XXVI 



MAGNETIC ROCKS 



367 



are found in the exposed rocky peaks of hills and mountains. 

 Another third are found where the rocks are bared in headlands, 

 coast cliffs, inland-bluffs, ridge-tops, and in the open basaltic plains 

 where trees are scanty. On the other hand, a third occur in 

 situations, as in wooded districts where the rock exposure is scanty, 

 when it is not easy to explain the polarity, unless it was developed 

 in clear districts that have since become covered with forest. 



Table showing the Relation between the Specific Gravity and the 

 Polarity of Volcanic Rocks. 



In no place did any evidence of the direct action of lightning 

 come under my notice. Mr. S. Skinner who kindly looked at a 

 few of these rocks says that he found no trace of fulgurites in them. 

 It is probable that here as in the mountains of Skye, as described 

 by Mr. Harker, these effects are the result of the general influence 

 of atmospheric electricity independently of the direct agency 

 of lightning. The frequency of polaric rocks in the highest peaks 

 of the island is very remarkable. Generally speaking, all the bare 

 summits of the mountains are polaric. In my experience there is 

 no exception. All the rocks obtained from the actual summits 

 show polarity. The variety of rocks thus affected is suggestive ; 

 and this chapter may be concluded with a brief reference to their 

 mode of occurrence on some of the mountain-peaks. 



In Mbatini, 3,437 feet in height, which is the highest mountain 

 of Vanua Levu, the pyroxene-andesite of which the bare rocky 

 peak is composed is somewhat weathered and has a polaric or 



1 These values represent the number of degrees that the magnetic needle is 

 repelled. The method is described above. A note on the average amount of 

 polarity found in all my polaric rocks is given at the end of the chapter. The 

 term " dacite " is here an equivalent of " felsitic andesite." 



