. Early geophysical papers of the Society of Exploration Geophysicists. Petroleum; Prospecting. i6 C. E. VAN ORSTRAND sea-level. At this depth, the pressure is supposed to be constant, that is, the weight of each column of rock per unit area extending from the level of isostatic adjustment to the surface of the ground is constant, regardless of the height of the column. This condition implies that the density of a mountain mass is less than that of the rocks beneath the adjacent plains or beneath oceans. Two hypotheses have been proposed to explain the geodetic facts. According to Pratt, the c
. Early geophysical papers of the Society of Exploration Geophysicists. Petroleum; Prospecting. i6 C. E. VAN ORSTRAND sea-level. At this depth, the pressure is supposed to be constant, that is, the weight of each column of rock per unit area extending from the level of isostatic adjustment to the surface of the ground is constant, regardless of the height of the column. This condition implies that the density of a mountain mass is less than that of the rocks beneath the adjacent plains or beneath oceans. Two hypotheses have been proposed to explain the geodetic facts. According to Pratt, the crust of the earth is of uniform depth, the density varying inversely as the elevations of the earth's surface, while according to the Airy, or the "Roots of Mountains" theory (n), the crust of the earth is supposed to be of variable thickness, and the mountains are supposed to float in the deep-seated magma just as icebergs float in water. The magmatic. 2720 3700 Fig. i.—Observed temperatures in Moffat tunnel, Denver, Colorado. rocks are supposed to be sufficiently plastic to yield to the pressure induced by the lighter strata above them and at the same time they are supposed to possess sufficient strength to carry the load imposed upon them. Quite curiously, advocates of each hypothesis have appealed to geothermal data as a means of deciding the issue. Osmond Fisher (12) contended that the isogeotherms beneath a mountain may pass from the convex to the plane type and ultimately become concave upward. Lees (13) rejects this hypothesis. He does not believe that a mountain has solid roots extending downward into a plastic substratum and that the isogeotherms pass from the convex to the concave type. In another publication, the writer will give a detailed application of Lees's equations to the temperature data obtained by the writer and Burgis G. Coy, resident engineer, in the Moffat tunnel. These observations are shown in Figure 1. The values, F. at the en- tran
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