318 ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



" such enormous pressures as those above calculated, even with 

 " the high temperatures which we have to expect in the interior 

 *J a fluid condition is conceivable. The hypothesis of a solid 

 " metallic nucleus in the interior of the earth has nothing con- 

 " tradictory in it, and indeed the phenomena of terrestrial mag- 

 u netism would appear to confirm this view. It is not to be 

 " doubted that the so-called magnetic storms have their seat in 

 " the atmosphere, or perhaps over it, and that the diurnal and 

 " secular variations of the magnetic elements are only to be 

 " sought in the exterior solid or solidifying crust of the earth. 

 *' If the seat of the greater part of the terrestrial magnetic 

 " power is in the earth's crust, then we must suppose such a dis- 

 " tribution of the magnetic fluid in it, as if on the average eight 

 " hard steel bars weighing one pound each, magnetised to the 

 " highest power, were present in every cubic metre. According 

 " to geological observation, however, we can scarcely suppose the* 

 " seat of the magnetic powei to rest in the earth's crust, since it 

 " does not seem to possess either a very great thickness, or a 

 " very intensive magnetism. According to an approximative cal- 

 " culation which my friend W. Weber has made, a globe of the 

 " hardest steel, magnetised to the highest degree, and having a 

 " diameter of nearly 476 (English) geographical miles, situated 

 " in the centre of the earth, would be able to produce the mag- 

 " netic phenomena which we observe on the earth's surface. In 

 " reality however these suppositions are not reliable, since we can 

 " neither expect to find hard steel nor a perfect magnetism in 

 " the centre of the earth. With less favorable circumstances 

 " than those above supposed, it would be necessary to assume the 

 " existence of a much larger solid globe in the interior of the 

 " earth in order to account for the magnetism on its surface. 

 " The radius of this globe would possibly extend far beyond the 

 " point at which, according to the calculations already mentioned, 

 " a density equal to that of metallic iron exists." 



In whatever degree Von Waltershausen's method of determin- 

 ing the earth's density at its centre, may be looked upon as 

 uncertain, it is scarcely possible to regard his theory of the 

 gradual increase of density as otherwise than, very reasonable. 

 Indeed since it is certain that the centre of the earth is much more 

 dense than the surface, it is scarcely possible to conceive how 

 the increase can take place otherwise than gradually. More- 

 over Laplace deduced a similar result from his investigations 



