292 Status of the Theory of Isostasy. 



tasy is assumed to be true on the basis of work previously 

 published by many investigators during the past half 

 century. The later sections will discuss in detail certain 

 recent adverse criticisms which may have seemed to some 

 readers to undermine the solid basis on which isostasy 

 was thought to rest. Those criticisms, in so far as they 

 are against isostasy rather than against some subordi- 

 nate or unessential hypothesis, are found here to be in- 

 valid. The conclusion therefore is reaffirmed that for 

 regions sufficiently broad, and to variable degrees of per- 

 fection, the evidence is now convincing that the high or 

 continental areas of the surface are underlain by lighter 

 matter than are the low, or oceanic, areas. This relation 

 is maintained closely enough over varied conditions to 

 lead to the further conclusion that the relationship is 

 one of cause and effect. Over broad high areas the 

 mean elevations are high because the crustal densities 

 are there low, and vice versa, the mean surface of 

 low areas is low because the densities there are high. 

 This is quite closely true for the relations of con- 

 tinents and ocean basins but to lesser and lesser de- 

 grees for the smaller and smaller subdivisions of 

 these major segments of the crust. The subject re- 

 mains problematic only in regard to closeness of ad- 

 justment and limits of area involved. The larger fea- 

 tures of the earth's surface are, therefore, sustained in 

 solid flotation, and at some depth the strains due to the 

 unequal elevations largely disappear, the elevations being 

 compensated by variations of density within the crust. 

 In consequence the subcrustal shell is subjected to but 

 little else than hydrostatic pressure. This conclusion 

 regarding equilibrium of pressure in a subcrustal shell 

 is embodied in the name isostasy, proposed by Dutton 

 in 1889, meaning equal pressures. Above this shell, 

 whose upper part though gradational is not more than 

 80 or 100 miles deep, every radial column of adequate 

 area, say 100,000 square miles, contains very nearly the 

 same mass as every other column of equal area, although 

 the mean surfaces of ocean and continental columns may 

 be several miles different in elevation. 



Thus far we may speak of the theory of isostasy and 

 regard its existence as demonstrated in the same way 

 that astronomers have demonstrated the existence of 

 small cyclic variations of latitude and the motion of the 



