606 HARMON LEWIS 



of the earth and a compensated segment, the surface of which is 

 not at sea level. This density difference will be called the "compen- 

 sating density difference." 



The "depth of compensation'''' for any segment of the earth is the 

 greatest depth below sea level at which there is a compensating density 

 difference. This is different from the definition of Hay ford which 

 makes the "depth of compensation" the depth "within which the 

 isostatic compensation is complete." The former definition allows 

 for the possibility of a compensation which is not complete. 



The "distribution of compensation" for any segment of the earth 

 is the manner of variation of the compensating density difference with 

 respect to depth. If the compensating density difference is uniform, 

 the distribution of compensation is uniform; if it is uniformly 

 varying from a maximum at the surface to zero at the depth of 

 compensation the distribution of compensation is uniformly 

 varying. 



The "degree of completeness of isostatic compensation" is 

 an expression used by Hay ford. After defining the depth of com- 

 pensation as quoted above, he says, "At and below this depth the 

 condition as to stress of any element of mass is isostatic; that is, 

 any element of mass is subject to equal pressures from all directions 



as if it were a portion of a perfect fluid In terms of masses, 



densities, and volumes, the condition above the depth of compen- 

 sation may be expressed as follows: The mass in any prismatic 

 column which has for its base a unit area of the horizontal surface 

 which lies at the depth of compensation, for its edges vertical lines 

 (lines of gravity) and for its upper limit the actual irregular surface 

 of the earth (or sea surface if the area in question is beneath the 

 ocean) is the same as the mass in any other similar prismatic column 

 having any other unit area of the same surface for its base." This 

 condition of course follows from Hayford's definition of depth of 

 compensation, but it would not hold for the definition adopted in 

 this discussion unless the compensation were complete. Hay ford 

 continues as follows: "If this condition of equal pressures, that is, 

 of equal superimposed masses, is fully satisfied at a given depth 

 the compensation is said to be complete at that depth. If there 

 is a variation from equality of superimposed masses the differences 



