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AN INTRODUCTION TO OCEANOGRAPHY

AN INTRODUCTION TO

OCEANOGRAPHY

WITH SPECIAL REFERENCE TO
GEOGRAPHY AND GEOPHYSICS

JAMES JOHNSTONE, D.Sc.

Professor oj Oceanography in the
University oj Liverpool

THE UNIVERSITY PRESS OF LIVERPOOL LIMITED
HODDER AND STOUGHTON LIMITED, LONDON

1923

Made and Printed in Great Britain by C. Tinling & Co. Ltd.
53, Victoria Street, Liverpool,
and at London and Prescot.

CONTENTS

Preface
Chapter I.
Chapter II.

IX.

The World- Ocean

page

Chapter III.

The Origin of the Oceans

The origin of the earth, 8 ; the gaseous-
molten hypotheses, 8 ; the tetrahedral
earth hypothesis, 10 ; spherical harmonio
deformation, 11 ; failure of these
hypotheses, 14 ; planetesimal hypothesis,
15 ; growth by accretion, 20 ; rotational
changes, field-tracts, 21 ; oceanic and
continental regions, 29 ; permanence of the
oceans, 3 1 ; smallness of these effects, 32.

The Depths of the Ocean

Estimation of depth, 34 ; sounding
machines, 36 ; the bottom gradients,
39 ; continental shelf, 40 ; continental
slope, 41 ; form of the ocean floor, 44 ;
contours, 46 ; the Antarctic Ocean, 47 ;
the Arctic Ocean, 48 ; the Atlantic Ocean,
60 ; the Pacific Ocean, 66 ; the Indian
Ocean, 58.

90;
92;
96;
97.

Chapter IV. The Sea Bottom

Sea bottom deposits, 62 ; original nature
of the deposits, 63 ; classification, 65 ;
categories of deposits, 66 ; httoral deposits,
68 ; shallow-water deposits, 68 ; terri-
genous muds, 71 ; neritic deposits, 74 ;
oceanic neritic deposits, 78 ; coral forma-
tions, 79 ; origin of coral reefs, 82 :
pelagic deposits, 88 ; plankton,
pteropod ooze, 92 ; globigerina ooze,
radiolarian ooze, 94 ; diatom ooze,
red clay, 96 ; pelagic oozes in general.

Chapter V. The Oceanic Margins

The foreshore, 103 ; foreshore gradients,
104 ; shallow-water bottom gradient, 106 ;
the marginal seas, 107 ; the epi-continental
seas, 108 ; the Mediterranean and relict
seas, 109 ; marginal sea-bottom deposits,
111; foreshore neritic deposits, 112;
shallow-water neritic deposits, 112; the
continental margins, 113; interior of the
earth, 114; isostasy, 117 ; earth-movements
and isostasy, 121 ; the shield-lands, 122 ;
the oceanic abysses, 122 ; the great features
of the continental margins, 124 ; the
Continental displacement hypothesis, 127.

34

61

100

viii CONTENTS

Chapter VI. The Chemistry of Sea Water .

Chemistry of sea-water, 133 ; composition
of sea-salts, 134 ; salinity, 137 ; variations
of salinity, 138 ; salinity charts, 140 ; the
gases of sea-water, 146 ; alkalinity, 148
photosynthesis and carbon dioxide, 151
distribution of gases in sea -water, 154
food substances in the sea, 156 ; origin of
the salts of sea water, 160.

Chapter VII. The Physical Characters of
Sea Water ....

Pressure, 165 ; temperature, 167 ; tempera-
ture variations, 170 ; oceanic isotherms,
174 ; ice, 184 ; vertical temperature vari-
ations, 188 ; seasonal temperature changes,
192 ; density of sea-water, 192 ; osmotic
pressure, 195; optical characters, 196.

Chapter VIII. The Tides ....

The tides from ordinary observation, 199 ;
rise and fall, 199 ; tide gauges, 200 ; sea
level, 201 ; range, 202 ; periodicity, 202 ;
springs and neaps, 202 ; types of tides,
204 ; tidal streams, 206 ; tide-generating
force, 210 ; aperiodic tidal variations, 217 ;
theories of the tides, 219 ; the equilibrium
theory, 220; the progressive wave theory,
220 ; stationary wave theory, 222 ; tidal
predictions, 225 ; empirical prediction
methods, 226 ; establishment of a port, 230 ;
prediction by harmonic methods, 239 ; the
" tidal satellites," 241 ; tidal constituents,
242 ; harmonic analysis, 245 ; harmonic
synthesis, 249 ; tides as marine agencies,
250 ; long period tidal variations, 251.

Chapter IX. The Oceanic Circulation

The general scheme, 255 ; causes, 257 ;
Ferrell's law, 260 ; effect of the earth's
rotation, 263 ; currents as they are, 263 ;
equatorial currents, 264 ; the effect of the
land, 265 ; depth, 266 ; salinity, 267 ; the
great oceanic currents, 270 ; Atlantic
currents, 270; Gulf Stream, 271 ; Labrador
stream, 271 ; Sargasso Sea and the Atlantic
stream, 271 ; South Atlantic currents, 273;
Pacific oceanic circulation, 276 ; the Indian
Ocean, 279 ; the Polar seas, 283 ; the
marginal seas, 285 ; the North Sea, 286 ;
seasonal variations, 287 ; mid-Atlantic
circulation, 288 ; the Atlantic gjnral, 289 ;
the vertical circulation, 293 ; methods of
investigation, 296.

131

165

199

CONTENTS ix

Chapter X. Secular Changes in the Ocean 301

The shield-lands, 303 ; oceanic depressions,
304 ; oceanic -continental margins, 304 ;
the Pacific Ocean, 309 ; the Atlantic
Ocean, 314 ; the Indian Ocean, 319 ;
History of the oceans, 321 ; Evidence
available, 321 ; paleontological evidence,
323 ; ancient land and sea, 325.

Appendix. Literature Consulted . . 337

Index ....... 343

PREFACE

The modem science of oceanography has been developed
to such an extent during the last twenty-five years that it
is now impossible to deal comprehensively with its results
in a book of any reasonable size. There is hardly any
limit to the amount of descriptive detail that might be
given with respect to such subjects as the currents of the
ocean and its tributary seas ; the distribution of the
various kinds of bottom deposits ; or the chemistry of
sea water in all parts of the world. Further, the science
blends into marine biology in such an intimate manner
that two lines of treatment are now quite necessary :
Physical Oceanography on the one hand and Hydrobiology
on the other. It has also its mathematical sides in the
modem theories of the tides and in hydrodynamics, and
for the study of these subjects a certain discipline and
technique are necessary that have been acquired by only
very few scientific men. Lastly the study of the ocean
can be approached from the aspect of geography and
geophysics.

Although an attempt has been made here to deal in a
general way with the science of oceanography, it is rather
with the outlook of the student of geography and geology
that the book has been written. Even then the method
of treatment is as general as possible, so that it is hoped
that a summary account of marine science from the
physical side has been provided, and that it will be useful
not only to those whose main interest is in geography
and geology but also to biologists — who, it may be noted,
have mainly been responsible for the development of our
science of oceanography.

A very great difficulty in such a book as this is the
illustrations. A multitude of charts and maps is quite

xii PREFACE

necessary in order that the presentation of the results may
be helpful in the highest degree. This wealth of illustration
is, of course, quite impossible at the present time, and so
the best kind of reader will have to make very frequent
use of a good modern atlas of physical geography, and he
will also have to consult the small scale Admiralty charts
ior details of ocean depths, currents, tidal streams, and
the like. The figures contained in the text are, of course,
in the nature of sketches, and the amount of reduction
and simplification that has been involved in their prepara-
tion precludes the statement of much desirable detail.
For this the reader must seek elsewhere, but he will
•certainly find the task of doing so interesting to a remark-
able extent.

No references are given in the body of the book, but I
have prepared a short Appendix which gives most of the
authorities consulted, and which will also be a guide to the
reader who cares to go further with the subject. The
books and journals mentioned are mostly such as are
■easily obtained in any good library.

J.J.

University of Liverpool,
October, 1923.

CHAPTER I

THE WORLD-OCEAN

When we look at the Ocean as it is represented on a
globe certain regularities become apparent : that is, the
distribution of land and water does not seem to be purely
arbitrary. Figures 1 to 6, which follow hereafter, are
made from photographs of a globe, and they may be
taken to represent telescopic views of the earth, seen, it is
true, under rather favourable conditions. These
appearances we must now discuss. Fig. 1 is a view of the
Antarctic hemisphere. Here we see a circumpolar land

Fig. 1. Telescopic view of the earth : the Antarctic aspect.

mass, the Antarctic continent. Although the land
boundaries of this are not yet precisely known, the depths
of the surrounding ocean indicate very clearly that it is

B 1

2 AN INTRODUCTION TO OCEANOGRAPHY

a truly continental earth feature. About it there is the
great southern ocean and this is nowhere less than 600
miles in width. Three meridians may be taken as dividing
it into as many sectors. These are :

(1) About 75° West longitude. This meridian passes
through Graham Land in Antarctica, and Tierra del
Fuego, in South America. Here the Southern
Ocean is most constricted, and it is also more shallow
than elsewhere. At its narrowest part it is about
600 miles wide.

(2) About 20° East longitude. This meridian passes
through the South pole and the Cape of Good Hope.
The Southern ocean is here rather over 2,000 miles
in width.

(3) About 140° East longitude. This passes through
the pole and the Australian Continent in the
neighbourhood of the Gulf of Carpentaria. In its
vicinity the Southern Ocean is about 1,600 miles in
width.

Thus there is a nearly symmetrical arrangement of land
and sea round about the South pole. There is a polar
land- mass of considerable elevation, and round it there is
a circumpolar oceanic region. Opening out from this,
between America and Africa is the South Atlantic Ocean,
and this covers a sector of 90°. Between Africa and
Australia, and covering a sector of 120°, is the Indian
Ocean, while between Australia and South America, a
sector of about 150°, is the Pacific. If the arrangement
were strictly symmetrical these three sectors would each
be 120°.

Fig. 2 represents the view seen where the North Pole
is in the centre of the field. Here there is very roughly
the opposite kind of distribution of land and water from
that which Fig. 1 represents. In the centre, round about
the North Pole, there is an oceanic area of considerable
dimensions and depths, and round this again is a circum-
polar land-mass, the principal boundaries of which are

THE WORLD-OCEAN 3

the Eur- Asian and North American Continents. The land
ring is broken through in three places : between Siberia
and Alaska, in the narrow Behring Straits, is the entrance
into the Pacific ; between Europe and the eastern coast
of Greenland is the Norwegian Sea (and the entrance to
the North Atlantic), while between Greenland and North
America is Davis Straits. These communications between
the Arctic, and the Pacific and Atlantic Oceans are, as
we shall see later, rather shallow and not truly " oceanic "
in character. They do not obscure our general picture.

Fig. 2. Telescopic view of the earth : the Arctic aspect.

which is that of a Polar oceanic area surrounded by
continental land masses with fringes of islands.

Three Oceanic " Gulfs " open out from the great
Southern Ocean.

Fig. 3 represents the Pacific water hemisphere.

Here we see the greatest oceanic area of all. The Pacific
Ocean is bounded by the coasts of America and Asia and
there is only the very small interruption formed by Behring
Strait. Then the Asian land boundary is continued across
to Australia by an archipelago round which the ocean is

4 AN INTRODUCTION TO OCEANOGRAPHY

relatively shallow. Below Australia the Pacific passes
into the Southern Ocean.

Fig. 3. Telescopic view of the earth : the Pacific aspect.

Fig. 4. Telescopic view of the earth : the Indian Ocean.

This is our second Oceanic Gulf.
The third Oceanic Gulf is the Atlantic one and this, like
the Pacific Ocean, joins together the Antarctic and Arctic

THE WORLD-OCEAN 5

Oceans. It is convenient, for reasons that will appear in
the next chapter, to regard it as consisting of two oceanic
areas, North and South Atlantics. To the south the
communication between Atlantic and Antarctic is very
wide and deep, while to the north the opening into the
Arctic is constricted and shallow. Also each of the
continents of Africa and South America presents towards
the other a bold promontory, Cape Verde and Cape San
Roque respectively, and these suggest the division of the
whole Atlantic into North and South regions.

Fig. 5. Telescopic view of the earth : the South Atlantic aspect.

Fig. 6 represents the North Atlantic Basin, directly
continuous on the south with the South Atlantic and
bounded on the north in the way that we have noticed.

Now it is obvious that there are certain features in this
distribution of land and sea on the surface of the earth
that call for explanation. These are :

(1) The north polar oceanic region placed antipodally
to the south polar continental region.

(2) The northerly, roughly annular, continental land
region which we set over against the southerly,
annular, oceanic region.

AN INTRODUCTION TO OCEANOGRAPHY

(3) The condition that the great oceans are (roughly)
antipodal to the great continental regions.

(4) The general forms of the continents — these diminish
in breadth from north to south (North and South
America and Africa) and, as a consequence, the
great land margins run obliquely towards and across
the equator.

Fig. 6. Telescopic view of the earth : the Xorth Atlantic aspect.

(5) A rough tri-radiate arrangement of land and ocean
round the poles. This is shown best by the ways in
which S. America, Africa and Australia are directed
towards the Antarctic Continent, but it is also
suggested in the contours of North America, Green-
land and Asia. These land masses are broad to the
north, where they form the boundaries of the Arctic
Ocean and they taper away towards the equator.

This arrangement is the reverse of that which we see in

the southern hemisphere.

CHAPTER II
THE ORIGIN OF THE OCEANS

The precise distribution of land and sea on the face of
the earth is, to a great extent, " accidental " : that is, it
is due to the operation of a great number of causes, no one
of which is much more important than any other. Round
the coasts of all the continents there is an extensive region
where the sea is not more than about 1,000 fathoms in
depth, and here the changes in the extent and form of the
land have been very frequent in geological, and even in
historic time. There is continual erosion of the coast and
the extent and rate of this depend on the nature of the
materials of the land. Thus shallow, submarine fiats are
formed on the one hand while, on the other, the eroded
materials may be carried away by the sea to form shoal^
elsewhere. Then there are movements of elevation and
depression, both of the dry land and of the sea-floor, and
the result is that the ocean transgresses on the land and
vice versa. So the contours of the continents fluctuate t6
a considerable degree and large tracts of the surface of
the earth are alternately land and water.

In spite of this flux certain broad, general features of
the earth are relatively permanent : they are the deep
depressions forming the oceanic basins and the great
continental plateaus and mountain ranges. These are
the characteristic and noticeable markings of the earth as
a whole. Even if they are not absolutely permanent, they
still maintain a recognisable form in spite of extensive
changes in the precise outlines of land and sea. They are
geological rather than geographical earth- features. They
are not " accidental " in the sense in which we have
employed the term above, but they are due to the operation

7

8 AN INTRODUCTION TO OCEANOGRAPHY

of several large causes which are predominant among the
numerous ones to which the fluctuations of land and oceanic
contours are due.

The origin of the earth. — These larger causes to which
continental and oceanic forms are due are cosmic ones, but
they must have terrestrial peculiarities. It has been noted
that the surface markings of the moon and the planet Mars
difEer notably from those of the earth. If those markings
indicate areas of elevation and depression (as in the case
of the earth), then the evolution of superficial planetary
features must have taken dif?erent directions in each case,
and we must seek for causes which have been in operation
on the earth, but not at all, or not to the same extent, in
its nearest neighbours in the solar system. Differences
may easily be found, as for instance, in the different
masses of the three bodies ; that of the moon being only
l/80th, and that of Mars 1/lOth, of the mass of the earth.
We have to deal, then, with factors which are both cosmic
and terrestrial ones. Hypotheses of the origin of the
continents and oceans must, therefore, depend, to some
extent, on the hypothesis of the origin of the earth itself
that we adopt.

