37 



A NATURALIST IN THE PACIFIC 



CHAP. 



which had been cleared of trees, has a repellent force of 10. The 

 polarity of the olivine-basalt from the well-wooded slopes of 

 Ulu-i-ndali, a range 1,100 feet in height on the east side of the 

 Wainunu estuary, is not so easily explained ; the intensity varies 

 from 8 to 28. Ngalau-levu, a hill 1,650 feet in height, rising 

 behind Lea on the south coast of Natewa Bay, is polaric in its 

 upper portion. Specimens of a hemicrystalline basic andesite, 

 somewhat scoriaceous, which form the agglomerate of the rocky 

 summit, have a repellent force of 18, whilst a similar rock from the 

 agglomerate of an exposed spur two-thirds of the way up the hill has 

 a force of as much as 38. A curious case of polarity is exhibited 

 in a bare tuff overlooking the Vui-na-Savu River between Rauriko 

 and Vitina. It is composed of a much weathered whitish trachytic 

 rock, which in appearance affords no promise of polarity, but has 

 the power of repelling the magnetic needle 2 to 3. 



Note on the Average Polarity (Repellent Power) of the Volcanic 

 Rocks of Vanua Levu. 



It would appear from the table given below that the difference 

 in the average polarity of acid and basic rocks is not very great. 

 The average for rocks with a specific gravity below 27 is about 

 10 ; and that for heavier rocks is about 14. The difference 

 mainly lies at the maximum end of each series, the capacity for 

 extreme polarity being, as before remarked, markedly greater in 

 the basic rocks. 



1 The mode of measurement is described on p. 366. 



