. Water Supply and Irrigation Papers of the United States Geological Survey . areaof the covering strata is certamente non meno di 1: 3,000, per lo spessore dell'età del risparmio del Potsdam è di circa 700 piedi, mentre la formazioneè nota per esistere come una roccia acquatica per almeno 400 miglia a sud del bacino idrografico. Lo spessore medio della pietra arenaria del Dakota è di circa 200 piedi, mentre la distanza dalla catchmentarea nelle Black Hills allo affioramento orientale a Yankton è di circa 350 miglia, in modo che il rapporto dell'area della sezione trasversale trasmittente degli strati porosi all'area di

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. Water Supply and Irrigation Papers of the United States Geological Survey . areaof the covering strata is certainly no less than 1: 3, 000, for the aver-age thickness of the Potsdam is about 700 feet, while the formationis known to exist as a water-bearing rock for at least 400 miles southof the catchment area. The average thickness of the Dakota sand-stone is approximately 200 feet, while the distance from the catchmentarea in the Black Hills to the eastern outcrop at Yankton is about 350miles, so that the ratio of the area of the transmitting cross section ofthe porous strata to the area of the covering strata is about 1: 9, 000.Thus if, on the averaire, the covering material had in the one case onlyj, , 1, , , , and in the other only 9-0V0 °f the transmission capacity of thewater-bearing stone, there would still be as great opportunity forthe upward escape of the water as for its onward movement. Thisupward seepage would cause motion of the water in the porous strata, so that motion is possible even if there be uo actual outcrop of thetransmitting rock. Anticline. Fig. 21.—Diagram illustrating how gas and oil have been retained beneath anticlines of shaleduring protracted geologic intervals, thus proving the substantial imperviousness of thecovering. Local upward seepage may explain, perhaps, some of the anomalousvariations in pressure of underground water in artesian basins. It iscertain, however, that upward leakage is entirely absent over largeareas. In the Ohio oil regions, for example, all of the oil and naturalgas, not being continually supplied to the rock as water is, would haveescaped ages ago if we suppose the most minute permeability to theoverlying strata of shale. The gas and oil are held under the an-ticline of shale as shown in fig. 21, just as air may be held under abasin inverted in a tub of water. It would seem equally reasonableto suppose that the briny water which underlies the petroleum hasoccupied its present position during the same geologic i

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• bookauthorunitedst
• bookcentury1900
• bookdecade1900
• bookyear1902