438 T, W. E. DAVID. 
over and preserved by a layer of siliceous material or of sand and 
gravels cemented together by silica. Siliceous springs on the — 
contrary tend to choke up their basins by continually depositing 
silica in successive films on their walls and around their margins, 
and not to form a crust over the surface and thus enclose supplies 
of water. He also stated that the artesian wells of Christchurch, 
New Zealand, were not in a true artesian basin, such as those of 
London and Paris, but that at the former place there is a bed of 
gravel and sand charged with water, overlaid by a less porous 
stratum and that it is thought that the water is in part squeezed 
up through the comparatively shallow bores by the superincum- 
bent pressure [just as water would be forced up through a perfor- 
ated board pressed down upon a sponge charged with water], and 
this idea is borne out by the shallow wells gradually giving out. 
The main bulk of the water is however probably forced up by 
hydrostatic pressure from the higher outcrops of the porous beds 
along the hills. 
Mr. L. WHITFELD said, if the theory put forward by Professor 
David is correct, it should be easy to test it. The highest point 
to which the water will rise at the bore can be easily ascertained. 
The lower end of the subterranean channel being open to the sea, 
the height to which the water can rise at the bore must in the 
absence of further supply be continually diminishing. The extent 
and locality of the intake area are given by Professor David and 
the rainfall upon it can be ascertained, and thus it can easily be 
determined whether a period of dry weather over the intake area 
is accompanied by a constant diminution in the height to which 
the water will rise at the bore, and whether a fall of rain causes 
a rise in the height to which the water will flow. Also by observing 
which bores are affected by the rainfall on particular spots of the 
intakearea, the direction of the subterranean flow can bedetermined. 
Professor Davin stated with regard to the first theory put for- 
ward by Captain F. B. Gipps that one cause of the rise of artesian 
water was gas pressure, that the instance cited, that of the artesian 
well at Kissingen in Bavaria, was of a very exceptional character. 
