Theory of Isostacy. 
469 
is involved in the first supposition, and hence we are driven to 
the ’second, i. e., that the continental and sea areas are ap¬ 
proximately in isostatic equilbrium. 1 
In other words, the continents are mainly sustained above the 
level of the ocean in the same way as a ship. The hull of a 
great vessel rises 20 or more feet above the surface of the water, 
because upon the average it is lighter than the water which 
surrounds it. As the ship is loaded it sinks deeply into the water; 
when emptied, it rises higher above the surface. 
Theory of Isostacy .-—In many discussions of isostacy it is 
assumed that the level of the sea is absolute, and that its sur¬ 
face is a safe datum plane. However, it is evident that all 
shifting of earth material, either vertically or horizontally, 
either by deformation or by denudation, results in changing 
the level of the ocean in the absolute sense, that is, the aver¬ 
age distance of its surface from the center of the earth. Thus, 
an important subsidence in one region may produce an apparent 
movement of all continental shores. If the subsidence be of 
the sea bed, this will result in apparent uplift of the continents. 
If it be of some continental region, this will result in the ad¬ 
vance of the sea over this region, and because of the consequent 
absolute fall of the sea, there will be an apparent small 
uplift of the great undisturbed remainder of the continental 
masses. 
Evidence of the existence of approximate equilibrium of con¬ 
tinental and sea areas has been found in gravity determinations, 
and especially in recent determinations of the force of gravity 
by pendulum experiments made by Putnam. 2 
As a result of these we find we cannot suppose that there is 
any such delicate adjustment of the earth masses as might be 
1 Appendix to Babbage’s Ninth Bridgewater treatise, by Sir John 
Herschel, 1837, pp. 212-213. A treatise on attractions, Laplace’s func¬ 
tions, and the figure of the earth, by John H. Pratt: 4th ed., 1871. On 
some of the greater problems of physical geology, by C. E. Dutton: Phil. 
Soc. Wash., Vol. XI, 1888-91, p. 53. 
s Results of a transcontinental series of gravity measurements, by G. R. 
Putnam, and notes on gravity determinations reported by G. R. Putnam, 
by G. K. Gilbert: Bull. Phil.[Hoc. Wash., Vol. XIII, 1895, pp. 31-76, PI. 
V, Figs. 1-3. 
