The Elements of economic geology

Bibliographic data

Monograph

Identifikator:: 1773832379

URN:: urn:nbn:de:zbw-retromon-172798

Document type:: Monograph

Author:: Gregory, John W. http://d-nb.info/gnd/11683014X

Title:: The Elements of economic geology

Place of publication:: London

Publisher:: Methuen

Year of publication:: 1928

Scope:: XIV, 312 S.; graph. Darst.

Digitisation:: 2021

Collection:: Economics Books

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Chapter

Document type:: Monograph

Structure type:: Chapter

Title:: Part IV. Engineering geology

Collection:: Economics Books

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

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