80 DONALD C. BARTON 
The regional variation of gravity (Ag,) is shown by the isogams of 
Figure 5. That regional variation shows the following significant 
characteristics: (1) a general, fairly uniform southward decrease in 
the intensity of gravity, (2) the suggestion of a broad, faint gravity 
nose through Landshut, and (3) slight curvature of the isogams as if 
around a basin, or else as if in conformity with another gravity nose 
through Augsberg. 
INTERPRETATION 
ISOSTATIC EFFECT 
A southward decrease in Ag, across the Munich basin would be 
produced by the root of the Alps if they are in isostatic equilibrium. 
According to the isostatic theory, the excess mass of the projection of 
the mountains above the general level must be compensated by an 
equivalent deficiency of mass in the root of the mountains. That sub- 
surface deficiency of mass would produce a gravity minimum which 
would be concentric with the mountains and which will extend out 
from the edge of the mountains for a distance equivalent to two to 
three times the depth of the center of gravity of the deficiency of the 
root. 
The isostatic effect of the Alps at a maximum could produce only 
a third of the observed variation of Ag,; and no such isostatic effect 
need be postulated to explain the observed variation of Ag. The re- 
sults‘of approximate calculations of the Ag, which would be produced 
by an isostatically compensated Alpine mass are shown in Figure 6. 
The following assumptions were used: (1) The excess of mass of the 
Alps above the general level of the surrounding country is compen- 
sated by a corresponding deficiency in the mass of the root of the 
Alps; (2) the mean density of the Alps above ground is 2.5; (3) the 
Alps consist of a prism infinite in the east and west directions, 120 
kilometers wide in north and south directions, rising 2.5 kilometers 
above the general level, and extending 25 or 50 kilometers below the 
general level; (4) the deficiency in mass is distributed uniformly 
through the vertical zone o to — 25 kilometers; uniformly through the 
vertical zone o to — 50 kilometers, uniformly through the vertical zone 
—25 to —50 kilometers, and uniformly through the vertical zone o to 
— 100 kilometers. It is evident frem Figure 6 that at a maximum, only 
a third of the observed variation of Ag, north-south across the basin 
could be produced by an isostatic deficiency of mass in the root 
of the Alps. If any of the calculated Ag, curves of the isostatic effect is 
subtracted from the curve of observed Ag,, a resultant curve will be 
obtained which will indicate that at Rosenheim and Holzkirchen, the 
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