The  consequence  is  a  swift  current  into  the  bay  at  flood-tide  and

Digitized  by  VjUUVlt:


174  LAKES

into  the  sound  at  ebb.  The  floor  of  the  British  Channel,  over
which  the  tidal  currents  run  very  rapidly,  is  swept  bare  of  sand,
which  is  carried  into  the  North  Sea.  By  such  means  as  this,  the  sea
cuts  away  the  land  to  depths  much  greater  than  unassisted  waves
can  effectively  reach.
Rocks  are  also  attacked  chemically  by  the  solvent  and  decom-
posing action  of  sea-water.  The  silicates,  such  as  the  felspars,
augite,  hornblende,  etc.,  are  attacked  much  more  rapidly  than  in
fresh  water.  In  shoal  water  and  on  the  shore  this  action  is  obvious
only  in  spots  sheltered  from  the  direct  assault  of  the  waves,  because
the  products  of  decomposition  are  immediately  removed  by  the  surf
and  the  mechanical  work  is  so  much  more  striking.  Limestone
coasts  suffer  from  solution  by  sea- water,  and  are  characterized  by
long  caverns  and  tunnels,  though  sea  caves  are  worn  by  the  surf  in
all  classes  of  rocks  In  the  profound  depths  of  the  oceanic  basins,
where  the  water  is  never  disturbed  and  where  its  motion  is  ex-
tremely slow,  chemical  activity  becomes  relatively  very  impor-
tant. Calcareous  shells  are  completely  dissolved,  and  the  volcanic
debris,  which  covers  the  sea-bottom  over  vast  areas,  is  disinte-
grated into  a  characteristic  red  clay.
3.  Lakes

In  comparison  with  the  long  life  of  the  earth,  lakes  must  be
regarded  as  merely  temporary  bodies  of  water,  which  will  sooner
or  later  disappear,  either  by  being  drained  of  their  waters  or  by
being  filled  up  with  the  sediments  which  are  washed  into  them.
