266 Arthur Holmes—Radio-activity. 
by the laborious studies of Hayford and Bowie.’ In India,? along 
the margins of the continents* and over the ocean basins,‘ a similar 
but less intimate association between terrestrial relief and underlying 
density has been recognized. At least for the broader features of the 
earth’s topography the theory of isostasy may therefore be Tee 
as established.° 
15. Lrrirs or Isosraric EQuinisRium. 
Hayford and Bowie, in their analysis of the results of the U.S. 
Geodetic Survey, found that by introducing the conception of isostatic 
compensation they were able to reduce deflection anomalies and 
gravity anomalies to respectively one-tenth and one-quarter of the 
value these anomalies would have had without applying the 
compensation hypothesis. 
The depth of compensation which most successfully reduces the 
anomalies is 122 km., though it should be noticed that the depth 
chosen may range from 66 to 305 km. and yet give an almost equally 
good reduction. Hayford suggests that ‘‘ the maximum horizontal 
extent which a topographic feature may have and still escape 
compensation is between one square mile and one square degree ”’ 
(op. cit., 1906, p. 169). This conclusion implies that the earth’s crust 
is so weak that the weight of accumulating sediments on the sea floor 
may well cause the latter to sink. Barrell, however, has made 
a careful study of the Nile and Niger deltas, and deduces from their 
thickness and great areal extent a much stronger crust than is possible 
on Hayford’s view of local compensation. Moreover, he demonstrates 
that the geodetic results of the United States, ‘‘instead of indicating 
local compensation to limits of less than one square degree, show on 
the contrary a ready capacity of the crust under the United States to 
carry over areas of from 5 to 10 or 15 square degrees, and 
exceptionally over even larger areas, departures from equilibrium 
greater than the mean” (op. cit., 1914, p. 165). Barrell supports 
his view of a strong crust by citing the immense departures from 
isostatic equilibrium exhibited by areas in India, Japan, and Norway, 
by the great basaltic dome of Mauna Kea, and by the Tonga Plateau 
and Deep. That the strength so manifested is greater than that of 
the surface rocks is only to be expected, for the experiments of 
Adams have proved beyond question that granite, for example, 
becomes increasingly stronger with increase of pressure.° There can 
be no doubt that North America is at present in a state of much 
1 Hayford, Proc. Wash. Acad. Sci., vol. viii, p. 25, 1906; U.S. Coast and 
Geodetic Survey, 1909 and 1910; Science, vol. xxxiii, p. 199, 1911. Hayford 
and Bowie, U.S. Coast and Geodetic Survey, 1912, Spec. Pub. 10. Bowie, 
U.S. Coast and Geodetic Survey, 1912, Spec. Pub. 12; Am. Jowrn. Sev., 
vol. xxxiii, p. 237, 1912. 
2 Burrard, Survey of India, 1912, Prof. Paper 12; Crosthwants Survey of 
India, 1912, Prof. Pap. 13. 
3 Schidtz, Skrift. Vedensk. Selsk. Christiania, 1908, No. 6. 
+ Hecker, Veréff. k. Preuss. geodat. Inst. Berlin, 1903, No. 11; 1908, No. 12. 
See also Bauer, Am. Journ. Sci.; vol. xxxi, p. 1,1911; vol. xxxiii, p. 245, 1912. 
° Gilbert, U.S.G.S., 1913, Prof. Pap. 85-C. Barrell, Jowrn. Geol., vol. xxii, 
Nos. 1-8, 1914; vol. xxiii, Nos. 1, 5, and6, 1915. 
§ Journ. Geol., vol. xx, p. 97, 1912. 
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