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

WATER SUPPLY 225 creeps along them by “ capillary attraction”; it may thus rise against gravity, but it cannot be forced through by pressure of a “ head ” of water. Larger tubes and spaces are super-capillary, and water is driven through them by gravity and gas-pressure. In small tubes, which may be compared to the small fissures and passages in rocks, the flow of water is controlled by friction, which increases directly with a decrease in diameter and with an increase in length, increases as the square of the velocity, and increases with the roughness of the inner surface of the tube. In tropical and warm temperate countries with an annual rainfall of less than 20 or 25 inches, and a fine-grained uniform soil, run-off and evaporation may remove the whole of the rainfall, and there be none left for percolation. Nevertheless the upper layers of the crust have been often represented as so charged with water that a deep well will be successful anywhere. A. Delesse (Bull. Soc. Géol. Fr. (2), xix, 1861, p. 64) estimated the amount of subterranean water as about equal to that in the oceans, and Slichter (U.S.G.S., Water Sup. Pap., No. 67, 1902, pp. 14-15) accepted a third of this quantity. The existence of this subterranean sea was based on the principle that a current of water uses the whole channel open to it. Thus when a stream of water enters a trough at one point and flows out at the opposite point, it does not pass straight across the trough ; the current widens and deepens till all the water shares in the movement. Hence it was held that water percolating through the crust must spread widely downward and sideways until it saturates the crust. Deep bore holes and mines, however, after passing through a wet zone often reach rocks that are quite dry, although they were deposited in the sea and must have been saturated with connate water. The slope of the water-table depends on the friction. Water poured into an empty U-tube rises to the same level in both arms, because the friction is negligible. If the tube be filled with sand, the friction is appreciable, and water poured into one arm rises slowly in the other; if the lower part of the tube be filled with clay the water only penetrates the clay by surface-tension, and the ‘‘ head ™ or pressure of the water has no effect, and none passes through the clay into the other arm.

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