1889.] Geological Origin of Terrestrial Magnetism. 



93 



netic iron-ore in basaltic rocks was then considered, and it was shown 

 that an average of 10 to 15 per cent, would express these proportions ; 

 and assuming similar proportions to exist in the earth's magnetic 

 magma, we should then have an effective terrestrial magnet of from 

 2|- to 3 miles in thickness. The actual magnetic magma or shell 

 might be very much thicker than that here assumed. 



Instances of polarity in basaltic masses at various localities were 

 adduced in order to illustrate the possibility of polarity in the internal 

 mass. The subject of the polarity of the globe was then discussed, 

 and it was pointed out how the position of the so-called " magnetic 

 poles " leads to the inference that they are in some way dependent 

 upon the position of the terrestrial poles. 



The author regarded the so-called "double poles" as merely foci 

 due to protuberances of the magnetic magma into the exterior non- 

 magnetic magma, and considered that there was really only a single 

 magnetic pole in each hemisphere, embracing the whole region round 

 the terrestrial pole and the stronger and weaker magnetic foci, and roughly 

 included within the latitude of 70° within the northern hemisphere. 



It was pointed out that the poles of a bar-magnet embrace a com- 

 paratively large area of its surface, and hence a natural terrestrial 

 magnet of the size here indicated may be inferred to embrace a pro- 

 portionably large tract for its poles. 



In reference to the question why the magnetic poles are situated 

 near those of the earth itself, this phenomenon seemed to be con- 

 nected with the original consolidation of the crust of the globe, and 

 the formation of its internal magmas. 



It was pointed out that, owing to the differences of temperature 

 which must have existed in the polar regions, as compared with 

 those of the equatorial, the process of solidification has been more 

 rapid in the polar regions than elsewhere, and it was inferred that in 

 the case of the magnetic magma the process of crystallisation and the 

 polar arrangement of the particles of magnetic iron-ore would proceed 

 from the poles towards the equator in a radial direction. The manner 

 in which the phenomena of magnetic intensity, and of the dip of the 

 needle at different latitudes could be explained on the hypothesis of 

 an earth's internal magnet, such as here described, was then pointed 

 out, and the analogy of such a magnet with a magnetic bar passing 

 through the centre of the earth was illustrated. 



The author then proceeded to account on geo-dynamical principles 

 for the secular variation of the magnetic needle, and also to show how 

 the objections that might be raised to the views here advanced, on the 

 grounds of the high temperature which must be assumed to exist at 

 the depth beneath the surface of the magnetic magma, could be met 

 by considerations of pressure, and on this subject read a letter which 

 he had received from Sir William Thomson, F.R.S. 



