Arthur Holmes—Radio-actiwity. 265 
VI.—Ranio-acriviry and THE Earru’s THEerMaL History. 
By ARTHUR HOLMES, A.R.C.S., D.1.C., B.Sce., F.G.S. 
PART Tit.! 
Radio-activity and Isostasy. 
14. Isosratic CompENsATION. 
f¥\HE distribution of land and sea implies that the earth’s outer shell 
is in a condition of approximate hydrostatic equilibrium. 
Otherwise the equatorial regions should be girdled by a continental 
protrusion, or else each of the polar regions should be occupied by 
a continental bulge. The observed fact that the inequalities of the 
earth’s surface exhibit neither of these conditions proves that the 
general ellipticity of the lithosphere does not differ greatly from that 
which would be assumed by a liquid spheroid having a similar 
distribution of density in depth. The incapacity—thus demonstrated 
—of the lithosphere to endure permanent stresses leads naturally to an 
inquiry into the conditions that maintain continents and mountain 
ranges above sea-level. Investigations based on the deviations of the 
plumb-line from the vertical and on the varying intensity of gravity 
indicate beyond doubt that the elevated tracts of the globe owe their 
support to a deficiency of density in their deep-seated foundations, 
while the great sunken areas owe their depression to a corre- 
sponding excess of density in the underlying rocks. Thus has arisen 
the conception of zsostasy, a word which was coined by Dutton *in 
1889 to express the state of hydrostatic* balance that maintains in 
position elevated and depressed columns of the lithosphere. Columns 
of equal cross-sectional area, though of different heights, may have 
the same mass on account of their respective densities, and if 
so, at a certain level beneath the mean surface of the geoid, each one 
will exert the same pressure on the zone below. 
The regional perfection of isostasy, and its local limitations, have 
been. demonstrated by the geodetic observations of nearly seventy 
years. Petit,°in 1849, found that the Pyrenees deflected the vertical 
much less than was to be expected. Archdeacon Pratt,®.in 1852, 
discovered a similar anomaly in the case of the Himalayas; and 
during the later months of the same year Airy,’ then Astronomer 
Royal, propounded a theory of compensation explaining the anomalies 
in terms of underlying density. Putnam and Gilbert® established 
a considerable degree of isostatic equilibrium for the United States in 
1895, and their conclusions have been verified in still greater detail 
? Parts I and II appeared in the GEoL. MaG. for February and March, 1915, 
pp. 60-71, 102-12. 
= Tat. Jeffreys, “ The Mechanical Properties of the Earth’? : The Observatory, 
491, p. 348, 1915. 
3 Bull. Phil. Soc. Wash., xi, p. 53, 1889. 
* Or hydrodynamic ? ? 
° C.R,, vol. xxix, p. 730, 1849. 
6 Phil. Trans. Roy. Soc., vol. exlv, 1885. 
” Thid. 
8 Bull. Phil. Soc. Wash., xiii, p. 31, 1895. 
