4
THE SEAS
.
'
=
Until quite reCently it was thought thatafter a‘depth of
'
1,000 fathoms was reached the slope of the=seaoor became
‘
almost imperceptible and was in most places not unlike a
'
vast
and
undulating plain. , That although it
stretched down hundreds of fathoms deeper, its extent was
—
so great that in most places we; should be unable to ap—
preciate any gradient whatever.
From about 2,000 fathoms
-
d0wnwards this very gradually shelving ocean bed was
known as the Abyss or Abyssal Plain.
’
had long been known that the whole bed of the ocean
was not absolutely at but presents considerable variation
in depth over large areas. There is, for instance, stretching
north and south through the whole Atlantic ocean a con—
tinuous ridge between
thousand and two thousand
fathoms_in depth, surrounded on either side by water down
to 4,000 fathoms deep (Plate 3).
From this ridge rise the
Dceanic islandsof the Azores, the Saint Paul Rocks, Ascen'
sion and Tristan d’Acunha.
But the introduction of echo sounding has made it
possible for us to- obtain many observations for
sounding made with a lead.
As a result our conception
'
of the, ocean bedwill become materially altered.
Although
overlarge areas the vast masses of silt and bottom deposits
'
tend to smooth out the surface, it is.now known that
-
in some regions there are mountain ranges, peaks and clis
_
just as we nd on land. Most
for instance, is the
’.
»
discovery of canyons 4,000 feet deep off New York.
'
,
The‘regio'ns in which the bottom lies belowa depth of
.
3,000 fathoms are knoWn as “
deeps.” The greatest depth
'_ yet Ïre00rded 'is_ 5,3 50 fathoms o the island of Mindanao
,
'
in the Philippines, in the Pacic. This enormous depth,
,
,
OVer siX and- a quarter miles, is hard to Visualize.
The Ï‘Ï
_
reader can'îaerhapsbest-realize it if he imagine that the
highest mountain in the world, Mount —- Everest,
.