he volume of the hydrosphere is a little
too great for the true basins, and it runs over, covering the borders of
the continents" (see Continental Shelf). Several theories have been
advanced to account for the roughly triangular shape of the continents,
but that presenting the least difficulty is the one expressed above,
"since in a spherical surface divided into larger and smaller segments
the major part should be polygonal, while the minor residual segments
are more likely to be triangular."

As bearing on this geological idea, it is interesting to notice in this
connexion that the areas of volcanic activity are mostly where continent
and ocean meet; and that around the continents there is an almost
continuous "deep" from 100 to 300 m. broad, of which the Challenger Deep
(11,400 ft.) and the great Tuscarora Deep are fragments. If on a map of
the world a broad inked brush be swept seawards round Africa, passing
into the Mediterranean, round North and South America, round India, then
continuously south of Java and round Australia south of Tasmania and
northward to the tropic, this broad band will represent the encircling
ribbon-like "deep," which gives strength to the suggestion that the
continents in their main features are permanent forms and that their
structural connexion with the oceans is not temporary and accidental.
The great protruding or "squeezed" segments are the Eurasian (with an
area roughly of twenty-four, reckoning in millions of square miles),
strongly ridged on the south and east, and relatively flat on the
north-west; the African (twelve), rather strongly ridged on the east,
less abruptly on the west and north; the North American (ten), strongly
ridged on the west, more gently on the east, and relatively flat on the
north and in the interior; the South American (nine), strongly ridged on
the west and somewhat on the north-east and south-east, leaving ten for
the smaller blocks. The sum of these will represent one-third of the
earth's surface, while the remaining two-thirds is covered by the ocean.
The foundation structure of the continents is everywhere similar. Their
resulting rocks and soils are due to differential minor movements in the
past, by which deposits of varying character were produced. These
movements, taking place periodically and followed by long periods of
rest, produce continued stability for the development and migration of
forms of life, the grading of rivers, the development of vari