40 C. E. VAN ORSTRAND 
isothermal surface corresponds very closely to the highest point on the 
“Second Wall Creek sand”. Extending laterally from this point, the 100 and 
120°F isothermal surfaces tend to parallel the surface of the sand as shown in 

Joe of Lower Broun zone 
Fig. 4. Long Beach Dome, Los Angeles County, California. 
Fig. 5.* In general, the isothermal surfaces are flatter than the surfaces of the 
strata and the flatness increases with diminishing depth until a point is 
reached near the surface of the ground at which the isothermal surfaces 
tend more or less to conform to surface topography. As the effects of surface 
ry oe — FEET 

SEA LEVEL 

' 4MILES 
= 
Fig. 5. Cross-section of Salt Creek Dome, Wyoming. 
topography, however, are practically negligible in this field, we must look to 
other causes for an explanation of the marked rise of the thermal surfaces as 
they pass over the dome. 
The Long Beach dome‘ presents a problem in which surface topography is 
*W. T. Thom, Jr., Relation of earth temperatures to burried hills and anticlinal folds. 
Economic Geology 20, 524-530 (1925). 
4A. J. Carlson, Geothermal variations in oil fields of Los Angeles Basin, California. Bull. 
Am. Assoc. Petroleum Geologists 14, 997-1011 (1930); Geothermal conditions in oil producing 
areas of California. Am. Petroleum Institute, Production Bull. No. 205, 109-139 (1930). 
184 
