Rocks and Regional Magnetic Disturbances. 535 



of the surface rocks, makes it probable that the interior is largely 

 metallic, and it is possible that;, even at depths less than that assigned 

 to the magnetic floor, iron may exist in large quantities with the very 

 high permeability it possesses at high temperatures. 



Lastly, the basic rocks of Mull give a much smaller mean value of 

 K than those of Skye and Ardnamurchan. If the underground rocks 

 were as permeable as these, all difficulty would vanish. 



On the other hand, there are some considerations which point to the 

 opposite direction. Thus, Professor Judd informs me that he leans to 

 the opinion that'in igneous magmas subject to the great pressures which 

 obtain at considerable depths iron tends to form silicates rather than 

 magnetic oxide. If this is so, the permeability of the surface rocks 

 may be a maximum rather than a minimum, unless native iron exists 

 in large quantity at great depths. The effect of pressure on perme- 

 ability is an unknown factor, which might support or weaken the 

 argument. 



Points such as these are, however, outside the scope of this paper. 

 But, though it is obviously unwise to be dogmatic on a question which 

 is still surrounded by difficulties, I think that the result of the present 

 enquiry is much in favour of the rock-magnetism theory of regional 

 magnetic disturbances. 



