366 A NATURALIST IN THE PACIFIC chap. 



inferred that the degree of intensity of the exciting cause required 

 to give polaric powers of a certain value to an acid rock, like a 

 dacite, would be much greater than that necessary to endow a 

 basalt with equal powers. We should not expect to find the 

 same amount of polarity in the bare rocky peaks of two adjacent 

 mountains, where one was of dacite and the other of basalt ; and, 

 other things being equal, if two mountains had been exposed for 

 ages to the same conditions, we should regard the polaric powers 

 of the two as nature's equivalent values for the work of atmo- 

 spheric electricity, on the two rocks in question. We have two 

 such mountains in Vanua Levu in the case of the adjacent peaks 

 of Ngaingai (2,448 feet) and Navuningumu (1,931 feet) which are 

 about 2j miles apart and possess similar bare rocky pointed 

 summits. I take it that the polaric power of 25" of the dacite 

 (sp. gr. 2'57) in the first case is equal to the power of 90° of the 

 basaltic andesite (sp. gr. 2"82) in the other. In the dacitic peak 

 of Ngaingai and in the basaltic peak of Navuningumu we can 

 measure what work atmospheric electricity can accomplish in the 

 course of ages in the magnetisation of rocks. The other conditions 

 being taken as about the same, the main determining difference is 

 to be found in the rock-characters. 



In the table on the opposite page we have a series of volcanic 

 rocks placed according to their specific weights, which range from 

 2"5 to 3'o, and in the second column are shown their relative 

 polaric powers as indicated by the number of degrees the north 

 end of the magnetic needle is repelled by the corresponding pole 

 in the hand-specimen. For this purpose a magnetic needle 

 2^ inches in length (strictly speaking 6'5 centimetres) was em- 

 ployed, a card marked in degrees being placed beneath. The 

 north pole of the stone was placed in contact with the north end 

 of the needle, and after the needle had become stationary in its 

 new position a reading was taken. 



These polaric rocks came under my notice over most of the 

 island. They are infrequent in the district between Undu Point 

 and the Wai-ni-koro River, where, however, acid tuffs are largely 

 exposed ; and I did not find them in the Natewa Peninsula east of 

 Lea, their absence from my collections made in the Mount Freeland 

 range being remarkable. But it is probable that this is due to the 

 surface conditions, since dense wood covers the slopes, and bare 

 rocky peaks are rarely to be seen. 



With regard to the influence of locality on the occurrence of 

 polaric rocks, the results may thus be classified. About one-third 