The gaseous-molten hypotheses. — The classical hypothesis
of the origin of the earth is, of course, the Kant-Laplace
one, and its main outlines must be familiar to the reader.
It leads us to consider an earth which was originally
gaseous, and then molten, and very much larger than it
now is. As this fluid globe cooled it contracted and,
therefore, its rotatory speed increased so that centrifugal
forces developed. Modern mathematical research has
greatly modified the crudities of the original hypothesis
and has shown that the rotating molten earth must have
become pear-shaped — the stalk of the pear being a
protuberance on the equator. This knob became detached
when the rotatory velocity exceeded a certain limit, and
so the moon originated, repeating the process by which
the earth itself became detached from the parent, solar

THE ORIGIN OF THE OCEANS 9

mass. Tidal evolution followed : the moon, in its primitive
orbit, set up enormous tidal waves in the body of the fluid
earth, and these set up friction which had the threefold
effect of reducing the rotatory speed of the earth, reducing
the rotatory speed of the moon and increasing the distance
of the latter from the earth. Moon and earth cooled down
still further and skinned over, so to speak, by the formation
of solid, rocky crusts. For a time these crusts must have
been highly unstable, so that they became broken up again
and again, but by and by they became permanent and the
water vapour previously held in the earth's atmosphere
condensed to form the primitive ocean. The crater-like
elevations on the surface of the moon may be taken to
represent the results of the original instability of the crust
and the " boiling- out " of the original gases held in the
fluid body. The mass of the moon is, of course, too small
to generate a gravitational field intense enough to hold
water vapour and the other gases that now form the
earth's atmosphere. Sub-aerial or aqueous erosion did not
therefore, occur, nor was there chemical action by an
atmosphere, and so the original markings have remained.

The consequence of this hypothesis is the idea that the
earth was originally intensely hot and has been losing heat
throughout its geological history. That it is still hot inside
is shown by observations of subterranean temperatures.
This rise of temperature as we descend into the interior
of the earth is very variable : in Great Britain it may
increase at the rate of 1° F. for every 30 feet, or 1° F. for
every 130 feet. Taking an average, however, and adopting
the very risky method of extrapolating from the curve
showing the gradient, we can find what is the expected
temperature at the earth's centre, and so get some quite
improbable value, such as 350,000°F. There is, of course,
no reason why we should not conclude, with just as much
probability, that the heated regions beneath the earth's
surface are strictly local, not continuous with each other,
and at no great depth. That there are localised regions

10 AN INTRODUCTION TO OCEANOGRAPHY

where the temperature is high enough to hold rock in the
fluid condition is quite certain : volcanic phenomena
demonstrate this, but that the whole earth at a depth of
50 to 100 miles is so intensely hot as to be potentially
fluid is really a deduction from the gaseous-molten
hypothesis of origin. If — as is undoubtedly the case — the
assumption involves us in serious difiiculties we are quite
at liberty to reject it on the nature of the evidence in its
favour. Now the parent hypothesis, and the observed
temperature increase with increasing depth, have left us
with the very generally accepted belief that the earth as a
whole, was originally intensely hot, is still very hot inside
and is, and has been, cooling rather slowly. The superficial
layer is, however, cold although the internal mass is hot
and is cooling. Therefore the core must still be contracting
while the crust has ceased to contract merely because the
core is losing heat. It follows, then, that the crust must
fall in, so to speak, upon the shrinking interior and,
therefore, become crumpled, folded, buckled or otherwise
deformed. Here we have a basis for an hypothesis of the
origin of the oceanic depressions and the continental
elevations.

The tetrahedral earth hypothesis. — Admitting, then, that
the earth as a whole is, and has been losing heat, we have
an hypothesis which, merely as a consequence of that
assumption, professes to account for the existing series of
large elevations and depressions of the crust. A closed
tube collapsing under external pressure sometimes assumes
a shape which is roughly triangular in section and a hollow
globe similarly collapsing is said to cave in so as to
approximate in shape to a regular tetrahedron. In such
a solid there are four faces each of them an equilateral
triangle in shape, there are six edges of equal length and
four solid angles. If the earth tended to contract, on loss
of heat, in such a way we should expect to find that the
central parts of the tetrahedral faces would be nearer to
the centre than are the solid angles, or coigns, and so the

THE ORIGIN OF THE OCEANS 11

water of the ocean would gravitate towards the central
faces and the land masses would be placed round the
coigns. One such prominent land-mass is the Antarctic
Continent, and so we place a coign of the tetrahedron there.
Then the three meridians diverging from the centre of
this are approximately fixed in position, for evidently one
must pass through the southern apex of South America,
and the other two must be at angles of 120° distant on
either side. The other three coigns will then be situated
on these meridans and roughly in N. latitude 33°. If they
are joined by great circles the trace of the enclosed
tetrahedron on the surface of the earth is complete.

By no possibility can a regular tetrahedron be traced
on the earth's surface so as to approximate to this
hypothetical distribution of continents and oceans. The
proof of this will be shown by trying. In order, even to
make a rough approximation, the tetrahedron must be
deformed, and with this sacrifice of its attractive simplicity
the hypothesis loses much of its interest. But, apart
altogether from this practical test we are not certain that
its basal assumption — that the earth as a whole is losing
heat and contracting round its centre — is justified. And,
that being so, the lack of approximation of theory and test
seems to be fatal.

The hypothesis of spherical harmonic deformation. —
Starting with the hypothesis of the molten earth the
present distribution of land and water may be explained
in another way (which appears to be exclusive of the
tetrahedral hypothesis). In the molten condition the earth
body was, of course, far from being rigid. It protruded
equatorially in a very oblate, spheroidal shape and then
calved off the moon from a part of its equatorial bulge.
That left it asymmetrical along both the polar diameter
and along another at right angles to this. As it cooled and
contracted it would become more rigid, but until its
material became as unyielding as if it were solid granite,
the centre of gravity of the earth could not have been its

12 AN INTRODUCTION TO OCEANOGRAPHY

centre of rotation. This is because the material must have
been crushed under its own gravity. As the earth rotated
it must, therefore, have wobbled.

Assuming, then, that its centre of gravity was not the
same as the centre of figure, certain consequences may be
traced. If the centre of gravity were nearer to the North
than to the South Pole, water would tend to accumulate
on the former region, while at the latter, land would
protrude. If it were slightly pear-shaped so that the axis
of rotation passed through the long diameter of the pear,
water would accumulate round the " waist " of the figure.
The equatorial bulge would not be a simple swelling but
a series of low, annular ridges with furrows between them.
Further, it may be assumed that the substance of the
primitive earth was heterogeneous and, that being so, the
heavier materials would tend to fly out towards the
periphery by reason of centrifugal force. Thus the figure
of the earth, as it progressively stiffened would take a
certain form such that some regions on its surface Avould
be nearer to the centre of gravity than some other ones.
So there would be areas of depression and elevation, and
the water of the ocean would tend to accumulate on the
former. The mathematical investigation of the origin of
these elevations and depressions, as a consequence of the
assumptions made, cannot be reproduced here, and it may
be sufficient to indicate that it leads to a distribution
rather like that which now exists. We must, however,
take some liberties with geographical features : there
should be land at the North Pole instead of an obvious
oceanic depression and the boundary between the
continental and oceanic areas has to be taken at a depth
given by the 1,400-fathom contour line. There must be a
considerable degree of " smoothing " of the shapes of the
continents. When these qualifications are considered
along with the original assumption of an earth body,
solidifying from the molten state, the hypothesis loses
much of its appeal. Nevertheless it has some plausibility.

THE ORIGIN OF THE OCEANS 13

at all events, and it may be that it does not depend
absolutely upon a juvenile, molten earth. Like the
tetrahedral theory it presents us with an earth in which
the oceanic depressions are permanent surface features :
that is, they owe their origin to the causes that have
produced the solid earth-body.

Now it is certainly the case that the centre of figure of
the earth is not the centre of rotation. Very careful
observations made during several decades have shown
that the latitude of a place is not constant in its value.
Latitude is a measurement from the zenith, that is the
point in the celestial sphere cut by a prolongation of the
earth's axis, and so it follows that the latter is not quite
fixed. In other words the poles are points that shift
about on the earth's surface.

This shifting of the pole is, of course, a very small
motion : its effect is that the latitude of a place varies
by about 0*27 second of arc, which means a shifting of
the pole by about 30 feet, or so, periodically. The shifting
follows a path on the earth's surface which is a sort of
complicated spiral, so that it is not cumulative. But
another motion enters into the total effect. There is a
small periodic change in the latitude which affects all
stations simultaneously, and this is to be explained by
supposing that the centre of rotation shifts up and down
and along the axis of rotation by about 10 feet. As it
shifts, so the axis sways upon the shifting point, and so
we get the total effect.

It was known that there would be such a precession of
the earth's axis, and (before it was actually observed) it
was predicted by Euler and given a period of 10 months :
that was on the assumption that the earth-body was
absolutely unyielding. Every 10 months, then, the north
and south poles would have rotated in small circles of
about 30 feet in diameter, always pursuing a re-entrant
path. The actual period is, however, about 14 months,
and this is because the earth is not perfectly rigid, but

14 AN INTRODUCTION TO OCEANOGRAPHY

really yields a little to the tide-generating force set
up by the combined gravitational field of the sun and
moon. Such earth-body tides have actually been
observed.

Failure of the gaseous-molten hypothesis. — There are,
of course, huge difficulties in accounting for an original,
intensely hot, immensely attenuated nebula with a move-
ment of slow rotation. But even when we concede the
existence of this nebula further difl&culties arise. If it
is given certain provisional dimensions, a mass, a rotational
speed and a physical state (a temperature, etc.) its further
development may be deduced. The original crudities of
the hypothesis have been removed. The nebular matter
that formed the planets could not have been cast off in
the form of rings, on the Saturnian model, but was calved
off in the way suggested above : this follows from Sir
George Darwin's mathematical investigation. But even
so the process must have had certain dynamical
consequences, and these ought to be traceable to-day in
the distances of the planets from the sun and the distance
of the satelUtes from the planets ; in the masses and
rotational speeds of the various bodies ; in their
revolutionary paths, etc. Observation is now fastidious
enough, and mathematical analysis sufficiently penetrating,
to show that the consequences to be expected are not
apparent to observation. Therefore the nebular hypothesis,
even its later modified form, cannot be sustained. It
leaves us also with th^ notion of an earth body which was
originally intensely hot and fluid, and which has long
been cooling and contracting. If that were so, certain
consequences are to be expected : the amount of contraction
ought to be calculable within certain limits. There should
be limiting values to the amount of contraction of the
earth's crust which results from the shrinking away from
it of the contracting core. Somewhere in the materials
accessible to us there ought to be relics of the original
crust that finally solidified. The hypothesis, in fact, must

THE ORIGIN OF THE OCEANS 15

come to such a state that it becomes the subject of physico-
mathematical investigation, and so far as that has proceeded
it does not support the gaseous- molten conception. It
must be again noted that there is no really convincing
evidence of an earth with an intensely hot interior, now or
in the past, and there is no good evidence that the earth
body is cooling and not actually getting warmer.

The planetesimal hypothesis of the earth's origin. — The
above hypothesis, as it is elaborated by Chamberlin and
Moulton, makes a new start. Given the discrete universe*
we have the picture of a very large number of massive
bodies pursuing paths which are Uke those of the molecules
in an imperfect gas. Originally the rather crude idea of
stellar collisions between these bodies was entertained.
But the distribution of the stars is such that there are
prodigious distances between them on the average — that
is the density of the visible universe is exceedingly small.
Therefore, the probabihty of colUsions can be calculated,
since we know a certain amount as to the velocities of the
stars and their spacing apart from each other. This
probability of colhsion is very small.

But the probability of a near approach of two stars is
much greater. By " approach " we mean that they may
come so near as to perturb each other. It is permissible
to assume a certain physical state of the perturbing stars :
such a state as that of the sun. Here we have an intensely
hot body from which eruptions (as shown in the solar
prominences and sunspots) occur with, considerable force.
The heated and expansive material of the sun is held by
its own gravitation, so that erupted substance mostly
returns to its surface. If, however, another cosmic body
of sufl&cient mass were to approach the sun near enough it
would set up a tide in the solar mass and cause an eruption.

* Some hundreds of millions of cosmic bodies of rather small
dimensions instead of one large body. Why ? There is some reason
for it, Lodge says. To ask for the reason leads on into philosophical,
rather than cosmogonical speculation.

16 AN INTRODUCTION TO OCEANOGRAPHY

With a close enough approach disruption of the solar body-
would occur.

There is, in fact, a limit within which two cosmic bodies
could not approach each other and still hold together by
their own gravities. The moon is far outside this limit,
with respect to the earth, yet quite significant tides are
set up by it in the yielding ocean and there are even very
small tides in the solid earth. Given, then, a very close
approach on the part of the sun and another cosmic body
of much the same order of mass, and fragmentation of one
or both bodies must ensue. Such fragmentation might
be the first stage in the evolution of a new planetary
system.

Just as the model for the Kant-Laplace nebular
hypothesis was given in the ring system of Saturn, so the
model for the planetesimal hypothesis is afforded by the
spiral nebulae which are such conspicuous features of
the heavens. It is not to be thought that these spiral
nebulae are really stages in, or actual examples of the
evolution of planetary systems. Their distances and
dimensions appear to be very great so that we see in them
something incomparably greater than a planetary system
of the order of magnitude of our one. Possibly they may
represent stellar galaxies, or universes similar to the
system that we see grouped about the Milky Way. In any
case, however, they have suggested the possible mechanisms
of a planetary evolution.

That mechanism is fairly simple (at this stage of the
hypothesis, at all events).

One of the two cosmic bodies (which are of nearly equal
magnitude) we may call the " eruptive sun," and the
other the " projectile." They are assumed to approach
each other, not " dead on " but nearly so. As they come
within each other's gravitational field they will take
hyperbolic paths about each other, approaching from, and
then receding to, very great distances. If they are
approximately in the same physical state the same series

THE ORIGIN OF THE OCEANS

17

of events must occur in each. Here, however, we trace
only the events that occur in the eruptive sun, and it is
assumed that the latter is in rotation. When the projectile
approaches to within a certain distance, which will depend
on the masses of the two bodies, the attraction will become

Fig. 7. Stages in the disruptive approach of two stellar bodies.

SO great that the eruptive sun will become partially
disintegrated. As in the case of an earthly moon-tide,
there will be two protuberances, one on either side, and in
the line joining the two stars, and at a certain moment
each of these will be projected as an explosive " bolt."
This is the condition represented by Fig. 7, D, I.

i

18 AN INTRODUCTION TO OCEANOGRAPHY

Then, as the projectile and eruptive sun swing round
each other into positions II, the process will be repeated
and a further pair of bolts will be projected. In the
meantime the eruptive sun will have rotated on its own
axis and the first pair of bolts will have swung out of line
with that joining the two bodies. The result will be the
disposition shown in Fig. 7, D, II. Passing on the same
thing will again occur and, because of the rotation of the
disruptive body, the bolts will have swung still further out
of line so that, as Case III represents, the two trains of
ejected matter will now have assumed the characteristic
spiral arrangement. We may suppose that there were
initially as many distinct bolts as there are planetary
bodies in the evolving system. The first embryo form of
the juvenile system will then be as Fig. 7, D,III represents.
Here we have two trains of ejected matter partially
wrapped round the parent sun as the two curved arms
characteristic of the spiral nebular form.

Of course there will be great irregularities in the process.
In general it may be expected that the nearer are the two
bodies to each other the greater will be the quantities of
matter ejected. While the latter are being belched out
and are still within the strongly developed double
gravitational field set up by the projectile and the eruptive
sun, they will have gravitational tendency due to their
mass. They will have a proper motion of their own due
to the impetus given to them on eruption. They are sure
to have some rotatory motion about their centres of gravity
because of the irregularity of their ejection and the attrac-
tions of the parent stars. As a rule a considerable degree
of dispersion of the ejected material may be expected so that
a true nebula, in which there is the large central body
and the knots, or nuclei, of condensed matter would result.

