DEPTH   OF   THE    OCEAN  27

Deeps. — There  is  a  special  interest  attached
to  the  greater  depths  of  the  ocean,  so  the
writer  some  years  ago  called  all  those  areas
where  the  depth  exceeds  3000  fathoms  (18,000
feet)  "  deeps,"  and  gave  them  distinctive
names,  generally  after  exploring  ships,  navi-
gators and  scientific  men,  thus  following  the
practice  of  naming  mountain  peaks.  Fifty-
seven  such  "  deeps,"  based  upon  some  500
soundings,  are  now  known  :  32  in  the  Pacific,
18  in  the  Atlantic,  5  in  the  Indian  Ocean,  and
2  in  the  Southern  Ocean.  Their  positions  are
shown  in  Plate  II.  They  vary  greatly  in
outline  :  in  some  cases  they  are  very  irregular
or  basin-like,  but  often  they  are  trough  or
trench-shaped  and  relatively  close  to  continent-
al land.  Some,  like  the  Valdivia  Deep  in  the
Southern  Ocean  and  the  Murray  Deep  in  the
North  Pacific,  are  believed  to  cover  a  large
area,  while  a  few  are  very  small,  being  based
on  single  isolated  soundings.  The  total  area
covered  by  these  deeps  is  altogether  only  about
7  per  cent,  of  the  ocean-floor.  Occasionally
a  cone-like  elevation  with  a  depth  on  its
summit  of  less  than  3000  fathoms  rises  from
the  centre  of  a  deep.
