Chap. 8] MAGNETIC METHOD 401 



the anomaly curve correctly for strongly magnetic rocks, it has been known 

 to fail in several cases as far as the magnitude of the calculated anomalies 

 is concerned. This has led several investigators to assume abnormally 

 high susceptibilities, as in the case of the Kursk, and other magnetic 

 anomalies associated with iron ores. In at least one instance the assump- 

 tion of high susceptibility could not be maintained after the rock specimen 

 had been tested (Kursk). Hence, it appears that the fault lies with the 

 theory and that susceptibility alone is not sufl&cient to describe the effects 

 of the surface in all instances. As a comparison of the hysteresis curves of 

 Figs. 8-11 and 8-12 shows, the susceptibility alone would be sufficient 

 in the latter case to express the effect at the surface, whereas in the former, 

 it would not suffice. In the first case, the intensity of magnetization 

 would have to be represented by a function of the form ^ = ^' -f /cH 

 while in the latter the mere product ^ = kH as given in the preceding 

 theory expresses the situation satisfactorily. The tabulations given on 

 pages 315-16 for the coercive forces and remanent magnetization d' of 

 rocks indicate that (1) the ratio of coercive force and maximum suscepti- 

 bility for rocks is much greater than for commercial iron and steel; (2) for 

 many metamorphic and eruptive igneous rocks the remanent magnetiza- 

 tion is greater than the product of susceptibility and (earth's) field. Thus, 

 in some instances, the induced magnetization may be neglected in com- 

 parison with the remanent magnetization (see formulas on pages 382 and 

 387). 



It is possible to make allowance for such conditions by adding, in the 

 formulas of the preceding section, the remanent magnetization to the in- 

 duced magnetization. Since the remanent magnetization is a vector whose 

 direction may or may not coincide with the direction of induced magnetiza- 

 tion, it must be resolved into its longitudinal and transverse components. 

 For thin ore bodies, igneous sheets, and the like, and for low suscepti- 

 bilities, the transverse components of both induced and remanent magneti- 

 zation are usually negligible, so that from formula (8-55) : 



AH= -?^(.Zo±^;) 

 AZ= -^^(KZo±d[), 



► (8-63a) 



where 3i is the longitudinal remanent magnetization and the ± sign indi- 

 cates normal or abnormal polarization, respectively. 



For a wider vertical body the transverse components of both induced 

 and remanent magnetization may become important. Hence, the anoma- 

 lies follow from formula (8-616) and are, if both the longitudinal (3i) and 



»* Slichter, op. cit., 250. Haalck, Gerl. Beitr., 22, 241-255 and 385-399 (1929). 