The bolts, on ejection, would have a temperature
approximating to that of the eruptive sun, and they might
reasonably be expected to be vaporous. But there would
be such a vast expansion of this gaseous material that

THE ORIGIN OF THE OCEANS 19

cooling would be a verv rapid process, and condensation
to the liquid or solid condition would soon occur except in
the central parts of the nuclei. The system that would
come into existence, therefore, would be (1) the central,
partially disrupted sun, still in its original physical state
and having lost only a rather small fraction of its mass ;
(2) a number of bolts arranged at first in two spiral arms
but gradually settling down into regular orbital
revolutionary paths round the central body ; (3) a vast,
widely extended cloud of wholly condensed matter in large
or small fragments or particles, and also following orbital
paths round the central body, or the individual knots,
or the entire system. These are the planetesimal bodies.
They would be cold. The nuclei or knots of the bolts
would, for a time, retain much of their heat — how much,
and for how long, would depend on their masses.

Such a process, if it were accessible to our observation,
would perhaps be represented by the phenomena seen in
a " new star." Here we can observe a very sudden
increase in the luminosity of a star that was previously
very faint. The latter we may suppose to have been in
the physical condition of our own sun, perhaps even
colder and undergoing condensation : perhaps even
" skinned over " and with a solid, glowing crust. The
approach of another similar star, or even of one of the cold
and dark bodies that we know to exist, would initiate the
series of huge eruptions suggested above and so there
would be an enormous increase in the luminosity. It is
characteristic of new stars that the light emitted rapidly
decreases and is variable in the course of the decrease,
and this is easily explained by supposing that the ejected
matter quickly cools so that it forms a rather dense,
nebulous mass of absorbent matter surrounding the glow-
ing central, eruptive body and the incandescent bolts.
This would be in slow rotation ; it would not be uniformly
wrapped about the central body and bolts, and so the
absorption of Ught would not be uniform.

20 AN INTRODUCTION TO OCEANOGRAPHY

The process of accretion. — Such may reasonably be
supposed to be the genesis of the evolving planetary
system. This would come to consist of a glowing central
sun having a mass which would be (as in our own case)
very much greater than that of all the other bodies budded
off from it. Round this there would be a greatly extended
nebular cloud consisting mostly of cold and dark cosmic
" dust," that is, discrete solid bodies of magnitudes
varying from that of molecules perhaps up to or even
greater than those of the largest meteorites that fall to
the surface of the earth. In this nebula there would be
a number of " nuclei," each representing a separate bolt.
These planetary nuclei would be in orbital revolution ■
about the central sun, being held by the gravitational
field of force of the latter, and they would also be in
rotation about their own axes.

Each of them would set up its own gravitational field
and draw to itself the cosmic dust, or planetesimal bodies,
that were at first in the condition of orbital revolution
about the parent body. Thus each nucleus would sweep
space about itself, gathering up the planetesimals and
growing in mass and volume with this process. A certain
condition of the evolving planet may be imagined in this
preliminary phase : it would be much smaller than in the
final one, but would be growing all the time. At first the
infalling planetesimals would form a light, porous stratum
or shell, but this would soon settle down by its own
gravitation ; pack together closely, and finally harden by
compression as it underwent burial under the further
infalls of the surrounding planetesimal matter. At first
the growing planet would not be able to set up a
gravitational field strong enough to hold the lighter gases :
this is the stage attained by the moon, while it is perhaps
the case with Mars, which appears to have an atmosphere
consisting of carbonic acid gas and water vapour. The
gases that now form the earth's atmosphere, or are
combined with mineral matter in its crust, as well as the

THE ORIGIN OF THE OCEANS 21

water of the oceans, must have originally been part of the
nebular mass, held there by the gravitational field of the
central body. But as the mass of the earth increased in
consequence of the continued infall of planetesimal bodies
it would, by and by, become great enough to collect the
gases that how surround it. Before that, however, it
would capture water vapour and this, at first, would be
largely incorporated in the porous, growing, planetary
substance.

Changes in rotational speed. — Given that the ejected
bolt of solar matter had a certain initial, rotational speed
it appears that we must postulate changes in this.
Planetesimals would fall in from all sides, some overtaking
the young earth and falling into it so as to communicate
momentum to it, while others would meet it and so fall
as to arrest its rotational speed. There might be such a
distribution of the planetesimals that these effects would
cancel each other, but it seems most reasonable to expect
such irregularities in the nebula that the infall of matter
would sometimes accelerate the terrestrial, rotational speed
and sometimes retard the latter. At all events this effect
may be assumed in the provisional stage of the hypothesis.
It must have had momentous consequences, if it
occurred.

Changes in rotational speed must, then, have affected
the figure of the earth. At present the equatorial semi-
diameter of the earth is about 13"4 miles greater than the
polar semi-diameter. Even now an increase of rotational
speed would tend to increase the equatorial protuberance
and accentuate the polar flattening — axidiviceversa. Given,
then, that the primitive nebula consisted of planetesimal
materials irregularly scattered along the great tracts
diverging spirally from the central body, it seems likely
that the accretions that the earth received would at one
time increase its rotational speed and, at other times,'
retard it. Therefore the figure of the earth (which must
also be assumed to have been more yielding in its earlier

22 AN INTRODUCTION TO OCEANOGRAPHY

stages than it now is) must have oscillated and taken on
greater or less oblateness from time to time.

Yield-tracts in the growing earth. — Next a " mechanism "
must be assumed such as to allow the earth body to change
its shape as it also changed its rate of rotation. Stresses
would be set up in it as the rate changed and these would,
on the whole, be greatest near the centre and least at
points near the poles. They would, in general, diminish
from centre to surface. Given, then, a change in the
rate of rotation and a stress will be set up. In order that
this stress may be relieved material must be shifted from
the polar to the equatorial regions, or vice versa. If the
rotational speed increased there would be a tendency to
further flattening of the polar regions and so the crust
there would be compressed from above downwards. At
the same time the equatorial crust would tend to become
more protuberant, and so it would be in a state of tension
from above downward. If the rotation slackened the
reverse would be the case. Somewhere between the poles
and the equator there would be zones of no stress (of this
kind) and the semi-diameter there would remain the
same.

If the earth body were liquid, or even somewhat viscous,
these stresses would be relieved by an actual transport of
material, the molecules of the earth substance literally
slipping on each other, or flowing in the same way that the
ocean yields to the tidal generating force. We must
assume, however, an earth-body that is solid and rigid in a
high degree and incapable of viscous flow. Material must,
therefore, be shifted in some other way. Under great
mechanical pressure, assisted, it may be, by heat, tracts
of " rock-flow," schistosity, or other similar conditions are
estabhshed. In that kind of metamorphism seen in
schistose, gneissose, or slaty rocks, planes of cleavage are
, set up and the rock splits easily along these planes (which
are initially perpendicular to the direction along which
the pressure is exerted). Such cleavage planes are formed

THE ORIGIN OF THE OCEANS 23

by the mechanical re- arrangement of the particles of the
rock in thin sheets, or laminae, or by the re-crystalhsation
of the rock substance in such a way that the long axes of
the platy or columnar crystals set themselves at right
angle to the pressure. Then the rock yields and great
masses become displaced, folded, ruptured, faulted, etc.,
in such ways as to relieve the stresses by actual dislocation
and fracture and consequent re-arrangements of the
material of the crust.

Directions of the yield-tracts. In such speculations as
this some analogy suggests the form of the hypothesis.
Assuming an increase of rotational speed the crust at the
polar regions becomes compressed, or it may be thought
about as being stretched everywhere from the pole along
the meridians. A common response to stresses distributed
in this way is a cracking along three lines radiating from a
point at angles of 120° : this gives rise to such rock forms
as the familiar six-sided basaltic columns, which are due
to contraction of a slowly cooling rock mass. A tentative
explanation of the earth body accommodating itself to a
change of rotational speed supposes, then, that a number
of great yield-tracts will tend to radiate out from the poles
meridionally towards the zone of no stress — the " fulcrum-
zone," in Chamberlin's terminology — and this we may
take to be somewhere about 30° in N. and S. latitudes.
First we may consider a symmetrical arrangement of the
yield-tracts and then consider how this is to be modified.
There are, then, three meridianal lines radiating out from
each pole at angles of 120° apart, and terminating at 30°
N. and S. latitude. There will be a difference of 60° in
longitude between the positions of these lines in the north
and south hemispheres. Oblique hues are next drawn
from the extremities of the northern meridianal ones to
meet the southern meridianal lines. Thus the earth is
divided up into six quadrilateral segments, each of which
is bounded by parts of great circles. The reader should
trace these lines on a globe.

24 AN INTRODUCTION TO OCEANOGRAPHY

It has been noted that the stresses set up by changes in
the rotational speed of the earth, or changes in the intensity
of the tide-generating force are greatest towards the centre
of the earth-body and least at the surface. Therefore, we
have to consider, not only the surface of the earth, but its
whole body down to the centre. The three pairs of
quadrilateral areas represent, therefore, the bases of three
pairs of four-sided pyramids, or wedges, having their
apices touching each other at the earth's centre, their sides
formed by great planes passing through the centre and
their bases on the surface. These pyramids will sway on
their centres, moving against each other, and it will be
seen, on reflection, that this kind of segmentation lends
itself to the changes in the figure of the earth that must
occur as the results of the change in rotational velocity.
We assume now that such a segmentation existed during
the period of earth-growth by accretion and that
oscillations in rotatory speed occurred, in the course of
which the earth-body yielded to the changes in stress.
Along the planes which bounded the quadrilateral segments,
therefore, there were regions or tracts of fracture and
readjustment. The yield-tracts on the surface would,
then, be somewhere near the meridianal lines radiating out
from the poles, and near the oblique lines crossing the
equator.

Now it is clear that very many factors must have
influenced the precise course of the surface yield-tracts.
The latter would not be lines, so much as rather broad
regions of fracture and disturbance, and it is not to be
expected that they would take the symmetrical courses
indicated above. They would not be straight, nor would
the angles of the poles be 120°, but something rather more
or less. What we have to expect, in general, is a rough
correspondence to the ideal scheme, and also yield-
tracts subsidiary to those indicated. Further, as the earth-
body became more rigid, in the course of its growth and
consolidation other yield-tracts must have developed. It

THE ORIGIN OF THE OCEANS 25

will, however, be sufficient to show that there is really a
rather striking correspondence between the postulated
theory and the results of actual observation. This
correspondence is shown in the following Figures, 8 (1) to
(9). These are modified from Chamberlin's figures : the
angles between the meridianal lines, and so the directions
of the oblique ones depart from the theoretical scheme,
but not to such an extent as might be thought necessary.
The heavy continuous lines, then, represent these modified
yield-tract directions ; the heavy, broken lines continue
them under the ocean floor, and the dotted lines are
subsidiary yield-tracts not contemplated in the symmetrical
scheme.

The actual earth yield-tracts.

(1) South polar region. Fig. 8 (1). — There is little doubt
about our choice of the directions the tracts ought to take
here. The Antarctic Continent is placed nearly
symmetrically above the pole and three great land masses,
S. America, Africa and Australia are placed round it in
positions that approximate to equal spacing about a
centre. There are indications of land connections (see
Fig. 11) between the Antarctic Continent and the land
regions that bound the great Southern Ocean, and so we
have little difficulty in drawing the first three, basal,
meridianal yield-tracts. They radiate out to Cape Horn,
Cape of Good Hope and Tasmania. But obviously a
subsidiary yield-tract is indicated in the position of New
Zealand : this is represented by the dotted line,

(2) South Atlantic aspect. Fig. 8 (2).— The Antarctic
yield-tract directed towards S. America is seen to bifurcate
not far from the fulcrum-zone in about S. Lat. 30°. One of
the oblique lines connecting it with the corresponding
northern yield-tract runs along the eastern side of S.
America, and its course underneath the tropical Atlantic
is indicated by the heavy broken line. The other obhque
line runs along the western borders of S. and N. America
(see Fig. 8 (3)). The second meridianal line that joins

26 AN INTRODUCTION TO OCEANOGRAPHY

Antarctica with Africa bifurcates into its two oblique
prolongations : which embrace Africa along its eastern and
western coastal regions. On the north the oblique
connecting line running up the western side of Africa is

Fig. 8. The Chamberlin yield -tracts. Various views of the earth.

seen to join a radial one running down from the N. Pole.
Antillean region. Fig. 8 (3). — Here again two of the
postulated meridianal tracts in the high N. and S. latitudes
are shown, as well as the oblique one that bounds the
Pacific coasts of the Americas. But two subsidiary yield-

THE ORIGIN OF THE OCEANS 27

tracts are indicated by dotted lines : one of these is
raeridianal and runs down from the North Pole through the
Gulf of St. Lawrence and the great American lakes towards
the Floridan coast, while another is obUque and, together
with the postulated line, bounds the north-east coast of
S. America and the Antillean region — an area, as is well
known, of great earth-disturbance.

The Eur-African Quadrilateral. Fig. 8 (4).— All the
yield-tracts represented here are postulated ones, but the
angles formed between them diverge (though not greatly)
from the theoretical values. The Antarctic-African
meridianal, the Arctic meridians embracing the W. side
of Europe on the one hand, and striking down through
Asia, on the other, and the two African oblique yield-
tracts are shown.

East Indian yield-tracts. Fig. 8 (5). — Here we see the
counterpart to the Antillean region. There are shown (a)
the postulated N. and S. meridianal and oblique lines, but
the figure also represents the subsidiary meridianal tract
striking from Antarctica through New Zealand, and a
subsidiary northerly tract descending through China.
These are joined together, in a regular way, by a subsidiary
oblique tract. ■ This together with the postulated one in
this region include between them the East Indian area —
one of evident disturbance — in the same way as postulated
and subsidiary tracts embrace the Antillean area.

The western borders of the Atlantic and Pacific. Figs.
8 (5) and (6). — These figures represent an obvious symmetry
which is suggestive in a high degree. The East Indian
region with its pair of parallel yield-tracts bounds the
western side of the coalesced N. and S. Pacific Oceans.
The West Indian region, also with its pair of parallel yield-
tracts similarly bounds the western side of the coalesced
N. and S. Atlantic Oceans.

The Indian Ocean region. Fig. 8 (7). — Here we again
have the postulated yield-tracts : on the south, two
meridianal ones joining Antarctica with Africa and

28 AN INTRODUCTION TO OCEANOGRAPHY

Australia ; on the north, the meridianal tract striking down
through Asia and, running obliquely across the Equator,
the two connecting lines that frame the Indian Ocean.

The South Pacific region. Fig. 8 (8).— Both postulated
and subsidiary tracts are required here. On the S. W.
Antarctica and New Zealand are joined by the subsidiary
south meridianal tract, and on the N. W. the broken line
represents the oblique connecting line that goes under the
Pacific Ocean : this is the approximate line of coalescence
of N. and S. Pacific. On the east the yield-tracts are all
postulated ones.

The two Pacifies. Fig. 8 (9). — Here the junction of the
N. and S. Pacific Oceans is represented by the drowned
oblique yield-tract shown by the broken heavy line. On
the S. W. the border of the N. Pacific is formed by the
southerly, subsidiary, meridianal yield-tract.

The reader is strongly recommended to trace these lines,
as they are shown in Fig. 8, on a small globe : then he will
easily see how very helpful is the Chamberhn hypothesis.
He must again note what are the assumptions made :

(1) An earth growing by the accretion of solid
planetesimals and with a rotational speed which
oscillates because of the collisions of the infaUing
bodies with it ;

(2) The consequences of this — stresses set up by the
tendency of the earth-body to assume spheroidal
shapes corresponding with its changing rotational
speeds ;

(3) The further consequences — " rock-flows " and tracts
of yielding of the earth body in its adaptive changes
of figure ;

(4) A possible " mechanism " permitting of this adaptive
change of figure — the segmentation of the earth body
by the establishment of three pairs of quadrilateral
wedges extending down to the centre.

