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76 H. S. WASHINGTON ISOSTASY AND ROCK DENSITY 



The normative method for arriving at the average density of a region 

 is described in detail. This method consists in calculating the norm of 

 the average rock of the region and, from the known densities of the nor- 

 mative minerals, calculating the average density. The average densities 

 of many areas over the globe, including the earth as a whole, the conti- 

 nents and ocean floors, and many countries and States, resulting from 

 these calculations, are given in Table III, and their relations to the 

 corresponding altitudes are discussed. 



The physical method of calculating the average density, which consists 

 in averaging the specific gravities determined by the balance, is discussed 

 and some results are given. It is shown that this method is not capable 

 of yielding such consistent and reliable results for the present purpose 

 as the normative method. 



The depth of the "isopiestic level," or "depth of compensation,'' at 

 which all the various columns of. inversely varying densities and heights 

 balance each other or exert the same pressure, is discussed. The depth 

 is calculated from the known average altitudes and from the average 

 densities calculated by the normative method, using different sets of 

 areas. It is shown that the normative method is capable of yielding- 

 reliable results with proper data, and that the most probable value on this 

 basis for the isopiestic depth is 59 kilometers, whiclp. agrees well with 

 Bowie's value of 60 kilometers, arrived at by gravity methods at stations 

 distributed over the United States, but which differs from that of 96 

 kilometers derived from mountain stations and accepted by Bow T ie as the 

 best. It is shown that the former value is probably the more correct. 



The correspondence between the distribution of comagmatic regions 

 and areas of positive and negative gravity anomalies in the United States 

 is briefly discussed. 



Introduction 



According to the theory of isostasy, the outer part of the earth, the 

 "crust," is in a condition of more or less perfectly balanced equilibrium, 

 and tends to act as if it were made up of columns of different densities, 

 in the sense that, speaking generally, the higher the topography the 

 lighter the material beneath it, and the lower the surface relief the denser 

 the underlying matter. The crust is thus assumed to be made up, as it 

 were, of long-light and short -heavy columns, which balance each other; 

 so that it is upheld, not by the principle of the arch, but by what may be 

 called flotation, although the material beneath is solid, or essentially so. 



Observations with the pendulum and the plumb-bob have shown long 

 ago that in many cases the lower the altitude the greater the force of 



