MAGNETOMETER SURVEYS IN CALIFORNIA 1361 
tinguish them strikingly from the rest of the formation. Two such 
beds were observed in the Cretaceous, of which the course is indicated 
by lines connecting high points in the profiles (Fig. 4), and one such bed 
was found in the Briones. As some of the magnetic anomalies by which 
these beds differ from the associated rocks amount to as much as 300 
gammas, nearly one hundred times the least reading possible with the 
instruments, they are very conspicuous markers, easily recognizable with 
the magnetometers. The effect of these magnetic beds is very large and 
sharply marked where they crop out, and becomes progressively less 
intense and more widely distributed as the depth of cover increases. Their 
effect is clearly evident under the deepest cover existing in the Walnut 
Creek Valley. This depth is not known, but, as indicated by water-well 
evidence, is probably more than 150 feet. 
POSO CREEK, SAN JOAQUIN VALLEY, CALIFORNIA (FIG. 5) 
In the vicinity of Poso Creek, the magnetometer outlined an out- 
standing magnetic “high” indicating that a strongly magnetic mass, 
such as gabbro, has been uplifted close to the surface. Confirmation 
of this fact is.supported by the drilling of a well on the flank of this mag- 
netic “high” which entered plutonic rocks at 2,700 feet. The uplifting 
of this gabbroid mass caused a fault on its north side. Figure 5 shows 
that in working with such large bodies of magnetic material the effect 
of the deeper rock can be corrected out. This picture presents the small 
anomaly, caused by a fault, separated from a large regional variation, 
caused by a very magnetic large mass in the underlying basement com- 
plex. 
VENTURA COUNTY 
The survey shown on Figure 6 comprises an area of about 35 square 
miles south and east of Oxnard, California. It is a relatively sandy flat 
basin flanked on the northeast by the Camarillo Hills and by Round 
Mountain, an isolated igneous hill in the southeastern part. 
There is no doubt that the volcanic rocks are the chief causes 
of magnetic disturbances in this area. The effect of concentrations of 
magnetic minerals in terrace material is small and local—in few places 
more than 20 gammas—and does not mask the major disturbances. 
Results of the survey are shown on Figure 6, which gives contours 
of vertical intensity at 10-gamma intervals. As volcanic rocks are ex- 
posed on the southeast rim of the basin, and not at the north, the large 
disturbances in this part of the Ventura basin are probably the result of 
underlying bodies of volcanic rock. This interpretation was later sup- 
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