EARTH'S CRUST WASHINGTON". 315 



As far back as 1852 J. H. Pratt suggested that there was a defi- 

 ciency of gravity beneath the Himalaya Mountains, basing this on 

 the anomalous behavior of the plumb bob. 40 He also propounded 

 the view that the heavier portions of the earth's surface were sinking. 

 Later, Dutton 41 first clearly expounded the idea that the various 

 portions of the earth's surface, being laterally unlike or heteroge- 

 neous, are in a delicately balanced condition of equilibrium, so 

 that the lighter portions (those of less specific gravity) tend to rise 

 and the heavier (those of greater specific gravity) tend to sink, 

 the various portions thus balancing each other. This theory, later 

 called isostasy (meaning equal standing), was taken up by Hay- 

 ford, and he and William Bowie, of the United States Coast and 

 Geodetic Survey, and others, 42 have done much to develop it, espe- 

 cially as regards its application to gravity problems. While still 

 in dispute, especially as to some details, it is now a well-recognized and 

 generally accepted geodetic theory. 



It will be well to quote in part Hayford's definition 43 of isostasy. 

 Assuming a condition of lateral heterogeneity, he says: 



Different portions of the same horizontal stratum may have somewhat dif- 

 ferent 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 [depression], relatively speaking. 

 The bumps on this supposed earth will be the mountains, the plateaus, the 

 continents, and the hollows [depressions] will be the oceans. The excess of 

 material represented by that portion of the continent which is above sea level 

 will be compensated for by a defect of density in the underlying material. 

 The continents will b'e floated, so to speak, because they are composed of rela- 

 tively light material, and, similarly, the floor of the ocean on this supposed 

 earth [will] be depressed because it is composed of unusually dense material. 

 This particular condition of approximate equilibrium has been given the name 

 " isostasy." 



As has been noted above, in the case of northern India it has long 

 been known, from pendulum determinations of gravity, that in 

 many portions of the earth the observed force of gravity does not 

 correspond with that calculated from the form of the geoid, after 

 making corrections for the influence of topography (such as the 

 attraction of near-by mountain masses) and for the elevation of 

 the station above sea level. Thus, it has long been known that 

 gravity is on the whole greater over the ocean than over land areas, 

 and this has naturally been connected with the fact that the rocks 

 of oceanic islands are mostly basaltic and therefore heavy. 



40 Cf. Iddings, J. P., The Problem of Volcanism, p. 64, 1914. 

 *» Dutton, C. E., Bull. Phil. Soc. Wash., xi, p. 51, 1889. 

 42 For some references, see Iddings, op. clt., p. 65. 



« Hayford, J. F., The Figure of the Earth, p. 66, 1909 ; Cf. Bowie, W., U. S. Coast and 

 Geodetic Survey, Special Publication No. 40, p. 7, 1917. 



