MAGNETOMETER SURVEYS IN CALIFORNIA 1353 
2. The feature sought must have broken or deformed the strata 
so as to have caused a considerable difference in elevation of some bed 
of marked magnetic characteristics within the limits of the survey. As 
an illustration, it is a waste of time to try to trace magnetically a fault 
of small vertical displacement in the Kern River formation, which has 
little magnetic contrast. 
Field parties —Our practice in California is to have a party consist- 
ing of two geologists, one car, and two vertical magnetometers. This is 
found more economical because two readings with different instruments 
can be taken at the same time within a short distance of each other and 
their values averaged if within a few gammas. This method establishes 
the value of the point without further checking, and informs the oper- 
ator immediately if either balance is out of adjustment. 
INTERPRETATION 
Presentation of results —In order to interpret the results of a magnetic 
survey, it is necessary to show the values determined by the work in their 
true relation to topographic and known geological features on a map of 
the area surveyed. We have used the following methods. 
1. Peg model of vertical intensities. If enough points have been 
occupied, the tops of the pegs show a magnetic surface from which much 
can be inferred about the course of faults, magnetic marker beds, and 
other structural features (Fig. 1). A model of this type is inconvenient 
to make, transport, or file, and can be shown in the report only by pho- 
tographing it. 
2. Celluloid profile model of vertical intensity. This is made by 
placing a large sheet of celluloid over a map and cementing to the celluloid 
cover profiles cut from the same material representing the vertical in- 
tensities to scale above each station. 
This type of model is more useful than the foregoing because the 
celluloid is transparent and does not obstruct a view of the map as the 
pegs do. It is serviceable as a guide in preparing magnetic contour maps 
because the magnetic surface can be seen without use of the imagination. 
It has the disadvantages that it can not be easily transported and can 
be included in reports only by photographing it (Fig. 4). 
3. Magnetic contour maps. In this method of presentation, con- 
tour lines are drawn connecting points of equal vertical intensity exactly 
as contour maps of topography are made. Contour maps are easily 
made and do not greatly detract from the original map on which the con- 
tours are drawn. The contours show all the major features and a great 
many of the smaller features. 
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