Thus a certain series of zones, meridianal and oblique
(with reference to the equator) are assumed. Along these

THE ORIGIN OF THE OCEANS 29

zones the earth-body yields to stresses (in so far as the
yielding can be traced on the surface). Assume now that
yielding takes place most frequently along the margins of
the continents. This is indeed fairly certain, for there we
find the greatest inequalities of level, the continental
plateaus descending fairly steeply, to the oceanic abysses.
There also we actually find regions of obvious earth
disturbance, volcanic and earthquake areas and great
mountain folds.

Continental and oceanic areas are, therefore, bounded by
a series of zones which are mostly parts of great circles.
These zones *are representable by a theoretical scheme.
The actual zones, in so far as they can be traced on the
earth's surface, diverge somewhat from the theory, but
the extent of divergence is not very great when we consider
that the causes assumed are certainly far less complex than
the actual ones. And quite unimportant changes in the
angles assumed, with the recognition of subsidiary yield-
tracts, lead to a fairly satisfactory correspondence of theory
and observation.

Oceanic and Continental regions. — We find, therefore,
that a series of meridianal and oblique yield-tracts divide
up the surface of the earth into six great, roughly
quadrilateral areas. Some of these are continental and
others are oceanic. The oceanic areas are arranged in
pairs. North and South Atlantics, North and South
Pacifies and the Indian Ocean which is southerly to a
corresponding area of predominant depression occupied by
the Mediterranean, Baltic, Black Sea, Caspian, Red Sea
and Persian Gulf. North and South Atlantics have
coalesced just as North and South Pacifies have done.
There is a further symmetry with regard to the situations
of each pair of oceans : the North Atlantic lies obliquely
to the west with respect to the South Atlantic, and this is
also the case with the North Pacific in respect to the South
Pacific. The Mediterranean-Black Sea basins lie obliquely
to the west with respect to the Indian Ocean. There is

30 AN INTRODUCTION TO OCEANOGRAPHY

little geological evidence of the former existence of the
two Pacifies, but the forms of the Atlantics, and the
markedly volcanic and seismic nature of the ocean floor
along the region between the Straits of Gibraltar and the
Antilles, suggest strongly that this has been a zone of
yield of the earth's crust.

Some of the great areas delimited by the yield-tracts are
depressions beneath the mean earth level, and are, therefore,
oceanic, while others are elevations, and, therefore, are the
loci of continental plateaus. These differences are to be
explained, and two new processes are invoked for this
purpose : (1) the prevalent atmospheric circulation and
(2) the disintegrative effect of water upon the elevated
land regions. The general form of the atmospheric
circulation is that of an imperfect eddy over each of the
great oceans, just as in the case of the oceanic circulations
themselves. -Such circulations are modified to an enormous
extent by the distribution of land and sea, but their prime
motive forces are independent of this distribution. In
general, equatorial regions must be those of high tempera-
ture and ascending movements of ocean water and
atmosphere, while sub-polar regions are characterised by
low temperatures and descending motions. Further, the
rotation of the earth on its axis has a deflecting effect such
that currents of water and air tend always to turn to the
right-hand side in the northern, and the left-hand side in
the southern hemispheres. We must assume that
tendencies to these gyratory movements of both the ocean
and the atmosphere existed in the juvenile earth during
its phase of growth by the accretion of planetesimal
particles.

If that is so there would be a tendency for the heavier
particles to settle in the central parts of the great
atmospheric eddies and, therefore, these regions of the
earth's surface would be likely to come to consist of
material of higher specific gravity than the surrounding
ones. As soon as this became the case water vapour

THE ORIGIN OF THE OCEANS 'SI

drawn to the surface of the growing earth would gather
over the places where the force of gravity was greatest.
Thus the loci of the primitive oceans ought to have been
roughly marked out in the earlier phases of the earth's
growth.

Whenever rain began to fall on the surface of the early
contmental plateaus, a process of disintegration of the
primitive land would begin. Weathering and solution of
the rock surface would occur, and materials would be
transported from the land to the seas. It is probable
that this process was selective in the sense that heavier
materials would be transported from the land and lighter
materials left there, or on the shallow sea bottom fringing
the land. Thus the difference between oceanic and
continental regions — the former tending to become areas
of greater, and the latter of lesser gravitation would tend
to become accentuated. By some such cumulative
process as this, starting from a slight initial difference
that favoured the accumulation of w^ater on some of the
original areas deUmited by the great yield-tracts, the
oceanic and continental regions would become established.
This part of the theory is, however, not very satisfactory.

Permanence of the great oceanic regions. — It is to be
noted, at this stage, that the hypothesis of the origin of
of the oceans outlined in the preceding pages involves
the further hypothesis that the great oceanic depressions
have always occupied the positions that they now do.
This follows from the hypothesis of an earth-body
contracting into a tetrahedral shape from a state in which
it was intensely hot and also from the hypothesis of Love,
according to which " harmonic deformation " occurred as
the result of an earth-body rotating on a centre that was
not its centre of gravity. There are other reasons for
believing in the permanence of the oceanic basins, notably
that of the great difference in nature of the materials
forming the deep-sea deposits and those that are to be
found in the series of sedimentary rocks. This we shall

32 AN INTRODUCTION TO OCEANOGRAPHY

deal with in the following chapter. There is also the
theory of isostatic equilibrium between continental plateaus
and ocean bottoms (see Chapter IV), and this seems
incompatible with the idea that an earth area that is now
an oceanic abyss may, in past geological periods, have been
the locus of a continental plateau. The assumption that
the South Atlantic region, for instance, was at one time
a continental land mass is plainly an ad hoc hypothesis
designed to explain certain facts of distribution of animals
and plants, facts which may be susceptible of other
explanations. There is, indeed, much evidence that
sedimentary rocks situated in land areas far removed from
the sea, are marine in origin, but this may merely mean
that shallow seas transgressed, at one time, on former land
areas. This does not imply that the sites of those strata
were ever covered by sea of the depth of the present
oceans. The question of the permanence of the oceanic
basins will, however, be discussed more fully in the last
chapter.

The smallness of the effects dealt with in this chapter. —
It is very necessary to note that the deformations and
inequalities of the earth considered in this chapter are very
small indeed on the earth-scale. We shall see in the following
chapters, that the oceanic depressions, as well as the
continental elevations, are, in magnitude, only about
0"04 % of the earth's diameter. The " tetrahedral earth,"
with its coigns representing continental tablelands, and its
faces the oceans would, if figured on this page, be
indistinguishable from a perfect sphere. Even the
equatorial protuberance and the polar flattenings, which
are really very large features when compared with the
land and sea elevations and depressions, cause the figure
of the earth to deviate from true spherical form only to
the extent of about 0'3%. Other deformations suspected
to exist — the " pear-shape," and the "nipple" somewhere
in Africa — are most difficult to detect, much more to
measure precisely.

THE ORIGIN OF THE OCEANS 33

So also with the movements that we call " yielding "
of the earth-body. Such phenomena as are seen in
volcanic eruptions and earthquakes are indeed gigantic
and destructive when judged by their effects on human
constructions, , but they are the merest sweatings and
shivers when they are regarded as terrestrial effects.
Ocean tides have momentous consequences to sailors and
fishermen, but they only produce infinitesimal differences
of sea level when they are measured on the earth-scale.
Tides in the body of the earth itself are only tremors. On
the cosmic scale then, the causes that are competent to
elevate continents or depress ocean basins are exceedingly
small ones, and so the effects we have to explain do not
require the operation of factors that are necessarily very
great. The earth is, in fact, a body that is exceedingly
stable, and however striking its instabiUty may be from
the human point of view, it is infinitesimal from the cosmic
standpoint.

CHAPTER III

THE DEPTHS OF THE OCEAN

, From the point of view of the sailor, fisherman, yachtsman
or oceanographer great diversity of feature characterises
the sea bottom. In respect of level tliey distinguish
between abyssmal and shallow seas, flats, channels, gutters,
deeps, shoals, ridges, bars, etc. In respect of the nature
of the bottom there are equally marked differences : hard
or soft, boulders and stones, gravel, sand, mud, sludge,
ooze, coral, etc. These features are, however, inferred
rather than seen, and the impression obtained when one
looks at extensive areas of sea bottom, laid bare by, a very
low spring tide, is that of monotony and sameness rather
than diversity. Compared with the land there is a striking
uniformity of level and the character of the materials laid
down on the sea bottom may appear to differ very little
over very great areas. That variety of depth which the
experience of the fisherman or sailor enables him to
visualise, depends rather on its utilitarian than on its
absolute value. A few feet more or less in depth may
make a very great difference from the point of view of the
navigation of a vessel ; while the nature of the bottom
itself, boulders, sand, v/eed, or mud, may be all important
to the success of a fishing operation. Even the
oceanographer usually exaggerates the '* bottom relief " as
well as the differences in the nature of the bottom deposits.
It is very important to have this influence of scale of value
in mind when one is considering the morphology of the
ocean floors.

The estimation of depth. — The practicable methods of
finding the depth of the sea are :

(1) The traditional hand and deep-sea sounding lines,

34

THE DEPTHS OF THE OCEAN 35

Here the apparatus consists of a hard, closely made, well-
stretched rope of small diameter marked at various intervals
by pieces of leather, cloth, etc., worked into the strands of
the rope. The sinker is a heavy piece of lead (10 to 14 lbs.
for a hand line of 25 fathoms and 28 to 30 lbs. for the
deep-sea line of 100 fathoms). The bottom of the lead is
hollowed out and the recess is filled with tallow. The
hand-line is simply kept in a loose coil, but the deep-sea
line is wound on a wooden reel held in a man's hands
when the sounding is being made. When the line is well
stretched and carefully marked the soundings obtained
are usually very accurate. The technique, however, of
sounding by these means is complicated and not easy to
acquire. :

(2) The pressure tube is used by large vessels that are
compelled to make soundings while still under way. A
steel rope consisting of a strand of a few wires is generally
used, and this is coiled on a reel which forms part of a
machine fixed to the deck of the ship. A glass tube of
rather small bore, and with one end open and the other
sealed up, is attached to the sounding line a little way up
from the lead. The latter is thrown overboard and goes
down to the bottom at an acute angle to the sea level, so
that much more line is out than is represented by the
depth of the water. When the sinker is felt to touch the
bottom the apparatus is hauled in and the depth is estimated
from the height to which the water has xisen in the sealed
glass tube. At the surface of the sea the pressure on the
column of air in the tube is, of course, one atmosphere.
Roughly 30 feet of water exert the pressure of one
atmosphere and from the relation, pressure x volume =
constant, the depth is found. The inside wall of the tube
is coated with a composition that is discoloured by contact
with sea water, and when it is taken aboard the fraction,
to which the original volume of air has been reduced by
the pressure of the column of sea water standing on the
sea bottom, is read off on a gauge calibrated for the

36 AN INTRODUCTION TO OCEANOGRAPHY

particular length of tube used. If, for instance, the
volume is reduced to one half, the pressure has been
doubled, and was exerted by about 30 feet of water (= 5
fathoms). If it is reduced to l/4th of the original volume
the pressure has increased to 4 atmospheres, that is
30 X 3 feet due to the water column and one atmosphere,
due to the air pressure. The depth is, therefore, 90 feet
= 15 fathoms, and so on. The method is suitable for
vessels that require only an approximate knowledge of the
water depth but is not accurate enough for scientific
investigations.

(3) Sounding Machines. Telegraph cable engineers and
scientific workers use what is practically a deep sea line of
very great length. This line is made of steel pianoforte
wire of rather less than one millimetre in diameter but,
nevertheless, exceedingly strong. There are various types
of sounding machines, but those which are most compact
and easily used are the Lucas forms. The larger
machine is represented in its essentials in the figure on
page 37.

The wire, which may be over six miles in length, is
wound evenly on a very strong steel drum working in a
triangular frame. It passes out over a leading-wheel
which is carried by a moveable arm, to the lower part of
which is attached a leather brake strap bearing on the
rims of the drum. When the sinker is running out its
weight is carried by the leading wheel, which is therefore
pulled forward. This throws back the lower part of the
arm and releases the brake strap. Whenever the sinker
touches the sea bottom its weight is taken ofE the leading
wheel, the two strong springs shown are then able to pull
back the arm and the lower part of the latter is thrown
forward and so pulls on the brake strap. Therefore the
machine stops whenever the sinker touches bottom.

At this moment, a counter, geared on to the leading
wheel, is read. The revolutions of the latter have been
recorded and the circumference of the leading wheel being

THE DEPTHS OF THE OCEAN

37

STrab

Caver-

Jdi^s of

Kuj. 9. Hydrographic apparatus. (1) The Lucas deep-sea sounding
machine ; (2) Detachable deep-sea lead ; (3) The Nansen-
I'ettersson water-bottle ; (4) Deep-sea reversing thermometer ;
(r») The Lucas bottom sampler ; (6) The Hudson collecting lead.

38 AN INTRODUCTION TO OCEANOGRAPHY

known the length of wire run out is found. As a matter
of fact the revolution counter is directly calibrated in
fathoms so that the depth is shown on it. The wire is
then wound back again by a motor geared on to a pulley
carried by the drum spindle, and as it comes in it is oiled
to prevent rust and " laid " uniformly on the drum by a
suitable mechanism.

The sinker is about 40 lbs. in weight, but in a deep
sounding the weight of wire run out is also very great.
In reeling-in there would, therefore, be a considerable
strain on the wire — enough, perhaps, to break it — and
so it is necessary to detach the sinker. Fig. 9 (2)
represents the quadruple-tube sinker with its releasing
mechanism. When the apparatus touches bottom the
four short tubes plunge into the ooze and fill themselves,
forcing up a weighted rubber washer that tends to close
their upper, open ends. At the same time the steel spring
shown throws off a ring from the hook attached to the
wire and the apparatus turns over on its side. The sinker
is now free to fall away from the sounding tube and the
latter is hauled up to the surface. At the same time the
rubber washer shuts down on the upper open ends of the
tubes and this prevents the ooze from being removed.
Sometimes a single steel sounding tube, of a foot or more
in length, is used. Sometimes this contains a glass tube
which is filled by the deposit. In this way any evidences
of stratification of the latter can be observed. Fig. 9 (5)
represents the Lucas grab. This goes down with its jaws
open and when it touches bottom the pressure of the
latter trips a Httle brass tumbling block within the jaws.
The latter are thereupon closed by a powerful spring.
Fig. 9 (6) represents another form of sounding tube.

In all cases the steel sounding wire itself is not directly
attached to the sinker but to a fathom or two of " stray
line " (strong, thin rope). The latter falls on the sea
bottom but the wire is kept straight and thus kinking is
avoided.

THE DEPTHS OF THE OCEAN 39

(4) Sounding by sound waves. Since the war of 1914,
methods of finding the depth of the ocean by quite a
different means have been developed. An apparatus
attached to the bottom of the ship makes a small, sudden
explosion, thus setting up a sound wave which travels
outwards along concentric, spherical fronts. On reaching
the sea bottom this wave is reflected and the echo is
transmitted directly back to the surface, where it affects a
receiving apparatus on the ship and makes a signal. The
moments of the explosion and of the receipt of the reflected
wave are recorded very precisely, and half of this interval
of time is that required for a sound wave to travel through
a column of water of the depth in situ. The rate of
transmission of a sound wave through water depends on
the temperature and salinity and so corrections have to
be made.

(5) Sounding by an electrical oscillator. This mechanism
is referred to in connection with tidal observations.

