OCEANIC MOVEMENTS. 75

 

valleys, though less marked in this region, are still dis-
tinguishable.

The true bed of the ocean begins at a considerable dis-
tance from the eastern coast of North America. For dis-
tances of from 75 to 100 miles, the depth scarcely exceeds
600 feet ; but from this point it descends, by steep terraces,
to profound depths.

The British Isles are connected with the continent of
Europe by a large submerged plateau, which underlies
nearly the whole North Sea, and extends for considerable
distances off the western and southern coasts. The depth
of this part of the ocean is nowhere very great.

204. The Indian Ocean.—The shape of the
shore-line is, in general, triangular. This ocean
has no connection with the Arctic, but is entirely
open on the south, where it merges into the great
water-area of the globe: the basins of the Ant-
arctic and Pacific.

Shape of the Bed.—A submarine plateau extends to the
south off the western coast of Hindostan. Its summits
form the Laccadive, Maldive, and Chagos Islands, and pos-

sibly extends in the same direction as far as Kerguelen
Island.

205. The Antarctic and Arctic Oceans.—The
shore-line of the Arctic has the shape of an ir-
regular ring. The shore-line of the Antarctic is
probably of the same shape.

But little is known concerning the beds of these
oceans. From the very limited land-areas south
of lat. 50° S., the bed of the Antarctic is presum-
ably deeper than that of the Arctic, except toward
the south pole, where it is probably shallower.

206. Ooze Deposits—Foraminiferal Land.—
The reef-forming coral polyps are not the only
animalcule the accumulation of whose bodies
after death add to the land-masses of the earth.
Deep-sea soundings show that over extended areas

Fig. 70. Foraminifera,

the floor of the ocean is evenly covered with a
creamy layer of mud or ooze, which, like the
deposits of the coral animalcula, is composed
principally of carbonate of lime. This ooze con-
sists almost entirely of microscopic skeletons of a

 

group of animalcule known as the Foraminifera,
from the great number of perforations or open-
ings in their hard parts. These animalcule are
so small that 1,000,000 are equal in bulk to only
one cubic inch. They appear to live in the layers
of water near the surface, and after death to
fall gradually to the bottom of the sea. Sound-
ings show their presence over very extended
areas.

Many of the very deep parts of the ocean’s bed
are covered, not with foraminiferal deposits, but

‘with a layer of red mud composed of finely-di-

vided clay. Its origin is probably as follows:
In very deep parts of the ocean before the fora-
miniferal deposits reach the bottom their limey
matters are dissolved, and the undissolved parts
form the deposits of fine red mud.

—0 Stoo

xX CHAPTER IL.

Oceanic Movements. ~

207. The Oceanic Movements can be arranged
under three heads: waves, tides, and currents.
Waves are swinging motions of the water,
caused by the action of the wind. Their height
and velocity depend on the force of the wind, and
the depth of the basin in which they occur. The
stronger the wind, and the deeper the ocean, the
‘higher the waves and the greater their velocity.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 71, Ocean Waves,

Height of Waves.—Scoresby measured waves in the
North Atlantic 43 feet above the level of the trough.
Waves have been reported in the South Atlantic, off the
Cape of Good Hope, between 50 and 60 feet high. Navi-