Chap. 7] 



GRAVITATIONAL METHODS 



291 



denser Jurassic rocks, it could be outlined by mapping bedrock topography. 

 In one of the profiles the lignite did not come up against the edge of 

 bedrock. A broad gradient maximum, due to the superimposed effect of 

 the lignite edge and the sloping bedrock surface nearby, was observed. 



The torsion balance is also useful for outlining placer channels. Edge 

 and Laby investigated a portion of the Gulgong field in New South Wales 

 with the object of determining the edge of a sloping channel filled with 

 alluvial strata, underlain by granite. (Seismic data for this field are 

 shown in Fig. 9-41.) A similar application of the torsion balance was 



^AriO 



Bt(k5a» 







40 

 30 



W 

 



-M fefmlMs 

 50 



Fig. 7-124. Underground torsion balance observations at Beienrode, Germany (after 



Birnbaum). 



made by McLintock and Phemister"^ who surveyed the buried channe^ 

 of the Kelvin River near Drumry. Surveys of salt deposits have been 

 made not only in connection with oil exploration but also for mining pur- 

 poses. An example is A. H. Miller's survey of the Malagash salt deposit,"^ 

 which revealed a dome-shaped anomaly of 3 milligal maximum ampli- 

 tude. The thickness of the salt is about 300 feet. Fig. 7-123 shows a 



1" W. F. P. McLintock and J. Phemister, Roy. Soc. (Edinburgh) Trans., 66(1), 

 141-155 (1929). 



1" A. H. Miller and G. W. H. Norman, A.I.M.E. Tech. Publ., No. 737 (1936). 