Methods (1) to (3) are obviously rather laborious. In
the time of the Challenger expedition, hemp rope was used
for the sounding line, and some hours were required in
order to make a single sounding. The use of steel wire
has shortened the time very greatly, but still the ship has
to be stopped and so manoeuvred as to keep the sounding
line vertical while it is running out. This makes sounding
in deep water a very laborious operation, if the observations
have to be made at close intervals, and the result is that
there are everywhere great areas of ocean bottom which
have not been sounded at all. Method (4) allows of the
depth being determined while the ship is on her passage,
and the last method gives a continuous record. So far,
however, these latter means of deep sea soundings have
not been completely developed.

The bottom gradients. — Ocean soundings have been made
most frequently in rather close proximity to the land —
in the neighbourhood of rivers, harbours, anchorages, bays,
estuaries and, in general, in waters most used by fishing,

40 AN INTRODUCTION TO OCEANOGRAPHY

and other vessels. The result is that the depth of the
sea is known in considerable detail, in many parts of the
world at all events, where it is less than about 100 fathoms.
Here there is much diversity of feature in the bottom,
considerably more than Avhere the depth is greater than
about 100 fathoms. There are shoals, channels, bars,
gutters, ridges, etc., and the charts representing these are
always more complex than they are for the truly oceanic
regions. Just now, however, we neglect this variety of
feature in shallow water and consider rather the average
gradients.

The continental shelf. — Round the margins of all
continents and continental islands there is a zone of sea
where the bottom slopes dowm on the average very slowly
to somewhere about 100 fathoms. If we " smooth out "
the smaller inequalities (channels, shoals, etc.) we find
that the inclination downwards is very small — usually less
than about 1° (1 ft. in about 57 ft.).

The depth of 100 fathoms as the limit seawards of the
continental shelf is rather arbitrary and is taken because
it is a round, easily remembered number : in different
continental regions it varies very greatly. The width of
this zone of shallow water is also highly variable. Out
from Achill Head (on the West Coast of Ireland), for
instance, it is only about 30 miles wide, but due west from
Land's End the depth of 100 fathoms is reached only at
about 200 miles. All the North Sea and most of the Irish
Sea are less than 100 fathoms deep, and only in a very few
places round the British Islands is the sea deeper than this.

Beyond this rather vague boundary the inclination
downward of the sea bottom becomes rather greater
towards a depth of about 1,000 fathoms, and then it becomes
greater still. If we join together all places where the water
is 100 fathoms deep and then draw a smoothly running,
irregular curve, passing as nearly as possible to all these
points, we obtain the " 100-fathom contour line." Doing
the same thing with regard to the 1,000, 2,000, and 3,000-

THE DEPTHS OF THE OCEAN 41

fathom soundings, we get a series of curved lines which
run, very roughly, parallel to each other and to the general
continental outlines. Measuring the average distances
between these we then find that the 2,000 — 3,000-fathom
contours are rather closer together than the 2,000 and 1,000
contours, and still more so than the 1,000 and 100 contour
lines. On the whole, then, the gradient downwards of the
sea bottom is greater just about the 1,000-fathom contour
line than it is anywhere else. This part of the sea bottom
is called the Continerdal Slope.

The continental slope. — It is usual to represent the
continental shelf and slope by means of imaginary sections
taken vertically to the surface of the sea. Generally the
vertical scale (of depths) is made much greater in these
sections than is the horizontal one, and when this is done
the figures often represent both the shelf and the slope in
a striking way. But the distortion of the two scales may
convey rather misleading ideas both of the relative depth
of water and the bottom gradient.

In Fig, 10 the sections are drawn to the same scale of
distances both horizontal and vertical : in each case one
millimetre represents a mile. Further, the figures represent
the curvature of the earth's surface, on approximately the
same scale. Now it is by no means easy to find data on
the published charts that will show the continental shelf
and slope, because there is generally a great paucity of
soundings outside the 100-fathom contour line and so the
inequalities of sea bottom, if they exist, must often be
" averaged out " in the process of making the sections.
But even when we do find data that enable us to construct
a section, it is often the case that it is very difficult indeed
to say where, precisely, the limits of the shelf should be,
or where the steepest gradient, or continental slope, should
be placed.

Fig. 10 (1) represents a section of the Atlantic Ocean
taken from the West Coast of Ireland in Lat. 51° 30' N.,
at a place where there is a prolonged continental shelf.

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^

^

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0

0

^1

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1;

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V.

<5

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kl

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THE DEPTHS OF THE OCEAN 43

Out to 50 miles from the coast the sea does not exceed
]00 fathoms in depth, but it is difficult to see any decided
change in gradient at about this depth and from there out
to about 220 miles from land the bottom slopes down
rather gradually to about 1,000 fathoms. Between about
220 and 240 miles out there is, hownever, a change in gradient
and this is quite noticeable on the section. Here the
average inclination downward is about one in ten (an angle
of 5J°). This is the continental slope, and beyond it the
depth and the average gradient do not change much.
That is, at a distance of about 250 miles from land in this
region the ocean " abyss " is reached.

Fig. 10 (2) represents a section taken along lat. 10° S.,
that is, out from the Peruvian coast, and nearly at right
angles to the general trend of the western coast of S.
America. It also includes the adjacent land (stippled part
of the section), thus passing through the Andes, where the
highest altitude on the line of the section is about 15,600
feet. The section thus represents the great " Pacific geo-
syncline," and shows, at the best, the transition from an
oceanic depression to an elevated continental region. At 20
miles seawards the depth is 50 fathoms, at 90 miles it is
1,000, and at 200 miles we attain the Pacific oceanic abyss
at a depth of about 2,500 fathoms. Now we see here that
the average inclination both of the " Pacific Slope " and of
the adjacent ocean floor does not appear to be particularly
striking. Neither is there any place where the sea bottom
is flat enough to constitute an obvious shelf, nor so much
steeper than the average as to give us a pronounced
continental slope.

Fig. 10 (3) is taken east from the Japanese coast in lat.
30° 10' N., and here one of the greatest oceanic deeps
is represented. At a distance of about 30 miles from the
coast we find a depth of 100 fathoms, at 55 miles 1,000.
at 80 miles 2,000, and at about 100 miles a depth of 4,000
fathoms. The inclination downwards is, so far as one can
see, nearly uniform out to a depth of about 2,000 fathoms.

44 AN INTRODUCTION TO OCEANOGRAPHY

but from there the bottom descends more steeply to a
depth of 4,000 fathoms. This is, however, not the typical
Pacific Ocean floor but is the bottom of a " deep," a place
where an exceptionally great depth, over a very limited
area, has been observed. So we cannot, in this section,
point to any obvious continental slope.

Fig. 10 (4) illustrates the slope and depths off an oceanic
island, and represents a very different condition from that
characteristic of the continental margins. The island is
the Funafuti coral atoll in the Pacific Ocean, and the
section is taken east and west (magnetic) across it. Going
from west to east we find a depth of 500 fathoms at one
mile from the rim of the atoll, 1,000 fathoms at 2^ miles,
and the ocean floor is reached at a distance of about six
miles. Here we find no indications of a shelf and, indeed,
the gradient downwards appears to be steepest close up to
the outside of the atoll. Within the latter the lagoon is,
of course, only a few fathoms in depth.

In all these cases (Fig. 10), the section is made from the
data given on the Admiralty charts. Contour lines are
drawn and then the horizontal distances are taken from
the latter. Because of the paucity of soundings there is
always considerable difficulty in drawing the contours, and
more than one way is usually possible.

The form of the ocean floor. — There is, therefore, a
general slope downwards from the land to the ocean bottom.
This slope is not uniform but varies in an .extraordinary
way though, looking at it in a very general manner, there
is, first of all, a very gradual descent down to a limited
depth which varies between about 100 and 1,000 fathoms
and, next, a rather steeper gradient down to about 3,000
fathoms. The fii-st, very gradual slope downwards may
be called the continental shelf, and the second, steeper
descent is the continental slope. Outside the latter the
ocean bottom is, on the average, a rather flat plain presenting
verv little relief. The gradients are, as a rule, exceedingly
small, far less than the ordinary railway ones, and they

THE DEPTHS OF THE OCEAN 45

would, in general, be imperceptible to the e^'e if the ocean
bed were dried up. On the great scale, however, these
very small gradients give the ocean floor certain forms
which we must now consider.

Terminology. — The various terms which are most

commonly used are :

Continental Shelf {Plateau ; Continental Schelf). The

bottom out to about 100 fathoms, or to any other

depth after which the gradient increases.

Plateau (Plateau). A flattened, extended, submarine

elevation.
Shoal {Hautfotid ; Grund). The most elevated parts of

a plateau.
Bank {Banc; Bank). A low non-rocky submarine

elevation.
Ridge {Crete; Rucke). A submarine elevation of

elongated form.
Rise {Seuil ; Schwelle). An extensive, gently sloping

elevation of the sea bottom.
Reef {Riff; Reef). A rocky, dangerous, submarine

elevation of elongated form.
Depression {Depression, Mulde). Any lowering of the

sea bottom in general.
Basin {Bassin ; Becke). A depression of approximately

rounded form.
Trench {Fosse; Grabe). Any marked, elongated

depression of the sea bottom.

Deep {Fosse ; Tiefe). The deepest part of a depression.

Generally all parts of the ocean floor greater than

3,000 fathoms in depth.

Contour lines. — The data for descriptions of the forms of

the ocean bottom are mainly the official charts published

by the various hydrographic departments (principally

Great Britain, Germany and the United States of America)

and the soundings made by deep-sea expeditions. Near

the land, in shallow water of less than 100 fathoms, the

soundings are generally " reduced " to what they would

46 AN INTRODUCTION TO OCEANOGRAPHY

be at low water of ordinary spring tides. In extreme
eases, therefore, the depths of water may be some 5 or 6
fathoms greater than those marked on British Admiralty
Charts. (See pp. 203-6).

Such charts are covered by a multitude of figures which
are difficult to visualise as a whole. Therefore contours
are drawn. On British charts these are usually drawn
at 5, 10, 20, 50 and 100 fathoms. A contour line on a
chart is usually defined as " a line joining all points where
the depth is the same," but this is not quite the case.
The contour passes as nearly as possible to all the points
where depths of (say) 50 fathoms are marked but, as a
rule, it would go near points where (say) 49 or 51 fathoms
are recorded. In areas where there are few records the
contour line will, so to speak, " split the differences " :
thus it would pass nearly midway between positions where
the depths are recorded as (say) 36 and 60 fathoms, but
it would be nearer to 60 than to 36. It should be clear to
the reader that contour lines are generally approximations
and that two persons constructing them from the original
data would not in general get quite the same results.

Further, the shape of a contour line will depend on the
scale of the data : the more numerous the latter the more
complex will be the shapes of the contours. In small-
scale charts the contours are generally lines of sweeping
curvature, and there is a tendency for the successive ones
(1,000, 2,000, 3,000 fathoms, say) to " flow," so to speak,
roughly parallel to each other. In large-scale charts as,
for instance, those representing the sea bottom in the
vicinity of an important harbour, such contours as the
5, 10, 20-fathom ones are often very complex. They may
interdigitate, or dovetail, and there may be numerous
*' islands " of shallower or deeper bottom.

Then the contours may be drawn for soundings in
fathoms or metres. A fathom is nearly 2 metres and may
be taken as such when we merely want a broad view of the
features of the ocean bottom in depths of, say, 0 to 50

THE DEPTHS OF THE OCEAN

47

fathoms. But when an ocean is contoured, first by 100,
1,000, 2,000 and 3,000 fathom-lines and then by 200, 2,000,
4,000 and 6,000 metre-lines the results will not be quite
the same.

The student should endeavour to think in terms both of
fathoms and metres, and he should look on bathy metric
contours merely as a general aid to visualising the forms
of the sea bottom. He should not omit to study the
uncontoured charts, trying to draw the contour lines for
himself. Such work is most interesting.

We next consider small-scale charts of the various ocean
bottoms.

Fig. 11. The region round the South Pole. The land outline, where
known, is indicated by a heavy, continuous line ; the probable
land outline is shown by heavy dots. The land (known and
probable) is stippled. The zone between the land outline
and the 1000-fathom contour is stippled. The soundings
are in fathoms.

The Antarctic Ocean. — Fig. 11 is made from the latest
Admiralty charts, but it will be seen that our knowledge
of the immediate surroimdings of the South Pole is still
very incomplete. Obviously there is an Antarctic Continent

48 AN INTRODUCTION TO OCEANOGRAPHY

which is deeply indented at two places, the AVeddell Sea
and the Ross Sea. The boundary of the continent, for
those places where land has not actually been seen, is taken
to be near the great ice-barrier. The whole continent is
covered by a permanent ice-cap which projects out into
the Southern Ocean and breaks away as the great tabular
icebergs.

The 1,000-fathom contour line round Antarctica is very
imperfectly charted because of the paucity of soundings.
Where it is not well known, it is represented by a broken
contour line in Fig. 11. In some places even the 2,000-
fathom contour is badly known. Clearly, however, our
available data indicate two conditions that are very
important: (1) the relatively shallow sea (between 1,000
and 2,000 fathoms) between Graham Land and Cape Horn,
and (2) the relatively shallow sea (also 1,000 to 2,000
fathoms) between the Antarctic Continent and the
Australian land.

The Arctic Ocean. (Fig. 12.) — The same conventional
markings are employed. The land surface is left white
and the continental sea margin (out to 2,000 metres in this
case) is stippled. The shading represents, by its intensity,
the depths. We see, then, a very extensive sea-area
where the depth is everywhere less than 2,000 metres.
On this are situated all the North American Islands,
Greenland, Iceland, the Faeroes, the British Islands, the
North Eurasian Islands, etc. Centrally to this archipelago
is the North Polar Basni where depths exceeding 3,000
metres are to be found. The precise western boundaries
of this basin are uncertain, for want of sufficient data,
but its general form must be substantially as shown in the
figure. It is clearly shut off from the Pacific by the
shallow water in and near Behring Straits, both on the
Arctic and Pacific sides : in the Straits themselves the
sea is only 40 to 60 metres deep. It is shut off from
the Norwegian Sea by water of about 800 metres in depth.
The Norwegian Sea is itself a basin of at least 2,000 metres

THE DEPTHS OF THE OCEAN

49

in depth, and in it there are two large depressions where
depths of over 3,000 metres are to be found.

Fig. 12. The Arctic Ocean.

Between Greenland and North America there is Baffin
Bay, containing a trough where the depth varies between
about 1,000 and 2,000 metres. The Norwegian Sea, which

50 AN INTRODUCTION TO OCEANOGRAPHY

interposes between Greenland and Norway is, itself, shut
off from the Atlantic by the great rise which connects the
British Islands with Iceland and Greenland. Along a
narrow ridge joining the Shetlands, the Faeroes and
Iceland the sea is only about 400-500 metres in depth.
The existence of this Wyville- Thompson Ridge has important
consequences for the general circulation of water in the
North Atlantic and North Polar Oceans.

The general picture of the Arctic Ocean is, therefore,
the reverse of that of the Antarctic. In the former we
have a nearly land-locked ocean of considerable area and
depth. It is shut off, in respect of the circulation of the
bottom water, by the restricted Atlantic and Pacific
openings, where the sea everywhere is much less than
1,000 fathoms in depth.

The Atlantic Ocean. — Fig. 13 represents in a simplified
way, the main features of the Atlantic Ocean in respect of
its depths. The contours that are drawn are 2,000, 4,000,
5,000 and 6,000 metres. The land surface is left white ;
the region between this and the 2,000-metre contour
(which we may regard as the Continental Shelf) is stippled ;
the region between 2,000 and 4,000 metres is lightly
shaded ; that between 4,000 and 6,000 metres is darkly
shaded and the " deeps," that is the isolated patches
where the depth is greater than 6,000 metres are cross
hatched. All detail other than this is omitted because
large scale charts are really necessary for its adequate
representation.

