Chap. 8] 



MAGNETIC METHOD 



427 



anticipated if basement rocks have been affected, if igneous sheets have 

 been intruded into the fault fissure, or if the fault plane has acquired con- 

 centrations of magnetic material or has been magnetized by other causes. 

 J. Jung and C. Alexanian^*® made surveys on faults at Allschwill (Meletta 

 agamst Cyrene marls), at Niederhaslach (Triassic Ume against sandstone), 

 at Aubure (granite against sandstone) and at Guewenheim (Meletta marls 

 against shales, and the like), and observed the typical depression in the 

 vertical intensity curve also noticed elsewhere. They came to the con- 

 clusion that this effect cannot be explained by the susceptibilities and 



Fig. 8-72. Magnetic survey of Walnut Creek fault, California (celluloid profiles on 



map) (after Lynton). 



disposition of the adjacent formations but must be an effect of the fault 

 plane itself. Similar depressions in magnetic curves corresponding to the 

 Leopoldsdorf and Sollenau faults in the Vienna basin were observed by 

 Forberger, John, and Petrascheck. 



3. Mapping of basement topography and of igneous intrusions. In 

 many oil-producing areas or prospective oil territories, structure in the 

 sedimentaries is controlled by the topography of basement rocks due to 

 deep seated folding or faulting, or by differential settling about pre-existing 

 basement highs. Intrusions of igneous rocks, such as plugs and batholiths. 



i««Ann. Off. Comb. Liqu., 4, 711-720 (1931). 



1" Akad. Wiss. Wien, Sitz. Ber., 143(1), (5-7), 137-145 (1934). 



