28 Wa Pr tENNY. 
Additional magnetic and geologic information is needed, however, 
to clear up the possible existence and geologic meaning of this sus- 
pected trend. 
CALIFORNIA! 
It seems probable that the main magnetic effects in California are 
due to deep regional structure with a northwest-southeasterly trend. 
This, interpretation explains the southwest and northeast directions 
of the most of the horizontal components, and is well established by 
the known regional geology. 
A magnetic high trend extends all along the California Valley from 
Red Bluff in the north through Stockton to Bakersfield in the south. 
A second parallel high trend may be noticed east of Bradley, San 
Lucas, Soledad, and Salinas. 
Farther south it seems possible to construct a high trend from 
Oceanside to Santa Monica and another parallel trend from San 
Clemente to San Nicholas. 
The magnetic stations are considerably denser in the valleys than 
in the mountainous regions of the Sierra Nevada and Coast Ranges 
or in the Mojave Desert. Inasmuch as the valleys represent sedi- 
mentary basins, whereas the mountainous regions are mainly com- 
posed of volcanic rocks, schists, gneisses, and highly metamorphosed 
sediments, it is surprising to note that the average vector in the val- 
leys is of about the same magnitude as the average vector in the 
mountainous regions. This circumstance may no doubt be explained 
in part by the geographic location and distribution of the stations, 
but it may possibly also have a deeper meaning, which the writer will 
try to explain. 
If the magnetic field is due to regional “‘structures” and if the cross 
sections in Figure 2 of ‘‘Decline of Great Basin’” are true, we should 
logically expect a magnetic high trend about 20 or 30 miles west of 
the San Andreas fault, over the Santa Lucia ‘“‘blocks,’”’ another high 
trend over the Sierra Nevada “‘block,”’ and a low trend in the Cali- 
fornia Valley, or sedimentary basin, between these two “blocks.” 
The cross section mentioned passes approximately through Sole- 
dad and slightly north of Madera, on our vector map. We notice a 
considerable difference between the actual magnetic data and those 
that would be assumed if based on the cross section. 
1 The local magnetic vectors are based on the information given in ‘United States 
Magnetic Tables and Magnetic Charts for 1925,” op. cit. 
2 J. Edmund Eaton, “Decline of Great Basin, Southwestern United States,” Bull. 
Amer. Assoc. Petrol. Geol., Vol. 16, No. 1 (January, 1932), P. 5. 
358 