Even the limited detail in Fig. 13 (which is only to be
regarded as a sketch chart) shows that the Atlantic differs
remarkably from the Pacific and Indian Oceans. This is
due partly to the much greater amount of investigation
that has been made in the Atlantic region and partly to
certain structural features which we shall consider in
Chapters V and X. Far too little oceanographic research
has been made in the Pacific and Indian Oceans, and some
speculations that are quite permissible with regard to

THE DEPTHS OF THE OCEAN

51

the Atlantic can hardly be elaborated in the cases of the
two other oceans.

Fig. 13. The Atlantic Ocean.

The salient features of the Atlantic, with respect to its
depth are as follows :

(1) The form of the Continental Shelf.

52 AN INTRODUCTION TO OCEANOGRAPHY

(2) The remarkable Central Atlantic Rise or Swell.

(3) The connection of S. America with the Antarctic
Continent.

(1) The Continental Shelf. The region of sea adjoining
the continental land and having a depth of less than 2,000
metres (approximately 1,000 fathoms) may, hereafter, be
regarded as the continental shelf. It is an area of
transition between truly oceanic and truly continental
regions. It may have been sea or land, and undoubtedly
it has been the locus of extensive geological changes.
Transgressions of the land over the sea and vice versa have
very frequently occurred in this region in the past. It is
seen in characteristic form all round the coast of Africa
and everywhere round the coast of South America, with the
exception of Patagonia. In these regions the shelf is
relatively narrow. Suppose that there is a similar wide
zone of land on the margins of the Continents and call
this the " continental rim " — then we see that both rim
and shelf might be now sea and again land, but that the
general form and extent of the continental elevations, and
the adjacent oceanic depressions would still be much the
same : that is in such regions as those that we have
mentioned, where the shelf has the characteristic
extension.

But off the coasts of North-East America and North-
West Europe the case is quite different : here the shelf is
greatly extended. Even between Florida and Newfound-
land it is wider than the average, and from the northern
shores of the Bay of Biscay on the one side, and from
Labrador on the other, the shelf extends all the way across
the North Atlantic Ocean. The two shallow inland seas,
Hudson's Bay on the west of the Atlantic and the Baltic
on the east, are situated on the shelf. The two deep
inland seas, the Caribbean and Gulf of Mexico on the west
and the Mediterranean on the east, on the other hand,
contain depressions that are considerably over 1,000
fathoms : they are sea regions of quite another character.

THE DEPTHS OF THE OCEAN 53

(2) The Central Atlantic Rise or Ridge. This is quite
the most remarkable feature of the Atlantic " bottom
relief." South of the continental shelf, where it bounds
Greenland, Iceland, the Faeroes and the British Islands,
there is an extensive area of sea bottom on which the
depth is between 2,000 and 4,000 metres : this has been
called the Telegraphic Plateau because it was here that
the first cables were laid. This now runs nearly south to
about lat. 12° N. and then it turns to the E.S.E to about
2° S. lat. where it again turns nearly due south. The
Rise is variable in width and tends to spread out laterally
at the north and south extremities and contract very
markedly at about the middle of its length. Further, it
is nearly equidistant, at all latitudes, between Europe and
N. America, and Africa and S. America. And, as the
westward " nose " of Africa protrudes a little northerly
of the easterly protruding nose of S. America, so the Rise
alters the direction to remain equidistant between North-
West Africa and Brazil.

Most of the Atlantic Islands are situated on the Rise.
To the north there is the plateau which carries the Azores.
Equatorially is a similar, but smaller plateau carrying
St. Paul Rocks. To the south are Ascension, Tristan
d'Acunha, Gough and Bouvet Islands. St. Helena is an
exception in that it rises out of the West African Basin,
while the Cape Verde Islands and the Canaries lie just
outside the continental shelf.

Nothing like the Central Atlantic Rise exists in the
Pacific or Indian Oceans. What it means, with regard
to the origin and morphology of the Atlantic Ocean is not
known, but a suggestion is made at the end of Chapter X.

(3) The American- Antarctic connection. The continental
shelf of the extreme S.E. Coast of S. America is widely
extended and projects down to the south-east : this is
the " Patagonian Shelf " and on it are situated the Falkland
Islands. Then the northerly directed peninsula of the
Antarctic Continent, Graham Land, is also prolonged to

54 AN INTRODUCTION TO OCEANOGRAPHY

the north-east as an extended continental shelf : on this
are the South Shetland, and South Orkney Islands, and
beyond these, and rising out of water which is between
2,000 and 4,000 metres in depth, are the Sandwich Islands
and those of South Georgia. It is not at all fanciful to see
here much the same disposition as in the case of the Greater
and Lesser Antilles in the West Indian region, that is a
great arc of elevated sea bottom bent out from west to
east. To this point, however, we return in Chapters V
and X. Clearly there is evidence of a former actual, or
at least, potential land connection between Antarctica and
S. America. This, it will be remembered, was assumed in
the Chamberlin hypothesis of the origin of the great
oceans.

The connections of the Central Rise with the Continental
Shdf. To the north the Central Rise expands out to form
the Telegraphic Plateau, and then this bathymetric region
(4,000 to 2,000 metres) thins out very greatly along the
Eur-African and American Atlantic margins. There is
really a very narrow zone of sea of this depth along the
eastern and western borders of the xitlantic, although it is
greatly extended to north and south. Now just as the
central rise becomes continuous with the 2,000-4,000 zone off
Labrador, on the west, and off the British Islands, on the
east, so it tends to join up in a similar way in the south.
OfE the Rio de la Plata there is a great extension to the
east of the zone of 2,000-4,000 metre bottom, and this very
nearly joins the Central Rise in about lat. 35° S. On the
other side of the Atlantic, at about the latitude of Walfisch
Bay, a somewhat similar tongue of 2,000-4,000 metre
bottom extends out to the S.W. and actually joins the
Central Rise between latitudes 30° to 40° S. Thus there
is a kind of barrier or sill extending almost uninterruptedly
across the South Atlantic between the parallels of 30° and
40° S. South from this the Central Rise terminates
abruptly in a broad tongue of relatively high sea bottom,
and south from which again is the South Polar basin.

|>o|

56 AN INTRODUCTION TO OCEANOGRAPHY

In describing these features of the Atlantic Ocean in
respect of its depth, it is impossible to avoid conveying a
somewhat exaggerated impression of the proportions of
depth of water to distance. The sketch chart, Fig. 13,
must necessarily do so, for the lateral scale is exceedingly
small and the markings used to indicate the variations
in depth do not suggest, in the least, the almost infinitesimal
thickness of the water layer on the scale of the diagram.
To correct this impression the reader must now consider
Fig. 14.

In Fig. 14 the depth of water is graphed to the same
scale as that of the horizontal distance. The earth is
supposed to be diminished to a sphere of about 4 metres
in radius and the actual curvature of this globe is
represented on the diagram. On this scale (about 1
millimetre to the mile) the width of the Atlantic in the
latitude chosen is about 3 metres, and so the section is
broken up into segments, the left hand extremity of each
joining on to the right hand of the segment just beneath.
The section starts in Portugal and is so directed that it
will pass through the Island of San Miguel in the Azores
Archipelago. The relatively deep water off the European
coast — the Cape Verde Basin — is well shown. Then comes
the gradual shoaling towards the Azores Plateau on the
Central Rise and the deepening again into the North
American basin. The thickness of the black strip is
proportional to the depth of water, and the thickness of
the dotted parts at the two extremities and in the
fifth segment is proportional to the height of the land
above sea level. The numbers placed above the black
strip represent distances in miles from the European
coast.

The Pacific Ocean. — In most respects the Pacific differs
remarkably from the Atlantic Ocean, and its depths do
not show that approximate symmetry which is indicated
in the latter region. Far less, however, is known about it,
both with regard to its present condition and its geological

THE DEPTHS OF THE OCEAN

57

history, but nevertheless the contrast with the Atlantic

is very striking.

There are three evident regions (see also Fig. 58) :

(1) The South-W ester n Pacific. Here we have what has

been called the debris of a former South Asiatic Continent.

Fig. 15. Sketch Chart of the Pacrlic Ocean. The projection is a
Mercator one and the depths are given in fathoms. The
stippled areas represent -lea bottom where the depth is less
than 1000 fathoms.

Australia and all the East Indian Islands are situated on
the West Pacific continental shelf. On this region of sea-
bottom there are, indeed, a number of small depressions
where the depth may exceed 2,000 fathoms, but these do
not obscure the obvious relations of the Australian and

58 AN INTRODUCTION TO OCEANOGRAPHY

Asiatic continental elevations. And though New Zealand
is separated from Australia by a large depression of over
2,000 fathoms in depth, these Islands are also situated on a
region of sea bottom with less than 1,000 fathoms of water
over it, and this region is much greater in area than that
of the New Zealand Islands. To the north of it are
numerous outlying areas of sea with less than 1,000 fathoms
of depth. Enclosing all this is the 2,000-fathom contour
line which, on its south margin, passes into the continental
shelf fringing the Antarctic Continent. Further, a
prominent tongue of sea bottom, bounded by the 2,000-
fathom contour line is thrust across and almost joins a
similar tongue protruded out from the continental shelf of
Western South America. A former connection with the
Antarctic Continent is therefore suggested and, possibly,
also one with South America.

(2) The Central Pacific. To the north and east of the
vestigial (or abortive) Austral- Asiatic Continent are a
number of patches of sea bottom where the water is less
than 1,000 fathoms in depth, and on these are the multitudes
of small islands that are so characteristic of the Pacific.
These are isolated from each other and do not show that
obvious continental relationship suggested by the Austral-
Asiatic Shelf, but there is the strong suggestion in their
distribution that they are outliers of the latter and belong
to the same general region in the geological sense.

(3) The North and East Pacific. To the north, east
and south-east of these Central Pacific shoal regions lies
deeper water. Here also are most of the " deeps " : that
is, depressions in the sea bottom of over 3,000, 4,000 and
even 5,000 fathoms. This region and its significance we
consider further in Chapters V and X.

The Indian Ocean. — Plainly the eastern and western
regions of the Indian Ocean differ from each other in
somewhat the same way as in the Pacific.

(1) Near Africa, and separated from it by water which
is less than 2,000 fathoms in depth, is the large island of

THE DEPTHS OF THE OCEAN

59

Madagascar. Between this and India are a number of
isolated patches of water that is less than 1,000 fathoms

in depth. These carry numerous islands : the Seychelles,
Maldives, Laccadives, etc. Here also we seem to see
outliers of a truly continental region.

60 AN INTRODUCTION TO OCEANOGRAPHY

(2) To the east are the deeper parts of the Indian Ocean.
The eastern margin is formed by the continental shelf
that bounds the Malay Peninsula, Sumatra, Java and
Australia, and here the declivity into depths of over 2,000
fathoms is steeper than on the western side. Here, also,
are the Indian Oceanic depressions of over 3,000 fathoms
in depth.

(3) Centrally and rather to the south is an elevation of
the bottom which approaches the South African continental
shelf. On this are several groups of islands : the Crozets,
Marions, St. Paul, Amsterdam, Kerguelen, etc. South of
this again is the great Southern Ocean.

In some ways, then, there are strong suggestions of
similarity between the Indian and Pacific Oceans and
equally strong points of differences between both and the
Atlantic. These resemblances and differences will become
the more marked when we consider the kinds of continental
margins that frame the oceans, as well as the general
schemes of the latter. This we shall deal with in Chapters
V and X.

CHAPTER IV
THE SEA BOTTOM

It is held, in this book, that the present continental
elevations and oceanic depressions are " permanent " in
the sense that they have occupied their present positions
throughout great parts of the geological period in Avhich
the sedimentary rocks have been formed. This does not
mean that the original oceanic floors, or the original
continental elevations are now accessible to observation.
Large tracts of the continental surfaces are covered by
rocks that have certainly been deposited at the bottoms
of shallow seas, and the ocean bottom in the region of the
continental shelf is covered with materials of the same
nature as those that are present on the continental land.
There are sedimentary and " igneous " rocks that have
been or are being eroded, and sediments that are in process
of being consolidated into rocks like those that are exposed
on the dry land.

So also the floors of the oceanic depressions are covered
with deposits that conceal the original earth materials
that existed there. Only about one to two feet of the
depth of this layer of sea-bottom deposit is known, and that
only at a very few spots. We have not even a hint, in
the results of observations, as to the depth of these deposits,
or as to the nature of the rock that lies underneath them.
Very little is known as to the rate at which oceanic bottom
deposits are being accumulated. Some few observations
made by the engineers of the telegraph cable ships suggest
that about one inch of deposit may be laid down in 10
years, but this is probably a maximum estimate and it is

61

62 AN INTRODUCTION TO OCEANOGRAPHY

certain that the rate of deposition is very much less in
some other places. One of the most interesting and
valuable results of oceanography would be a boring of the
ocean floor to a considerable depth but, at present, there
does not seem to be any practicable means of doing
this.

The study of sea-bottom deposits. — The methods of
collection have been described at the beginning of chapter
III. A deep sea sounding usually involves the collection
of a sample of the sea bottom at the same time, but this
is only possible when the deposit is mud, or ooze, or sand
capable of being forced up into the tubes of the sinkers
(Fig. 9 (2)). The grab (Fig. 9 (5)) is able to lift up small
pebbles, but any objects lying on the sea bottom and
bigger than what we call pebbles cannot be collected by
the apparatus described. Stones, boulders, large concre-
tions, large bones, etc., can be obtained by dredging, and
small dredges have been worked at practically all depths.
The process of using such apparatus is, however, very
laborious, and only an exceedingly small fraction of
the oceanic abysses have been explored in this way.
Dredge samples include everything that lies on the bottom
and, as an estimate of the area traversed by the apparatus
can be made, the sample obtained is roughly quantitative.
A true sample of the muddy or oozy deposit cannot,
however, be obtained by a dredge, as soft and fine material
is usually washed out from the apparatus to some extent
while it is being hauled up to the surface. For this purpose
the cup dredge. Fig. 9 (6), can be used.

Shallow seas are sampled in just the same way but with
modified apparatus — usually dredges with sharp, cutting,
front blades that dig into the sea bottom to a small depth,
and bags made of very closely woven, thick netting that
retains fine material to a certain extent. In such samples
the portion in the centre of the mass that fills the net is
usually taken since this is less subject to w^ashing than the
external parts. Grabs, such as that illustrated in Fig. 9 (5),

THE SEA BOTTOM 63

are, however, preferably employed in water of moderate
depth. The foreshore is, of course, susceptible of direct
observation and the methods of collection are obvious ones.

The description of a sea-bottom deposit is that of its
constituent particles. Mineral substances come from the
disintegration of rocks on the land, the broken-down
particles being transported by currents or wave action,
or blown into the sea from the atmosphere ; or they may
result from the breaking down of volcanic materials ejected
on to the sea bottom itself or reaching there after being
water- or air-borne. Organic materials are the hard parts,
or skelet-ons, of numerous species of plants and animals
inhabiting the sea bottom itself or living in the upper
layers of the ocean. The soft parts of these organisms
putrefy and pass into solution, but the living or siliceous
skeletons remain. Both mineral and organically formed
deposits undergo chemical change when they lie for a
long time on the sea bottom and so altered products,
concretions, etc., are to be recognised.

The analysis of a sea bottom deposit includes, then, the
identification of the constituent particles and of their mode
of origin — that is, the kinds of minerals present, whether
simply resulting from the disintegration of massive rocks,
or chemically altered ; the species of plants and animals
as deduced from a knowledge of the skeletons of the living
organisms and the chemical composition of the deposit,
as a whole, that is, for instance, the percentage of lime,
silica, alumina, iron, manganese, etc., in the dried material.
In addition to this treatment mechanical analyses may be
made, that is, the material may be described in relation te
the prevalent sizes of its particles, as, for example, in the
case of the sands that form the bulk of the shore deposits.
Such a description becomes necessary when we consider
the transport of fine sands, muds and oozes by the agency
of moving water.

