_oblate_ ellipsoid.

"We are far from meaning here to trace the process _by which_ the earth
really assumed its actual form; all we intend is to show that this is
the form to which, under a condition of a rotation on its axis, it must
_tend_, and which it would attain even if originally and (so to speak)
perversely constituted otherwise."[743]

In this passage, the author has contemplated the superficial effects of
aqueous causes only; but neither he nor Playfair seem to have followed
out the same inquiry with reference to another part of Hutton's system;
namely, that which assumes the successive fusion by heat of different
parts of the solid earth. Yet the progress of geology has continually
strengthened the evidence in favor of the doctrine that local variations
of temperature have melted one part after another of the earth's crust,
and this influence has perhaps extended downwards to the very centre.
If, therefore, before the globe had assumed its present form, it was
made to revolve on its axis, all matter to which freedom of motion was
given by fusion, must before consolidating have been impelled towards
the equatorial regions in obedience to the centrifugal force. Thus lava
flowing out in superficial streams would have its motion retarded when
its direction was towards the pole, accelerated when towards the
equator; or if lakes and seas of lava existed beneath the earth's crust
in equatorial regions, as probably now beneath the Peruvian Andes, the
imprisoned fluid would force outwards and permanently upheave the
overlying rocks. The statical figure, therefore, of the terrestrial
spheroid (of which the longest diameter exceeds the shortest by about
twenty-five miles), may have been the result of gradual and even of
existing causes, and not of a primitive, universal, and simultaneous
fluidity.[744]

Experiments made with the pendulum, and observations on the manner in
which the earth attracts the moon, have shown that our planet is not an
empty sphere, but, on the contrary, that its interior, whether solid or
fluid, has a higher specific gravity than the exterior. It has also been
inferred, that there is a regular increase in density from the surface
towards the centre, and that the equatorial protuberance is continued
inwards; that is to say, that layers of equal density are arranged
elliptically, and symmetrically, from the exterior to the centre. These
conclusions, however, have been deduced rather as a conseq