448 JOSEPH BARRELL 



with observations is that of the presence of a layer about 600 km. 

 thick, shghtly ductile (coefficient lo'-' to 10^''), existing beneath 

 an outer crust 120 km. thick.'' 



By postulating such a thick zone for isostatic flow, the viscous 

 resistances are reduced and solid flow is made easier. It also is in 

 conformity with the probability of a gradual change of physical 

 state from the rigidity above into a less rigid and less stable tract 

 and this in turn into a more rigid interior. Now if such a thick 

 viscous zone is incapable of supporting over broad areas loads 

 imposed by abnormalities of density above, it should also be incap- 

 able of supporting such horizontal inequalities of mass within it, 

 provided these are sufhciently large. But in order to produce the 

 same gravitative surface effects as more superficial masses, the 

 heterogeneities of this zone of viscous flow would in fact have to 

 be much larger. A cylinder of the dimensions postulated by 

 Gilbert, if of negative density and adjacent to another at the same 

 depth but of positive departure from the mean density, would tend 

 to be underthrust by the latter, and the denser would in turn tend 

 to be overflowed by the lighter. 



For these reasons it is not to be expected that the same depar- 

 tures from those densities giving isostatic equilibrium which could 

 exist in a rigid shell above would extend immediately below. 



Influence of centrospheric heterogeneity. — To test the question 

 of the influence of heterogeneity below the zone of compensation 

 a sphere will be considered. First, one whose center is at a depth of 

 319 km., 0.05 of the radius of the earth. As a second test, the 

 influence will be determined of a sphere whose center is at a depth 

 of 637 km., o. 10 of the earth's radius. For considering the attrac- 

 tion of a mass at points on the surface other than at the epicenter 

 it is more convenient to take the mass as having the form of a 

 sphere rather than a cylinder, since the mass of the sphere acts in 

 all directions as if concentrated at its center. This favors, further- 

 more, the accentuation of the effects upon the surface over what 

 they would be if the outstanding mass had a stratiform extension. 



'Dr. Willielm Schweydar, " Untersuchungen iiber die Gezeiten der festen Erde 

 und die hypothetische Magmaschicht," V erdfenilichmg des k.k. Prensz. geodat. Insti- 

 tutes, Neue Folge No. 54, Leipzig, 1912 (B. G. Teubner). 



