GEOLOGY. 421 



apart. The rise of density in this little difference of depth, if projected 

 to the center, as we are accustomed to project gradients of temperature, 

 etc., would give a density nearly twice that computed from the classic 

 law of density of Laplace. No account is here taken of mechanical 

 compression, for the specific gravities used as the basis of the estimates 

 were all taken under atmospheric pressure. IVIuch less was any 

 account taken of hjiDothetical quantities of metals or other specially 

 heavy material, for both these shells are formed of common rock. 



The two most abundant elements in the outer shell, oxygen and sili- 

 con, sometimes take the form of tridimite in the outermost shell but 

 not in the plutonic rocks, where the same elements appear as quartz. 

 This is commonly assigned to difference in the physical conditions of 

 the two horizons, especially pressure. Now, the specific gravity of 

 tridimite ranges from 2.28 to 2.33, while that of quartz is 2.65. This is 

 a density rise of 15 per cent between two horizons, both of which lie in 

 the limited zone made accessible by deformation and denudation. 



The most instructive data, however, are found in the successive 

 stages of progress in density developed in various kinds of silts as they 

 pass into various kinds of schists, and thence into various minerals 

 of the garnet group or allied groups of heavy minerals. The compres- 

 sion of the silts into shale, and even into the first forms of schist, may 

 be neglected, since a part of the increased density is due to the mechani- 

 cal elimination of porosity. But even after the early crystalline forms 

 have been assumed, there are notable increases of density in the still 

 higher stages of metamorphism from which the gametic minerals arise. 

 In the case of individual minerals these range from 36 per cent to 84 

 per cent, as shown by data assembled and correlated by Van Hise.^ 

 This rise of density is the result of the metamorphic reorganization of 

 very common and representative kinds of material. It shows further 

 that one order of metamorphic action may follow upon another. 



So great a rise of density in the shallow zone of observation as these 

 three specific cases imply, if extrapolated, leaves little ground for 

 question as to the competency of metamorphic action to yield all the 

 increase of density indicated by the mean density of the earth, when 

 allowance is made for simple mechanical compression and for such 

 partial segregation of the heavier material toward the center as are 

 compatible with the planetesimal theory and even postulated by it. 

 There seems no need to assume the presence of an amount of metal, 

 or other intrmsically heavy material, greater than is implied by the 

 planetary evidence already cited. Dynamic metamorphism is uni- 

 versally assigned to pressure and heat; these are the chief phases of 

 energy concerned in the earth's self-compression; it is only necessary 

 to assign them reorganizing functions in the depths of even less relative 

 effectiveness than they show in the outer shell. The force of these 



iC. R. Van Hiae, A treatise on metamorphism, Mono. U. S. G. S. xlvii, p. 299 et seq. (1904). 



