XXVI MAGNETIC ROCKS 365 



netite in the groundmass> On examining the character of the 

 non-magnetic rocks it appears that almost all belong to two groups 

 where magnetite might be expected to be scanty. The first in- 

 cludes the pitchstones or basic glasses, sometimes fresh, at other 

 times more or less palagonitised. The second comprises the highly- 

 altered basic rocks, where the ferro-magnesian silicates have been 

 replaced by viridite, calcite, and pyrites. It is not, however, to be 

 implied that rocks of these two kinds will not sometimes attract 

 the needle. Many do not, and those in my collection that do so 

 act feebly. 



Coming to the magnetic polarity displayed by some of these 

 rocks, when the ordinary hand-specimen behaves like a magnet in 

 attracting one pole of the needle and repelling the other, it is to be 

 at first observed that a rock can become polaric without being pre- 

 viously magnetic. Dr. Folgheraiter has observed polarity in the 

 case of fragments of ancient bricks and pottery ; and he has 

 described the same effect in the masonry of a house struck by 

 lightning. In one or two of the Vanua Levu acid rocks showing 

 polarity this can be also premised since magnetite is present in 

 very slight degree. 



Polarity is very frequent among the volcanic rocks of this 

 island. Out of 520 specimens in my collection, which was made 

 without any reference to this matter, 80, or 1 5 per cent, are polaric. 

 Of these seven-eighths are basic and the rest are acid rocks ; but 

 this proportion is partly accounted for by the far greater prevalence 

 of basic rocks in the island. The basic rocks showing polarity 

 include some of the heaviest olivine-basalts with a specific weight 

 of 3'0, as well as some of the lighter augite-andesites with specific 

 weight of 27. They comprise the coarse textured dolerite as 

 well as the vitreous pitchstone and include both scoriaceous and 

 amygdaloidal rocks. The polaric acid rocks are mostly referable 

 to the dacites, with a specific gravity of 2*5 to 2"6. 



Humboldt remarked long ago that there is no direct relation 

 between the degree of polarity and the specific weight. This is 

 well brought out in the table subjoined ; but it should be at once 

 observed that there is an indirect relation. Although when we 

 arrange the rocks in a series according to their specific weight we 

 find no corresponding relation in the amounts of the polarity, we 

 observe that the extent to which polarity can be developed is 

 markedly greater in basic than in acid rocks. From this it may be 



^ On p. 357 will be found some notes on the magnetic iron sand that occurs 

 in great abundance in river and stream beds. 



