any marked degree 

 the question of geological structure, although it does affect the questions 

 of water pressure and flow. It is to be hoped that at a later date some 

 information may be forthcoming on the purely hydraulic problems that 

 the wells of the area furnish. 



The sections exhibited on the plates will be taken in turn and their 

 special features indicated. 



Series No. 1. — From Islington to the Sea-coast at New Brighton. 



(Plate IX.) 



Islington is situated at an elevation of 112 ft. above the sea, and is the 



place furthest from the coas': at which artesian water has been obtained. 



An examination of the section will disclose that the beds are not laid down 



in any markedly regular sequeuce, but are of alternating layers of (;oarser 



