Nature and Bearings of Isostasy. 283 



forms were built up during earth growth, their nature 

 rooted deep in the planet, and their outlines, excluding 

 the shifting shallow seas, essentially constant through 

 all the ages. 



Some of the most significant tests of these hypotheses 

 are derived from the quantitative measurement of the 

 relations of the density of the crust to the relief of its 

 surface. 



From 1849 to 1855 it was shown by Petit, Pratt, and 

 Airy that certain mountain regions were matched in 

 their elevations by a deficiency in density. They floated, 

 as it were, in the solid earth for the same reason that 

 an iceberg stands above the ocean level because of its 

 lesser density. According as this relation is more or less 

 exactly attained, the strains imposed by the weight of 

 continents and mountains upon the interior of the earth 

 are largely eliminated. The geologic bearings of this 

 theory of the relations of relief to density were per- 

 ceived by Dutton, and in 1889 he coined for it the name 

 of isostasy. A few years later Gilbert and Putnam made 

 the first quantitative measurement of isostatic relations 

 in North America. In the past ten years, however, the 

 subject has been brought to a quantitative basis by 

 means of the geodetic data contributed by Hayford and 

 Bowie. Hayford reached the conclusion from his study 

 of deflections of the vertical that the average departure 

 of the surface from that level which would give perfect 

 isostasy was not more than 250 feet and that the hori- 

 zontal limits of such a departure were between one square 

 mile and one square degree. The work of the writer 

 has been based in considerable part upon a reinterpre- 

 tation of the data given by Hayford and Bowie, but has 

 reached the conclusion that the isostatic balance for 

 limited areas is much less complete than Hayford be- 

 lieved. The vertical magnitude of the departure from 

 isostasy permitted by the strength of the earth varies 

 inversely with the areal extent. An area two hundred 

 miles in diameter may stand probably with an average 

 elevation as much as 1,000 feet above or below that level 

 giving perfect isostasy, and loads several times as great 

 as this appear to be borne in places. 



Following this preliminary survey, a statement will 

 be made of the nature of the geodetic evidence upon 

 which the quantitative measurement of isostasy rests. 



The astronomic determinations of latitude and longi- 



Am. Jour. Sci— Fourth Series, Vol. XLVI1I, No. 286-October, 1919. 

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