ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 315 



must cause the bodies existing there to expand, and which must 

 thus neutralise much of the compression exercised on these bodies 

 by the masses lying above them. Finally, it seems that the com- 

 pressing powers of the superincumbent masses have b< j en somewhat 

 over estimated. The crust of the earth must be regarded as a 

 self-supporting arch, exercising a pre c sure only on the elastic fluids 

 occurring in it, and not as resting or floating on the fluid inlerior. 

 The latter has then only to bear the weight of the columns of lava 

 which may exist in the interior of volcanoes, and which is certaiuly 

 not inconsiderable. From these considerations it would seem per- 

 fectly correct to suppose that what tin crust of the earth wants in 

 density as compared with the whole mass of the planet, must be 

 made up by the centre. 



Naumann finds that assuming the average density of the earth's 

 crust to be 2*5, and the increase of density to take place accord- 

 ing to arithmetical progression, the density of the centre would 

 be 8*5, consequently considerably more than the specific gravity 

 of iron, and almost equal to that of cobalt. A similar calcula- 

 tion is the starting point for the theory already mentioned of 

 Sartorius Von Waltershausen. He finds the meau of the specific 

 gravities of orthoclase, albite, quartz, crystalline limestone and 

 mica to be 2*66, and assumes this as an approximation to the 

 mean density of the outer crust. Calculating, first, three fifths 

 of the total volume of the earth to possess this specific gravity, 

 he finally computes the density of the centre to be 9 585. He 

 moreover calculates the densities which exist at various depths 

 beneath the earth's surface. These depths, converted into English 

 geographical miles, with their calculated densities are as follows :— 



Miles from surface. Density. 



266 



34 2-79 



68 2-93 



103 3-07 Lime. 



137 3-20 Magnesia. 



171 3-34 



206 3-47 



240 3-6Q 



274 3.72 



309 3-85 



343 3-99 Alumina. 



686 5-15 Iron-oxide. 



1029 6-29 Tellurium,Chromiun3 



