Chap. 8] MAGNETIC METHOD 411 



were reached, with a bed of magnetite, dipping at an angle of about 60° 

 and having a thickness of some 220 m. The iron content varies from 26 

 per cent on the outside to 40 per cent on the inside of the bed. The dip 

 is likely to decrease and the thickness to increase below the depths reached 

 by the drill. 



In the interpretation of the magnetic anomalies and in the comparison 

 of the gravitational and magnetic results,^^^ difficulties were soon experi- 

 enced, since the magnitude of the anomaly could not be explained by the 

 material encoimtered by the drill. Tests of the quartzites brought to the 

 surface gave only 0.2 to 0.4 for the susceptibility. 



Calculations of the magnetic anomaUes by Gamburzeflf"' showed that an 

 agreement of theoretical and observed values could be obtained for a mag- 

 netization of 0.7 (which, with H = 0.5, corresponds to k = 1.4). Haalck"* 

 arrived at similar values from a comparison of torsion balance and mag- 

 netic effects, and Slichter^^^ concluded that the susceptibiUty of the Kursk 

 ore would have to be of the order of 2.17 to explain the anomalies."' 



This discrepancy between results of specimen tests and values obtained 

 from the anomaUes leads Haalck to believe that the anomalies are not due 

 to induction in the earth's field and that their real cause is a mag- 

 netite deposit of high iron content below the quartzites reached by the 

 drill. 



In another anomaly exceeding the earth's field, a better agreement was 

 observed between susceptibilities determined in the laboratory and those 

 obtained from the magnetic observations. Fig. 8-60 shows the magnetic 

 anomalies at Kiiruna in Sweden. The ore body is magnetite, about 85 m 

 thick, dips at 55** to the east, comes within 34 m of the surface, and is 

 known to a depth of about 200 m. The vertical intensity anomaly is 

 about 0.7 gauss; the horizontal anomaly —0.4 gauss. Carlheim-Gyl- 

 lenskold"^ determined in the laboratory an average ©f 0.8 for the sus- 

 ceptibility of the ore, and Haalck"^ concluded from calculations that induc- 

 tion in the normal earth's field can sufficiently explain the anomalies if 

 the susceptibility difference against the adjacent rocks is 0.5 to 0.6. Fig. 

 8-61 represents an underground survey made in the Kallmerberg district 

 in Sweden"^ and illustrates the relation between position of ore bodies and 

 horizontal anomalous vectors. 



"* See discussion on p. 401 in sec. E. 



"' Op. cit., 19(2/3), 210-230 (1928). 



"* Gerl. Beitr., 22(3/4), 241-255, 385-399 (1929). 



1" Op. dt., 251. 



"' See discussion on p. 401. 



"^ Magnetic Survey of Kiirunavaara (Stockholm, 1910). 



"8 Gerl. Beitr., 22(3/4), 241-255 and 385-399 (1929). 



"9 Lehrbuch prakt. Geol., I, 378-381 (1921). 



