Theory of Isostacy. 
47 & 
land results in a disturbance of isostatic equilibrium, the land 
becoming more and more deficient in gravity. At the same 
time the adjacent sea area may become overloaded. Where a 
sea border is subsiding, and the corresponding unloaded land 
area is rising, this shows that there is not exact isostatic equi¬ 
librium, but a disturbing stress, which surpasses the elastic 
limits of the rocks under the conditions in which they exists 
although the movement tends to prevent further departure from 
equilibrium. 
The ordinary explanation .offered for the sinking of the con¬ 
tinental border and the rising of the interior is the loading of 
the one and the unloading of the other. 
While it cannot be doubted that this is a factor in the process, 
it is by no means clear that this is the only or chief cause. As 
shown later, other forces are at work resulting in earth 
movements, and consequently in advances and recessions of 
the sea coast of the most complicated character. During these 
earth movements great stress-differences may be produced be¬ 
tween the borders and the interiors of the continents, and in the 
same direction as those of erosion. Under these conditions the 
loading of the sea borders and the unloading of the interiors 
may give the additional force necessary to produce a vertical 
stress-difference greater than the rigidity of the rocks can con¬ 
tinuously resist, and therefore inaugurate such movements as 
described. 
Where bordering the sea an area is under stresses so great that 
it responds by uplift or subsidence, because of its rigidity it 
may carry with it a smaller adjacent area not similarly stressed, 
and thus shift the sea shore. 
Where denudation of the continents and deposition in the ad¬ 
jacent sea beds are the only causes disturbing equilibrium, before 
movements begin a considerable slice must probably be removed 
from the continents, and a thick deposit be formed in the sea, in 
order to accumulate a sufficient stress-difference to overcome the 
rigidity of the rocks. How great this difference must be in 
order to act slowly can only be conjectured. 
It is possible that successive relative uplifts may result from 
the contractional forces, in connection with an alternation of 
