Magnetism of Basalt. 575 



successively with near ends 26*75 cms. from the magneto- 

 meter needle, gave deflexions as noted below on reversal of 

 their positions. 



The relative susceptibility of the bars was also tested in a 

 field of 40 c.G.s. units, the deflexions noted being those 

 obtained by reversing the field. 



Table I. 



Bar. 



Deflexion due 



to permanent 



magnetism. 



Deflexion in 

 reversed field 



of 40 C.G.S. 



Sgurr nan Gillean . . . 

 Storr 



mm. 

 61-3 



8-0 



70 











mm. 

 201 



2?1 



148 



45-8 



33 



Dunvegan 



Stromness Dyke 



Faule Ader 





A comparison of the two columns shows that whilst the 

 three Skye specimens are of medium susceptibility, the Sgurr 

 nan Gillean bar is, for rock material, very highly magnetized; 

 the Stromness bar is composed of a material of relatively 

 high susceptibility, and the Faule Ader bar is almost non- 



magnetic. 



Rock Sectiom 



Thin sections of the rocks Sgurr nan Gillean, Stromness 

 Dyke, and Faule Ader were examined, and all three were 

 found to contain crystals of magnetite as their chief magnetic 

 constituent uniformly dispersed throughout the sections ; in 

 the section of Faule Ader the linear dimensions of the mag- 

 netite crystals were about a sixth of those of the crystals in 

 the other two sections. Further than this, an examination 

 of the sections in the microscope gives no clue to the widely 

 different magnetic behaviour of these three rock specimens. 



Temperature Effect. 



The bars were next tested in the electric furnace in the 

 manner previously described, and curves were obtained which, 

 although exhibiting peculiarities, have a resemblance to the 

 temperature-permeability curve for magnetite, and those for 



