16 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 (11), 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 
Buck Creek 
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2400+67 HAST PORTAL AR ® Fura 
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TEMPERATURE °F. 
Zt 
2720 2700 2680 2660 2649 2620 
2580 2560 2540 2570 2500 2480 260 2440 2420 MOO 
STATION NUMBERS 
Fic. 1.—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, 40.0° F. at the en- 
trances of the tunnel, and 30.80° F. at the apex of the mountain were 
obtained by adjusting a straight line to the observed annual mean 
538 
