THE SEA. BOTTOM
55
sea, but this is not so because, as a result of the mixing of
the fresh water With the sea, the ne particles become
attracted electrically to one another and so form large
masses which readily sink just at the region of mixing of
the fresh water and the salt.
=
_
'
At about '100 fathoms the action of waves and of trans—
porting currents ceases
to be eective, and below this line,
which is called the mud—line
roughly to the
Continental edge, the particles come to rest permanently
and from this region outwards the bed of the ocean is covered
=
everywhere with mud.
Terrigenous deposits are found
both above and below this line, depending on its nearness
to land, and their nature naturally corresponds to the nature
of the land from which they are derived. ’ Between tide
marks and in shallow waters they consist of sand, gravel,
shingle and boulders‘ with 'mud in sheltered areas, rather
further from the shore are grave], Sands, banks of living
and dead shells, and, especially opposite estuaries, banks
of mud.
If the land is volcanic this is reected in the
volcanic nature of the deposits, if of coral origin the deposits
are formed of nely divided coral while, off continents, the
‘
deposits are usually quartz gravels and sands together with
marls.
The
terrigenous deposits beyond the mud-line
were
divided by Murray into ve categories—blue, red,
green, volcanic and coral muds, the rst three varying
somewhat in their chemical Content but having much in
Common while the nature of. the last two is clear.
_
_
areas far from land the dep‘osits on the sea bottom
consist exclusively of material which has dropped down
fro‘m the surface waters.
This second type of marine
deposit has been called Pelagic by Murray (Plate 19), and
is made
of volcanic dust which has fallen on to the surface
of ”the water fr0m the air, or of dead animals or plants
Which, as they, die, rain down upon the.bottomfar
below.?
55
sea, but this is not so because, as a result of the mixing of
the fresh water With the sea, the ne particles become
attracted electrically to one another and so form large
masses which readily sink just at the region of mixing of
the fresh water and the salt.
=
_
'
At about '100 fathoms the action of waves and of trans—
porting currents ceases
to be eective, and below this line,
which is called the mud—line
roughly to the
Continental edge, the particles come to rest permanently
and from this region outwards the bed of the ocean is covered
=
everywhere with mud.
Terrigenous deposits are found
both above and below this line, depending on its nearness
to land, and their nature naturally corresponds to the nature
of the land from which they are derived. ’ Between tide
marks and in shallow waters they consist of sand, gravel,
shingle and boulders‘ with 'mud in sheltered areas, rather
further from the shore are grave], Sands, banks of living
and dead shells, and, especially opposite estuaries, banks
of mud.
If the land is volcanic this is reected in the
volcanic nature of the deposits, if of coral origin the deposits
are formed of nely divided coral while, off continents, the
‘
deposits are usually quartz gravels and sands together with
marls.
The
terrigenous deposits beyond the mud-line
were
divided by Murray into ve categories—blue, red,
green, volcanic and coral muds, the rst three varying
somewhat in their chemical Content but having much in
Common while the nature of. the last two is clear.
_
_
areas far from land the dep‘osits on the sea bottom
consist exclusively of material which has dropped down
fro‘m the surface waters.
This second type of marine
deposit has been called Pelagic by Murray (Plate 19), and
is made
of volcanic dust which has fallen on to the surface
of ”the water fr0m the air, or of dead animals or plants
Which, as they, die, rain down upon the.bottomfar
below.?