28 HAYFORD 



If the heterogeneous material composing the earth were not 

 arranged in this manner at the outset the stresses produced by 

 gravity would tend to bring about such an arrangement. But 

 as the material is not a perfect fluid, as it possesses considerable 

 viscosity, at least near the surface, the rearrangement will be 

 imperfect. In the partial rearrangement some stresses will 

 still remain, different portions of the same horizontal stratum 

 may have somewhat different densities, and the actual surface 

 of the earth will be a slight departure from the ellipsoid of 

 revolution in the sense that above each region of deficient 

 density there will be a bulge or bump on the ellipsoid, and 

 above each region of excessive density there will be a hollow, 

 relatively speaking. The bumps on this supposed earth will be 

 the mountains, the plateaus, the continents — and the hollows 

 will be the oceans. The excess of material represented by that 

 portion of the continent which is above sea level will be com- 

 pensated for by a defect of density in the underlying material. 

 The continents will be floated, so to speak, upon the relatively 

 light material below them and, similarly, the floor of the ocean 

 will, on this supposed earth, be depressed because it is com- 

 posed of unusually dense material. This particular condition 

 of approximate equilibrium has been given the name, isostasy. 



Is the earth to-day in this condition? In connection with a 

 study of this question it is convenient to define two or three 

 phrases which will be found useful and in defining them to add 

 precision to our conception of isostasy. 



The adjustment of the material toward this condition, which 

 is produced in nature by the stresses due to gravity, may be 

 called the isostatic adjustment. 



The compensation of # the excess of matter at the surface (con- 

 tinents) by defect of density below, and of surface defect of 

 matter (oceans) by excess of density below may be called the 

 isostatic compensation. 



Let the depth within which the isostatic compensation is com- 

 plete be called the depth of compensation. At and below this 

 depth the condition as to stress of any element of mass is iso- 

 static, that is, any element of mass is subject to equal pressures 

 from all directions as if it were a portion of a perfect fluid. 



