1352 EDWARD D. LYNTON 
tiary or Cretaceous and contains irregular masses of basic volcanic rocks, 
in many places altered to serpentine, which have a magnetic susceptibility 
as great as 7,000 X 10°, so large as to obscure results on younger beds 
in the vicinity. 
Of the pre-Jurassic rocks in California, the granites, both of the 
Sierra Nevada and the Coast Ranges, generally have a susceptibility 
less than 200 X 10°, but in many places thev contain large bodies of darker 
rock of gabbroid appearance that are intensely magnetic—sufficiently so 
that a small hand specimen deflects a compass needle. Many of the 
metamorphic rocks of the Sierra Nevadas also are extremely and irreg- 
ularly magnetic. This must be kept constantly in mind in attempting 
magnetometer work in the San Joaquin Valley, particularly. 
The chief practical application of magnetic methods in California 
has been in tracing known structural features, such as faults and “mag- 
netic marker beds’’ which present eroded edges on the flanks of folds, 
from places where they can be seen into regions covered by alluvium. 
It is not safe in the present stage of the art to attempt to interpret mag- 
netic data without some support either from surface geology or from well 
logs. It is necessary always to keep in mind that the observed results 
are affected by all the material beneath, so that many anomalies caused by 
the formations of principal interest are obscured by unrelated conditions 
below them in rocks of greater magnetic permeability. For example, 
it would be very poor engineering to attempt to interpret in terms of 
structure either a magnetic or a gravity survey of an area known to be 
underlain by Franciscan rocks at a depth of less than 5,000 feet, because 
very magnetic and dense bodies of basic igneous rock are probably dis- 
tributed:irregularly within. 
Most work consists of measurements and mapping of the vertical 
component of the earth’s magnetic field. For recognizing faults not 
known by surface evidence, and for other special problems, measurements 
of horizontal intensity also are required, but are made only where the 
vertical data indicate that they are necessary for use in interpretation. 
Ordinary common sense should be used in laying out magnetic surveys, so 
that time will not be wasted in attempting to solve problems to which 
: the method is unsuited. For most useful results, the following conditions 
must exist. ; 
1. The rocks of economic interest in the problem must have mag- 
netic contrast, that is, must contain some beds known to be markedly 
more or less magnetic than the average. 
70 