The original nature of sea-bottom deposits.

(1) hiorganic Materials. This category includes

64 AN INTRODUCTION TO OCEANOGRAPHY

terrestrial, extra-terrestrial and chemically transformed
substances.

Terrestrial materials are such as come from the continental
land mass and are almost entirely the products of aqueous
denundation carried down into the sea by rivers as muds
and sands. They are shore-erosion materials resulting from
wave action on the rocks of the shores and distributed
mainly by tidal and wind currents ; volcanic maierials
coming from the disintegration of lavas ejected on to the
ocean floor by submarine volcanoes, pumice (either itself
or in disintegrated forms) settling on the sea bottom after
floating in the water, and volcanic dusts carried by winds
and finally settling on the surface of the sea.

Extra-terrestrial materials are very rare though of
immense interest. They are meteoritic dust coming into
the earth's atmosphere from outer space. They are
recognised in ocean-bottom materials as the " cosmic
spherules," Fig. 19. They are microscopic, roughly
spherical particles, black or bronze in colour, laminated or
crystallised in texture. They usually consist of iron or
of magnetic iron oxide. They are only to be found, as a
rule, in those deeply-laid deposits that accunmlate with
exceeding slowness, and they are recognised there because
of the relative scarcity of materials of other origin.

The chemically altered materials we consider later.

(2) Organic 7naferials. Those of vegetable origin are the
remains of calcareous algae such as the corallines, and
broken-down coccospheres and rhabdospheres. These are
relatively rare. Diatom oozes, made up of the siliceous
skeletons or frustules of these minute plants, occur much
more frequently.

Calcareous materials of animal origin are very much more
abundant. Here we have molluscan shells, and a great
variety of other calcareous skeletons. Siliceovs tnaterials
of animal origin are not so abundant, but nevertheless
constitute an important category of ocean-bottom
deposits.

THE SEA BOTTOM

65

Sea-bottom deposits : the Challenger classification. — The

classification made by Murray and Renard ought to be
known. It is :

Deep-sea
deposits laid
down in water
over 100
fathoms in
depth

Shallow- water
deposits laid
down in M^ater
less than 100
fathoms in
depth
Littoral
deposits laid
down on the
foreshore

Red clay
Radiolarian ooze
Diatom ooze
Globigerina ooze
Pteropod ooze
Blue mud
Red mud
Green mud
Volcanic mud
I Coral mud

Sands

Gravels

Muds

J Sands
I Gravels
I Muds

Pelagic deposits

Terrigenous
deposits

This classification considers both the immediate place of
origin of the deposits and the place of deposition in respect
of the depth of water. Terrigenous deposits are the results
of disintegration of the rocks that occur on the land.
These erosion-products are transported from the land by
rivers, ice and winds, and are then distributed on the sea
bottom near the land by wave-action, ocean currents, tidal
streams and winds. Therefore they are not found in bulk
at a very great distance from the land because of the
relative feebleness of these transporting agencies.
Nevertheless, very fine muds of land origin may be found
at considerable distances from the shore and thus the
terrigenous deposits are placed both inside and outside the
100-fathom contour line.

66 AN INTRODUCTION TO OCEANOGRAPHY

The pelagic deposits, on the other hand, have their
place of origin, not on the land but in the sea itself ; they
consist of the remains of animals and plants that live there,
and they are generally associated with masses of water of
great depths. But the organisms whose skeletons form
the pelagic deposits are really more abundant in relatively
shallow water than they are in the ocean over great depths,
and such materials as compose (say) globigerina and
diatom oozes are probably laid down on quite shallow
sea bottoms in much greater quantity than they are on
abysmal ocean floors. In shallow water, however, they
are covered up by the much greater quantities of terrigenous
materials resulting from land erosion and so they are
almost unrecognisable.

Here we are considering the oceanic depressions on the
great scale as contrasted with the great continental
elevations. Further, we are considering the continental
shelf, not as any particular gradient of sea bottom, or sea
floor of any particular limits of depth : it is the transition
region between the oceanic depressions and the continental
elevations and, during the vicissitudes of geological history,
it may have been now sea bottom and again land surface.
A classification of sea-bottom deposits which presents a
more illuminating point of view may, therefore, be
substituted for that of Murray and Renard.

Categories of sea-bottom deposits. —

fLittoral deposits

(1) Continental deposits -i Shallow- water deposits

[Terrigenous muds.
["Shallow- water neritic

(2) Neritic deposits < deposits

[Oceanic neritic deposits.

(3) Pelagic deposits The deep-sea oozes.

These various categories we now consider further.
Obviously the antecedent to the accumulation of very
extensive shore and shallow sea-bottom deposits is the

THE SEA BOTTOM 67

existence of a vast land area over which disintegration of
earth materials is proceeding on the great scale. That
means continental land, or at the least, extensive island
land areas ; great rivers with large volumes of water,
fairly rapid flow and extensive catchment areas ; large
glaciers capable of transporting heavy materials and with
notable power of erosion ; desert areas from which light
sand and dust may be blown by strong winds ; shallow,
marginal continental seas in which there are rapid tidal
streams capable of shifting bottom deposits, and where
wave-action may be exerted to the maximum degree — all
or some of these conditions. Obviously they are not
present on oceanic islands, and round these we find nothing
quite like the continental shelf and the characteristic
continental shallow-water deposits. Continental deposits
are not defined very precisely by the depth of sea in which
they are laid down or by the distance from the shore at
which they are found. There will be a grading of the
materials in respect of the average sizes of the constituent
particles, and this will depend on the strength of the
transporting agencies.

Neritic deposits are quite different in their nature.
They are organic in mode of origin, consisting of the dead
skeletons of marine animals and plants. There are very
clear distinctions between such deposits as shelly gravels,
or " coral " bottoms, on the one hand, and mud or sand-
banks, on the other. The former are called " neritic "
and the latter terrigenous. Neritic deposits laid down off
continental shores consist predominantly of the dead and
partially or wholly disintegrated skeletons of marine
animals and plants which have their most favourable
environment in shallow seas of relatively low salinity.
They are there in abundance because of the proximity of
the land. Neritic deposits off oceanic islands may be very
different in their nature, again because of distance from
extensive land areas. The differences between continental
and oceanic neritic deposits traces back to the natural

68 AN INTRODUCTION TO OCEANOGRAPHY

habits of the organisms whose dead skeletons form the
materials in question.

Pelagic deposits are due to organisms which have their
most favourable environment in truly oceanic regions.
They are independent of the land, living and reproducing
generally at or near the surface of the ocean.

The various kinds of sea-bottom deposits may now be
described, premising that for details of distribution and
identifications of the species of minerals and organic
remains the original memoirs must be consulted.

Littoral deposits. — These are the materials present on
the foreshore, that is, the region between high and low
water tide-marks. Characteristically they consist of
boulders, gravels, sands and muds. It will be convenient
to reserve a description of them for the following chapter,
where we deal with the continental margins.

Shallow- water deposits. — The most characteristic, most
abundant and most widely distributed sea-bottom material
is sand. In general this is the bottom material present in
most shallow seas. Its main source is the shore-land
itself, because the most abundant constituent of the latter
is generally a quartz- containing rock of some kind. Wave-
action, assisted by atmospheric agencies, breaks down the
shore materials, whether rock cliffs, boulder clay, or
whatever they may be. Stones and boulders suffer
attrition by being moved against each other by wave
action. Disintegration of the mineral constituents occurs,
and many of the latter undergo chemical alteration. The
more resistant species remain and undergo grading. Of
these resistant minerals the most abundant is quartz sand
which is, therefore, the characteristic bottom material in
shallow seas.

The grading of the shallow-water deposits is the most
important question involved in our discussion, but so
little adequate investigation has been made so far that
little can be said. In a very general way, however, we
find that the coarser materials, gravels of various grades,

THE SEA BOTTOM 69

occur nearest to the foreshore ; the coarser sands further
out to sea and the finer sands still further away from the
land. This is what is to be expected when we consider
the transporting agencies. The effect of the waves is
felt most strongly on the beach itself, then on the foreshore
and, finally, on the shallow sea bottom just off the land.
To what degree wave-action diminishes with increasing
depth of water is not precisely known, but in a sea of over
50 fathoms in depth the effect becomes slight. Tidal
streams are most rapid in shallow water near the land, and
particularly in bays and narrow estuaries, and they
diminish in velocity very greatly as the distance from the
land and the depth of water increase. Wind currents are
also most pronounced in water that is very shallow. In
general, then, the power of the movements of the sea to
shift and transport stones, gravels and sands decreases
with distance from the land. Further there is a resultant
effect of all these agencies. The general pounding action
of waves will take place at a certain angle to the shore line
and this angle will depend on local conditions. Tidal
streams reverse themselves in direction with every high
and low water change, but not completely, so that they tend
to transport materials along certain resultant paths.
Wind currents, too, have prevalent seasonal directions,
and it must generally happen that they also have a resultant
effect, taking the whole year into consideration, on each
part of a coast line.

Therefore, waves, tidal streams, wind currents, and the
marine channels of rivers transport the materials of the
shallow sea bottom ; so that they roll stones, gravels,
sands and muds in this way and that but, on the whole,
in a certain general direction dependent on the sum of all
the local conditions. The general directions will vary from
place to place, although they may, to some extent, be
seasonal. The same particles will be moved more rapidly
near the coast than off shore. Coarser particles will in
general, tend to remain on or near the beach and foreshore.

70 AN INTRODUCTION TO OCEANOGRAPHY

and finer ones will tend to be transported into deeper
water.

Large inequalities of the coast line, that is bold
promontories or headlands shaping out bays and estuaries,
influence the directions and velocities of the tidal streams.
Thus " tidal races " or local rapidly-running streams, or
slack ones, or eddies are established. On the sea bottom
beneath rapidly running tidal streams there will generally
only be rock, or boulders, or stones, or gravel, while beneath
large tidal eddies fine sand and mud will tend to accumulate.
Again unperiodic changes occur. Exceptional spates, or
freshets, or melting snow-falls may greatly increase the
volume and velocities of river streams in their estuarine
portions, and so sand-banks, embankments, etc., may be
temporarily breached and lead to changing river and tidal
channels. These react on the directions of the tidal
streams and so on the places in which coarser and finer
bottom materials tend to accumulate.

A grading into accumulations of coarse and fine gravels,
sands and muds occurs, then, as the result of the
disintegrating and transporting agencies which we have
very briefly mentioned. The distribution of the shallow-
water deposits in a particular sea area is, therefore, the
result not only of the nature of the rocks composing the
coast line, but also of the average effect of the transporting
agencies. The processes involved will be, as a rule,
exceeding complex, and their investigation must always be
difficult and laborious in the extreme.

The composition of a shallow-water deposit is studied by
identification of the rock fragments or mineral particles
contained in it. Quantitative separation of the species of
mineral particles may often be effected by the use of heavy
liquids of suitable densities ; by the use of an electro-
magnet where magnetic particles are concerned ; by
chemical analyses, etc. Grading of particles which are
mineralogically similar is effected by the use of metallic
sieves with apertures of known size (where the particles

THE SEA BOTTOM 71

are large) or by elutriation in water currents of known
velocities (when the particles are small ones).

The Terrigenous Muds — Muds are, in general, deposits
in which the particles are very small and rather
heterogeneous in respect of their chemical composition.
Very fine sands, with minute particles, are distinguishable
from muds and silts in that they consist predominantly of
quartz particles. In muds and silts alumina is present to
a much greater extent than in sands : this and the small-
ness of the particles is, perhaps, the only notable character
in which they differ. Clays are finer still than muds and
" bind " to a greater degree in drying. It must be noted,
however, that there are no satisfactory class-characters
that can generally be utilised in distinguishing between
muds, silts and clays.

Deposits that are variously coloured, very fine in respect
of the sizes of their particles, that remain easily in
suspension in sea water and discolour the latter, occur very
generally on the foreshore and at the bottoms of deep and
shallow seas. Neglecting the foreshore, in the meantime,
we consider the muds of the off-shore regions. Their
origin is, in general, the disintegration of continental and
volcanic rock, both sedimentary and plutonic. The
condition for their occurrence on the sea bottom is relatively
still water : still because of great depth, the absence of a
current or tidal stream, or the presence of an eddy, in the
central portion of which fine suspended particles settle to
the sea bottom. But there are no precise boundaries, as a
rule, to a mud patch and the transition between a typical
mud and a clean, angular, or rounded sand may be a very
gradual one. Both on and off the foreshore muddy sands
and silts, of very indefinite compositions as regards the
fineness of the particles and the proportions of the various
grades present, may occur. In shallow bays and estuaries
muddy water, during times of rough sea, or when
strong spring tidal streams are running, is familiar to every
one. This is the result of a natural process of levigation,

72 AN INTRODUCTION TO OCEANOGRAPHY

the lighter particles of the rather heterogeneous sea
bottom being borne by the motion of the water. The
process would, of course, lead to an ultimate grading of
the sea bottom in respect of the sizes of the particles were
it not that the heterogeneity along the coastal sea bottom
is continually maintained by the disintegration of more
earth materials.

Off the mouths of rivers, mud is also formed by a
process of physical agglutination. Fresh water entering
the sea in rivers contains colloidal " clay," that is the
aluminous particles are in a state which is not that of mere
physical suspension. They are too small to fall to the
bottom as sediments but they are not in a state of true
solution. When this river water mixes with that of the sea,
containing ionised sodium chloride in solution (see Chap.
VI) electrical changes occur and the colloidal clay passes
into the form of a true suspension. The particles of the
latter then sink as a sediment and the water becomes
" muddy " until the sedimentation is complete.

Outside the region of " shallow " water, where wave
motion becomes very feeble at sufficient depths and
where the tidal streams have very small velocities, fine
particles in suspension finally sink to the sea-bottom.
There is, then, a " mud line " round the continents and this
we may take to be somewhere outside the 100-fathom con-
tour— where exactly will vary to a great extent. As a
rough approximation we may put it near the outer margin
of the zones of " terrigenous " deposits on Figures 17 and
18, but it must be noted that observations on the distribu-
tion of sea-bottom deposits near the land are far too few to
enable us to chart the mud line in detail in any sea area.
However, somewhere towards the 1,000-fathom line the
gravels, sands and silts of the zone of terrigenous deposits
disappear and are replaced by the various muds of the table
on p. 65. But, in exceptional cases the terrigenous muds
may be found at far greater depths than 1,000
fathoms.

THE SEA BOTTOM 73

Blue Muds. These are fine muds that are black or
bluish-black in colour. Their limits of depth are so
variable that nothing more precise can be said than that
they occur towards the edges of the continental shelves.
Their origin is as we have stated above — that is, they are the
finer detritus resulting from the disintegration of the shore
materials or they are the result of the distribution seawards
of the colloidal clay brought to the sea by river water.
The colour is due to a process of chemical reduction leading
to the formation of ferrous sulphide. The typical blue
muds are very widely distributed.

Green Muds. These are only varieties of the general
deep-sea mud fringing the continental shelves. The
colour (which is rarely strikingly " green ") is due to the
presence of the interesting mineral called glauconite. This
is a hydrated silicate of aluminium, iron and potassium
but its chemical constitution is not satisfactorily established.
As a rule it occurs as casts filling the cavities of the shells
of foraminifera (see fig. 19) and thus in rather small particles.
Its mode of origin is not well understood but it is
very probable that it results from a process of organic
putrefaction followed by a chemical synthesis. It has
much significance inasmuch as it points to the way in
which potassium compounds are withdrawn from solution
in the sea. It is rather rare, far more than the typical
blue muds, but its distribution is wide. It is known
to occur off the Cape of Good Hope, off Eastern Australia,
off Japan, off the Atlantic coasts of the United States
of America and in the Ceylon Seas. In general it
is said (by Murray) to occur on the Continental slopes
off high and bold coasts where there is a seasonal alteration
in the strength of the ocean currents.

