232 ECONOMIC GEOLOGY 
Western Australia, and in Dakota and the adjacent parts of 
the United States, 
The London Basin consists of a synclinal of chalk between 
beds of clay, and when the chalk was first pierced many of the 
wells flowed at the surface. The water-level has however 
fallen, and the wells have become subartesian ”’ or * arte- 
soid,” as the pressure only raises the water part of the way 
and it has to be pumped to the surface. The water-level 
of the central London wells falls between 1 and 2 feet a year 
as much of the water is an old accumulation, or is water of 
cisternage. After it is exhausted these London wells will 
yield only the small supply afforded by percolation of water 
from the out-crop. 
All flowing wells and the flow of oil wells were once attri- 
buted to water-pressure. Some flowing wells are, however, 
due to rock-pressure. Venice stands on a sheet of clay 
containing lenticles of sand charged with connate water. 
If a bore enters one of these lenticles the weight of the 
over-lying clay and city squeezes out the water as out of a 
sponge. The well flows at first under high pressure, which 
falls as the lenticle is relieved of its surplus water. 
Rock-PressurE—Flowing wells due to rock-pressure may 
be illustrated by those at Kynuna in Queensland (Fig. 60) 
(Econ. Geol., ix, 1014, pp. 768-75). The water there comes 
from 22 thin layers of sand and sand-rock which are interstra- 
tified with beds of shale and occur at depths between 270 and 
2179 feet. The water from the first water-bearing layer rose 
only 40 feet in the well; from the next layer, at 420 feet deep, 
it rose 80 feet; and the rise increased with the depth until 
from all the layers below 1857 feet the water overflowed from 
the mouth of the well. The greater the rock-pressure the 
higher the water rises in the well. Many of the water-bearing 
layers are so thin that they must be small in extent and the 
pressure in the Kynuna wells cannot come from water in 
distant hills. It must be due to a local source, and as the 
aprise steadily increases with the depth, the discharge is 
doubtless due to the weight of the overlying rocks. 
Rock-pressure may also explain the loss of head and 
decline in flow of some wells in the Dakota Sandstone. Near 
Edgeley in North Dakota the hydraulic gradient in 1886 was 
about 4 feet to the mile; but in the past 40 vears it has