SURVEY OF MUNICH TERTIARY BASIN 83 
tion 4,000 meters thick; geologically, such a gradation in the Munich 
basin is most improbable. 
The west-southwesterly regional decrease of Z across the basin 
presumably must be produced by a west-southwesterly slope to the 
surface of the crystalline basement. Theoretically, that regional varia- 
tion of Z, could be produced also either by (1) a vertically uniform 
horizontal variation in magnetic permeability through a great verti- 
cal thickness, or (2) very special conformations of rock of high mag- 
netic permeability, within the basement. Neither of those two possible 
causes of the observed southwesterly decrease in the intensity of 
AZ, is probable geologically. Granitic-gneissic masses commonly have 
a higher magnetic permeability than most sediments. The surface 
on the crystalline rocks of the Bohemian massif dips under the Ter- 
tiary sediments. The most probable explanation of that observed re- 
gional variation of Z, therefore, is that the southwesterly slope of 
that surface presumably must continue across the basin under the 
Tertiary beds and under the Mesozoic beds. 
The failure of the gravity picture to reflect the southwesterly dip 
of the surface of the crystalline basement across the basin and the 
failure of the magnetic picture to reflect the southerly dip of the con- 
tact between the Mesozoic beds and the Tertiary beds is the effect of 
the independence between the density and magnetic properties of 
rocks. The crystalline rocks of the basement must have a high density 
and a high magnetic permeability. The Mesozoic sediments, or at least 
the Jurassic-Triassic sediments, must have a relatively high density, 
probably about the same as that of the underlying crystalline rocks, 
and presumably a low magnetic permeability. The Tertiary sedi- 
ments seem to have both a low density and a low permeability. The 
contact between the Tertiary and Mesozoic sediments therefore pro- 
duces a gravity effect but no magnetic effect; and the contact between 
the sediments and the underlying crystalline basement produces a 
magnetic effect but no gravity effect. 
The curvature of the magnetic isogams into a southwest trend in 
the southeastern part of the basin, in front of Salzburg, suggests the 
rise of the basement southeastward and the extension of the crystal- 
line rocks of the Bohemian massif southwestward under the Austrian 
continuation of the basin, perhaps to connect with the crystalline 
core of the eastern Alps. The moderate depth of the granite at Wels 
in Austria shows that the crystallines extend out at least to the mid- 
dle of the Austrian Tertiary belt.‘ 
4 That conclusion was reached before the writer came across Robert Schwinner’s 
“Magnetismus in Bohmischer Masse und Ostalpen,” Gerlands Beitrage zur Geophystk, 
Bd. 39, Heft 1 (1933), pp. 58-81. a 
