xxvi MAGNETIC ROCKS 363 



has long been known that granites, trachytes, &c., can possess 

 magnetic polarity l ; and the existence of this quality among acid 

 volcanic rocks is well shown in the case of the dacites in Vanua 

 Levu, rocks which compose some of the isolated mountain-peaks. 



One finds occasional reference to the highly magnetic character 

 of the rocks in oceanic islands of volcanic origin, but the nature of 

 the property is not always described ; and it is sometimes not 

 possible to gather from the data given whether the magnetism 

 affects the whole mountain mass, when it would be of the regional 

 kind, due probably to induction, or whether it is the simple 

 magnetic quality that almost all basic volcanic rocks possess on 

 account of the fine magnetite disseminated through the rock, or 

 whether there is evidence of a deposit of magnetite in the vicinity, 

 or whether it is a mere surface phenomenon confined to the bare 

 rocks of peaks and ridges, when such rocks, whether gabbro, 

 granite, basalt, trachyte, or dacite, display magnetic polarity. 

 Dana, with regard to the basaltic mountain of Tahiti, remarks that 

 the compass was often rendered useless by the local attraction of 

 the rocks, bearings taken being found to vary two to three points 

 on changing the position of the instrument. 2 Major Haig says that 

 the compass becomes perfectly useless anywhere in the neighbour- 

 hood of one of the mountain-masses or extinct craters in Mauritius, 

 and attributes this effect to the magnetite in the basalt 3 



On the summit of Mauna Loa in Hawaii, at the edge of the 

 great crater and in the vicinity of the site where Commodore 

 Wilkes carried out his pendulum observations in 1840, I found my 

 compass-needle greatly affected by local attraction, but I neglected 

 to inquire further into the matter. Judging from my sojourn of 

 twenty-three days on this mountain-top, thunder-storms are of very 

 rare occurrence there ; but the electric condition of the air is at times 

 very evident, and its physiological effects are somewhat distressing. 

 My blanket at night crackled in my hands and emitted sparks, so 

 that I could trace with my finger the letter A in phosphorescent 

 hues on its surface. 



That lightning is directly responsible in some instances for the 

 magnetic polarity of rocks in mountain-peaks is also well estab- 

 lished. It has been illustrated in an indirect fashion only last 

 year in the disaster on the Wetterhorn. Rocks partially fused by 

 thunderbolts and displaying polarity occur on the summit of the 



1 Some of the earliest observations were made on granites and trachytes. 



2 Geology of the United States Exploring Expedition, 1849, P- 2 94- 



3 Quarterly Journal Geological Society, vol. li., p. 469. 



