ever, most significant, for it throws light upon the history of
the earth's mass. Computation shows that the measure of flattening at
the poles is just what would occur if the earth were or had been at
the time when it assumed its present form in a fluid condition. We
readily conceive that a soft body revolving in space, while all its
particles by gravitation tended to the centre, would in turning
around, as our earth does upon its axis, tend to bulge out in those
parts which were remote from the line upon which the turning took
place. Thus the flattening of our sphere at the poles corroborates the
opinion that its mass was once molten--in a word, that its ancient
history was such as the nebular theory suggests.

Although we have for convenience termed the earth a flattened
spheroid, it is only such in a very general sense. It has an infinite
number of minor irregularities which it is the province of the
geographer to trace and that of the geologist to account for. In the
first place, its surface is occupied by a great array of ridges and
hollows. The larger of these, the oceans and continents, first deserve
our attention. The difference in altitude of the earth's surface from
the height of the continents to the deepest part of the sea is
probably between ten and eleven miles, thus amounting to about two
fifths of the polar flattening before noted. The average difference
between the ocean floor and the summits of the neighbouring continents
is probably rather less than four miles. It happens, most fortunately
for the history of the earth, that the water upon its surface fills
its great concavities on the average to about four fifths of their
total depth, leaving only about one fifth of the relief projecting
above the ocean level. We have termed this arrangement fortunate, for
it insures that rainfall visits almost all the land areas, and thereby
makes those realms fit for the uses of life. If the ocean had only
half its existing area, the lands would be so wide that only their
fringes would be fertile. If it were one fifth greater than it is, the
dry areas would be reduced to a few scattered islands.

From all points of view the most important feature of the earth's
surface arises from its division into land and water areas, and this
for the reason that the physical and vital work of our sphere is
inevitably determined by this distribution. The shape of the seas and
lands is fixed by the positions at which the upper le