GREATER PROBLEMS OP PHYSICAL GEOLOGY. 
53 
forces which determine the figure of the earth. That figure 
we know to be one which a liquid or viscous body of large 
size will take when subject only to the forces arising from 
rotation around an axis and to the mutual gravitation of its 
own parts. This form is an oblate spheroid. 
The spherical form, however, is only approximate. We 
find large portions of its surface protruding into continents 
and islands, while others are sunken to form oceanic basins. 
How did these inequalities arise ? If the form of the earth 
is nearly spheroidal why is it not exactly so ? It has always 
been supposed that this nearly spheroidal form implies that 
the earth, if not • liquid, is certainly not rigid enough to 
maintain any other form against the forces of its own grav¬ 
itation. Even if the earth were a mass of unbroken steel no 
great departure from this shape could be maintained for a 
moment. It would straightway collapse and flow into a 
spheroidal form. But if gravitation compels it to take a 
nearly spheroidal shape why should it stop short of making 
it perfectly so ? Perhaps it will be said that while the rigid¬ 
ity of rocks may be insufficient to permit a great deforma¬ 
tion of the normal spheroid it may be sufficient to permit a 
small one. Before discussing this point it will be necessary 
to introduce a consideration which has seldom been touched 
upon by geographers or geologists. 
If the earth were composed of homogeneous matter its 
normal figure of equilibrium without strain would be a true 
spheroid of revolution; but if heterogeneous, if some parts 
were denser or lighter than others, its normal figure would 
no longer be spheroidal. Where the lighter matter was ac¬ 
cumulated there would be a tendency to bulge, and where 
the denser matter existed there would be a tendency to flatten 
or depress the surface. For this condition of equilibrium of 
figure, to which gravitation tends to reduce a planetary body, 
irrespective of whether it be homogeneous or not, I propose 
the name isostasy. I would have preferred the word isobary, 
but it is preoccupied. We may also use the corresponding 
adjective, isostatic. An isostatic earth, composed of homoge- 
