238 Intelligence and Miscellaneous Articles. 



is to be found in the decomposition of many of the heavy silicates 

 by enormous pressure when the temperature of the menstruum in 

 which they were fused sunk below the melting-point of quartz. 

 An equivalent of oxide of lead and of crystallized silicic acid would 

 have their common volume increased about 14 per cent, on combi- 

 ning and forming lead glass. Now, at a depth of 1000 kilometres 

 below the earth's surface, the pressure is equal to about 300,000 

 atmospheres ; and accordingly the formation of a cubic inch of 

 glass by the union of quartz and oxide of lead would, in conse- 

 quence of the expansion it involves, be resisted by a force the ther- 

 mal equivalent of which may be represented by the heat expended 

 in melting 14 cubic inches of ice. A force of equal energy would 

 be exerted by the same pressure for the decomposition of a cubic 

 inch of silicate of lead, in the supposed locality, and for the crys- 

 tallization of the resulting silicic acid. As far less heat is evolved by 

 the union of the strongest acids and bases, and as a crystallization 

 or atom-arrangement can make no heavy demands on force, it is 

 reasonable to conclude that in the supposed case chemical affinity 

 would be overruled and that the silicate of lead would be decom- 

 posed. From similar estimates it would also appear that other 

 silicates, especially those of heavy metals, would undergo a similar 

 decomposition at great depths, and would part with their silica 

 when the temperature became low enough to allow its solidification. 



Another source of buoyant matter is to be found in the transfer 

 of silica from the heavy metallic oxides to the alkalies and other 

 strong bases. The light compounds thus formed would, according 

 to Delesse and Deville, contract more than other igneous rocks in 

 passing into a solid state ; and it is evident that in proportion to 

 this contraction will their production be favoured by pressure on the 

 decline of the primitive heat. The growth of a floating crust would 

 also be promoted by other circumstances. Of many of the metallic 

 oxides, the most infusible compounds are those in which the silicic 

 acid is very small or in a very large proportion. But the latter 

 bodies, which have almost invariably the lowest specific gravity, 

 have also their fusibility reduced most by pressure in consequence 

 of the contraction which they undergo in assuming a solid form. 

 On this point more satisfactory evidence may be obtained by an 

 investigation similar to that of Clausius, but in which the effects of 

 pressure upon fusion is determined from the change of volume and 

 the modulus of elasticity. 



Of the various products which separate from the subterranean 

 lava in cooling, the most dense parts would sink to the centre, 

 though solidifying in the uppermost stratum ; while the lighter 

 material, though taking the solid form at great depths, would rise 

 towards the surface. But the solidity of the light silicated matter 

 could be permanent only when kept under the influence of immense 

 pressure, by settling on prominent points which extend from the 

 inner side of the crust deep into the lava. The great centres of ac- 

 cumulation of this buoyant matter must be under continents, where 



