424 TriniHtirf ioitx. 



of accumulation near Birdling's Flat must be extremely rapid, as old maps 

 show that Lake Forsyth had in historical times a permanent outlet to the 

 sea, and later still the seas used to break over the narrow spit of shingle 

 which divided the lake from the sea ; and more recently, within the memory 

 of the present writer, this bank has increased in breadth by several chains. 

 This increase has taken place in spite of a probably downward movement 

 evidenced by the presence of submerged forests in Lake EUesmere. 



The absence of any similar spit on the south side of Banks Peninsula 

 between it and the plains is strongly suggestive that they have not been 

 beneath the sea since the present land-surface was formed, as spits would 

 have formed there if it had been depressed below its present level. The 

 presence of a strong northerly current closely hugging the coast, and pre- 

 venting the accumulation of silts and similar non-permeable beds inter- 

 stratified with the shingle, explains the general absence of artesian areas 

 to the south of the Rakaia. 



The volcanic mass of Banks Peninsula is a fundamental factor in the 

 formation of the Christchurch artesian area, since under its shelter have 

 accumulated the finer sediments interstratified with coarser material 

 without which artesian conditions would have been impossible. Imme- 

 diately to the north of it the width of the " flowing well " belt is the greatest, 

 and it narrows down as the distance north and south from it is increased. 

 No doubt the northerly extent would be greater were the hills of North 

 Canterbury further away, for the supply of artesian water falls off as they 

 are approached, and fails altogether in their vicinity. The genera! structure 

 of the area, as subsequent examination will show, is that of irregular beds 

 of shingle and sand parted by clay-beds. They are not arranged in basin 

 shape in accordance with the usual text-book diagrams, but in beds with 

 a general slope to the east. 



The water finds its way into the gravel-beds and flows seawards, and in 

 all probabiUty it has an outlet beneath sea-level to the eastward of the 

 present shore-line. Wlien a pipe is put down to the water-bearing stratum 

 the water follows the path of least resistance, and if the surface of the ground 

 is below the cfEective head of water the well will be a flowdng one. This 

 seems to be the principle on which the flow is dependent. 



It is probable that during the formation of the artesian beds there has 

 been a struggle between two forces — viz., the gradual sinking of the land, 

 as evidenced by the presence of peat-beds several hundreds of feet beneath 

 the present area sea-level, and the gradual building-up of the plain by aggra- 

 dation. In a limited area parallel to the coast-Hne marine beds may have 

 been laid down, and as sedimentation proceeded the area has been covered 

 up by shingle from the rivers. 



Peesent Surface Conditions. 

 The former conditions can be most easily realized by taking the present 

 features of the area. In the sea off the present coast-line are beds of sand 

 and mud ; in certain places, as the estuary of the Avon and Heathcote, 

 patches of both occur in close conjunction. This estuary tends to silt up on 

 account of the gradual increase in height of the half-tide flats by the action 

 of Zostera and other halophytes entanghng the finer sediment carried by 

 the rising tide and incoming fresh-water streams. Thus over the sand-beds 

 a coating of fine silt is deposited. Then again, along the coast and also in 

 difJerent places inland, probably marking old shore-Unes, are rows of sand- 

 dunes. Between them are usually peat-bogs of no great depth, the coating 



