SURVEY OF MUNICH TERTIARY BASIN 95 
LESSER MAGNETIC ANOMALIES 
The heterogeneous jumble of more or less irregular residual mag- 
netic maxima and minima within the Tertiary basin (Fig. 8) at first 
sight gives little suggestion of any fundamental underlying structural 
plan in connection with the anomalies. It is probable that a considera- 
ble part of the anomalies are not produced by geologic structure. 
Practically all detail magnetic surveys have many anomalies which 
seemingly are not produced by geologic structure. The data usually 
are not sufficient to determine what is the cause of these anomalies. 
Some of them presumably are apparent rather than real and are pro- 
duced by observational errors, by accidental coincidence of several 
surficial anomalies of the same sign in consecutive stations, and by the 
incomplete covering of the area by the survey and the consequently 
inaccurate inference in regard to the course of isogams in the un- 
mapped areas between surveys. Others of these anomalies presumably 
are produced by variations in the content of magnetite and ilmenite 
in the sediments. Many of the anomalies of Figure 1 presumably are 
of one of these types or a combination of these types. 
A certain more or less definite pattern does stand out, however, 
after closer |study of Figure 10 and suggests some fundamental un- 
derlying structural plan and cause for many of the anomalies. The 
magnetic axes are Shown on Figure 11. Some of them, such as the Alb 
and Danube axes, plainly are actual axes and must reflect geologic 
structure in the subsurface. Others of the lines may be the result of 
subjective connection of accidentally aligned independent local anom- 
alies. 
The Passau axis, however, can be seen probably to be the effect of 
differences of magnetic permeability in the basement. The Passau 
axis of maximum is well delineated. Its amplitude is above the average 
of the anomalies other than those of the Alb and Donau axes. It has 
been crossed by sufficient traverses to indicate that it does exist. But 
unfortunately it does not cross or approach any of the torsion-balance 
work. The eastern end of the maximum, as the B. M. I. mapped it, 
extends into the area of the outcrop of the granite and the meta- 
morphic rocks; and for a considerable distance in the area slightly 
farther west, the depth to the granite and the metamorphic rocks can 
not be great. A nose of metamorphic rocks projects from Passau east- 
ward into the area of granite exactly on the prolongation of the Passau 
axis of maximum. From the B. M. I. magnetic data and the other 
data which are available, genetic connection between that nose of 
metamorphic rocks in the granite can not be proved. But it seems 
617 