Red Muds. These are simply a variety of the general
continental slope mud coloured reddish by iron oxide.
Typical localities are the Yellow Sea and the continental
shelf off the coast of Brazil. Relatively to the blue-black
muds, the red variety is rare.

74 AN INTRODUCTION TO OCEANOGRAPHY

Volcanic Muds. The characteristic minerals in these
deposits are those arising from the disintegration of
volcanic rocks — thus there are liparite, basaltic minerals,
andesite, volcanic glass, broken down pumice, etc. Where
volcanic rocks occur on the coasts the weathering of these
produces volcanic sands and muds in the immediate
vicinity. So also the loci of submarine volcanoes are also
those of volcanic muds. But plutonic materials are very
widely distributed on the sea bottom. Dust is ejected
into the atmosphere during eruptions and may settle
everywhere on the surface of the sea. Pumice may
float on the sea for prolonged periods before becoming
water-logged and sinking to the bottom to disintegrate.

Neritic Deposits — The sea-bottom deposits considered
so far are mainly inorganic in nature : they consist of
land detritus, the disintegration products of " clastic "
agencies. They are transported from the land by rivers
and are distributed on the sea bottom by movements of
the ocean. The distributing agencies grade the deposits
to a certain extent.

Nevertheless almost any sample of sand or mud taken
from the foreshore or the adjacent shallow sea is sure to
contain materials that are organic in their origin and do
not come from the detritus of the land. Almost any such
sample will contain, perhaps, a few per cent, of calcium
carbonate, and microscopic examination will show, as a
rule, fragments or particles which can often be identified as
broken down shells or skeletons of various animals and
plants. Such mineral matter of organic origin may be
briefly mentioned : —

Molluscan shells and their fragments ;

The limy plates and spines of starfishes, sea-urchins,

ophiurids and crinoids ;
The calcareous tests, or skeletons, of alcyonaria,

polyzoa, gorgonids, etc. ;
The limy tubes of marine worms ;
The calcareous tests of foraminifera ;

THE SEA BOTTOM 75

The limy spicules of sponges, alcyonaria and tunicates ;
The calcareous exoskeletons, or carapaces, of Crustacea ;
Teeth, earbones, etc., of whales and sharks.
On a great scale — the calcareous skeletons of solitary
and reef-building corals.
The above are animal remains, and they consist of
calcium carbonate in the form of aragonite or calcite.
Siliceous animal remains are : — ■
The skeletons of radiolaria ;
The spicules of sponges.
There are also calcareous plant remains. These are : —
Calcareous skeletons of algae — the " corallines " ;
Coccospheres and rhabdospheres with their dis-
integrated products — the coccoliths and
rhabdoliths. (These are also calcareous algae).
The siliceous plant remains are: —

The skeletons or frustules of diatoms.
These materials are not restricted to any particular sea
bottom, with respect to depth : they occur both in
continental or pelagic deposits. In shallow water in
particular and, in general, on the continental shelf they
are smothered by the huge quantities of sand and mud
laid down at the same time.

In certain places, and on practically all coasts, however,
there are deposits which consist predominantly of some of
the kinds of organic remains mentioned above. This is
even the case with the deposits of the foreshore, but we
consider these in the next chapter. The accumulations
of calcareous materials of organic origin that we find here
and there on the sea bottom near the land are —

The Shallow-Water Neritic Deposits. The principal
deposits of this kind are the shelly gravels and sands that
are to be found at the sea bottom in depths out to (say)
50 fathoms. The characteristic remains in such are the
whole or broken valves of bivalve and univalve shellfish.

It is, of course, very rarely that a neritic deposit will
consist entirely of such materials and when this is the case

76 AN INTRODUCTION TO OCEANOGRAPHY

there is sure to have been some process of segregation at
work. To the same category of deposits belong shelly
sands which are the result of the attrition of shelly gravels.

Nullipore gravels and sands. Nullipores are calcareous
algae, that is, plants like the ord'nary seaweeds but
having the soft plant tissues impregnated with calcium
carbonate to the degree that the plant body has become
stony in texture. The two principal forms in British
waters are Lithothamnium and the ordinary nullipore —
Corallina.

Polyzoan gravels and sands. Polyzoa are colonial
animals of small size which have much the same external
appearance as the zoophytes. As a rule they form
incrustations on the surface of stones, gravels and even
coarse sands. They are never very abundant in a shallow-
water deposit in point of mass, but they may be very
noticeable because of the way in which they form thin
crusts on the surface of truly terrigenous materials.

The distribution of the shallow-water neritic deposits. This
is not arbitrary. Just as the distribution of the various
grades of gravel, sand and mud on the continental shelf
depends on the nature of the adjacent land, and on the
prevalent currents, tidal streams, winds, etc., so there are
factors which rule the distribution of the neritic deposits.
These are called the "ecological factors." There is
something in the geological nature of the inorganic materials
of the sea bottom ; or in that of the adjacent land ; some
particular set of currents or tidal streams that carry food
materials ; some favourable admixture of fresh and salt
water ; some particular materials in solution in the rivers
entering the sea in the neighbourhood ; a particular annual
range of temperature, or winter minimal, or summer
maximal values of the latter ; some set of currents or
tidal streams that carries eggs or larvae to a certain part
of the sea. All or some of these conditions may lead to
the establishment, throughout a great number of years,
of colonies of molluscs, nullipores, echinoids, etc., on

THE SEA BOTTOM 77

restricted parts of the sea bottom. In course of time
their skeletons accumulate to form a neritic deposit — one
which does not come from the materials of the land but
which is, to a predominant degree, conditioned by the
local peculiarities of the adjacent land.

The study of such ecological conditions has become
one of the most attractive departments of biology. The
methods are the ordinary ones of the field naturalist, but
they include rather specialised apparatus for collecting
the living animals and the deposits from the sea bottom.
The use of the fisherman's trawl-net (but with a very
thick foot-rope which will bump over stones and rough
ground), or of the old fashioned naturalists' dredge gives
us qualitative information as to the kinds of organisms
that inhabit the sea bottom. Quantitative estimates
are made by means of the Petersen " bottom-grab,"
an apparatus that scoops up a given patch of soft sea
bottom, say one-quarter square metre. The deposits
so obtained are then examined at leisure and the mineral
and organic materials present are identified, measured,
counted, etc. Most of the animals and plants living
on the sea bottom {demersal species) at some early stage
in their life-history live swimming or passively drifting
in the water and usually near the surface — this is their
pelagic or planktonic stage — and they are collected by
using a small conical net made of fine (miller's) bolting-
silk cloth. Such nets are towed slowly from boats, and
either at the surface of the sea, or at any convenient
depth down to the bottom. They can be so constructed
as to give approximately the volume of water that passes
through their meshes and the individual animals or plants
caught by them can be identified and counted. Finally
the physical conditions of the sea can be studied : the
directions of tidal streams by means of floats, etc., the
temperature of the water at the surface and various
depths by means of specially constructed thermometers ;
the salinity of the water and its composition in respect

78 AN INTRODUCTION TO OCEANOGRAPHY

of substances necessary for life (see Chapter VI),
are determined by chemical analysis. All such
investigations are seasonal ones since the conditions
are always changing.

Thus the investigation of marine' animal and plant
communities comes into relation with the study of the
neritic deposits now forming on the sea bottom. From
the results of such studies (which have not, so far, been
very extensively made) it becomes possible to reconstruct
the ecological conditions that prevailed in the past when
neritic deposits, such as may be found in the fossil form,
were being laid down.

Oceanic Neritic Deposits. — These occur in the neigh-
bourhood of oceanic islands. The main character of the
latter, from one point of view, is that they have no
continental shelf — that is no flat, or relatively fiat
margin of sea bottom round them. As a rule the sea in the
immediate vicinity of an oceanic island sinks down more
steeply towards the ocean bottom than does that along a
continental margin. The slope is never "precipitous " to
the oceanic abyss but it may often be 1 in 10 and rarely
as steep as 1 in 1 . The islands themselves are usually of
volcanic origin ; they are small ; there is no great surface
exposed to subaerial or submarine erosion and great
rivers are absent. Their material, therefore, makes
but a small contribution to the sea bottom in the
neighbourhood, and the latter is what we term neritic.
There are volcanic sands and muds, and along with these
materials are the remains of organisms that live in the
water of the ocean and have much the same general
biological characters as those that contribute to the
formation of the deep sea oozes. In fact what we have
in the neighbourhood of an oceanic island is an oceanic-
insular, rather than a continental-terrigenous, inorganic
sea bottom, along with a pelagic deposit. The pelagic
constituents are not, as a rule, disintegrated to the same
extent as are those that we fmd at the bottom of the deep

THE SEA BOTTOM 79

oceans. The shells and other remains are usually cleaner
and more nearly entire and there is less muddy or clayey
constituents. Such sea-bottom materials in the immediate
vicinity of oceanic islands are, therefore, mixtures in
various proportions, of volcanic and coral sands.

Coral Formations. — True corals, as apart from the
"corals" that are often marked on the official charts,
or the deposits so named by fishermen (which are often
nullipores or polyzoan remains) are animals belonging to
the group called Hydrozoa. They are often solitary,
living on the sea bottom as units. Mostly the hydrozoa
are the common " zoophytes, " that is, colonial groups.
The individuals or zooids, or polyps, are connected together
by a common fleshy material and are arranged in variously
formed communities simulating plant forms. The common,
fleshy, connecting material, and the greater parts of the
bodies of the polyps are invested by a stiff, flexible cuticle
formed of chitin. The colonies are rooted to stones on
the sea bottom. Zoophytes are an important group of
animals in shallow water. Because of the perishable
nature of the chitinous investing skeleton they leave little
or no remains, to contribute to the formation of the deep
or shallow-water deposits.

Instead of a chitinous cuticle many of these animals
secrete a calcareous one (just as many marine algae, or
polyzoa do). These are the calcareous corals. A number
of them form very massive colonial skeletons which grow
to form the typical reefs.

Reef- building corals have rather a limited distribution, as
will be seen by consulting Figures 17 and 18 at the end of
this chapter. They are quite absent over the greater part
of the Atlantic, being found only in the West Indian region
and of? Bermuda. They are highly characteristic of the
Australian Seas and of what we have called the Central
parts of the Pacific and Indian Ocean. Figs. 17 and 18
give only their general distribution : on a larger scale
chart very numerous patches of coral sea bottom would be

80 AN INTRODUCTION TO OCEANOGRAPHY

marked. But, in broad terms, coral reef formations are
characteristic of the Central and Western regions of^jth^^
Pacific and Indian Oceans and curiously deficrent in the
Eastern regions. They are quite absent in the Northern
and Southern latitudes. This distribution is bound up
with that of oceanic temperature as we shall see in Chapter
VII.

Coral Reefs. These are to be regarded rather as great
geological features than neritic bottom formations,
nevertheless their nature is described by the latter term.
Massive coral reefs are present in the gigantic scale as the
Great Australian Barrier Reef. This runs for 1,200 miles
along the north-eastern coast of Australia at an average
distance of about 20-30 miles from the continental land.
Inside the Barrier there is a channel with a general depth
of about 15 to 20 fathoms, and within this again, and
forming its continental margin are a line of inner reefs.
Between the inner reefs and the land are shoal channels.

Fringing Reefs have much the same characters
represented on a much smaller scale than the great Barrier
Reefs. They may be placed on a foundation of
sedimentary or plutonic rock, or on a basis of old coral
rock. Abrasion of the foundation by wave action results
in the formation of a very shallow submarine flat bounded
shoreward by a low flat beach often terminated by cliffs
which may be undercut. On the outer edge of the
submarine flat the fringing reef is built up by the polyps.
As it is formed it becomes broken down by wave action
and fragments of the dead coral rock fall down into the
sea to form what is called a " talus." Often this seaward
slope of the fringing reef is called a "coral precipice,"
but the terms " talus " and " precipice " ought not to
suggest the land features to which they were originally
applied. The actual slopes of the " talus " and " precipice "
(when these can really be stated numerically) are, as a rule,
far less than the corresponding land ones. Such a slope
as that formed by loose stones and earth lying at their

THE SEA BOTTOM 81

*' angles of repose " on a hillside can hardly exist in the
sea, for wave action will break up and distribute the
materials of the "talus" and spread them out over the
sea bottom at an angle which may be a rather small one.
Precipices on the land may be actually vertical, but such
features are exceedingly rare on the margins of continents
and oceanic islands.

Between the fringing reef and the land the sub-
marine flat is usually deepened to the extent that it
forms a shallow channel navigable by smaTMboats. Just
how this deepening occurs is not quite clear. It jsascribed
to slow disintegration of the old coral rock foundation,
with the formation of coral mud. When there are tidal
streams these run with increased velocity, along the
submarine flat inside the fringingj"eef — the latter, it may
be stated, is usually broken do^^ii here and there so that
there is access of the sea to the shallow region inside the
reef. The disintegrated coral mud may, therefore, be
scoured away by rapid tidal stream, or even removed in
solution.

On the outside of the reef the coral polyps continue to
live and build fresh material because there is a plentiful
supply of food and oxygen — more than there is on the
inside margin. The deposition of new coral rock will
keep pace with the destruction of the material of the reef
by wave action. Abraded rock falls down the seaward
" talus^" continually adding to the latter, and evidently
the reef will tend both to increase in mass and to grow
seaward from the land. Thus the barrier form of reef
comes into existence. But when the formation becomes
sufficiently great its character must alter. The boat
channel between the fringing reef and the land will deepen
as coral mud is removed, and tend to become a passage
for oceanic water, and new reef building corals will tend to,
establish themselves so that the inner reefs become formed.

Atolls are reefs of roughly circular, crescentic, or arcuate
form. The profile of a typical coral atoll, that of Funafuti,

82 AN INTRODUCTION TO OCEANOGRAPHY

is represented, in section, in Fig. 10, 4. The gradient on
the outer margin of the reef is relatively steep, more so
at the extreme margin than ofE shore and, as a rule, a depth
of over 1,000 fathoms occurs on the convex side of the
atoll at a distance of a mile or m6re. On the concave
side, that is, within the encircling reef, the slope is slight
and the depth is only a few fathoms. Inside the lagoon
so formed the bottom consists predominantly of living
corals, but outside there is the usual " talus " formed by
the disintegration of the reef by wave action. This is
covered by coral sands containing the remains of pelagic
organisms.

Modes of Origin of Coral Reefs — The well-known
hypotheses of the mode of origin of coral reefs in general
are((lpthat of Charles Darwin and (2) the Murray-Semper
one. According to Darwin's hypothesis reefs are built
up on ocean bottoms that are undergoing depression.
The usually cited case is that of a volcanic cone which
is slowly being submerged. Round the margin of the cone
reef-building polyps are living and are raising the shallow
sea-bottom to near the surface and as the cone keeps on
sinking the growing reefs keep pace with it and maintain
their level at a fathom or two beneath the surface. Wave
action leads to disintegration and fragments of coral rock
are piled up to form a beach. Inside the latter a shallow
flat forms and this becomes a channel in the way that we
have indicated above. Thus a fringing reef is established
but with continued subsidence of the enclosed cone the
fringing reef becomes an atoll encircling a shallow lagoon.
It is not clear why the growing corals do not gradually
fill up the latter : to explain this we have to assume
death of the enclosed corals, disintegration of their
calcareous matrix and removal of coral mud either by
scour, or by solution, or by both processes. The hypothesis
involves a general depression of an enormous regiorL_gf
the Central and Western Pacific, but the assumption fits
in with the general idea formed from a broad survey of

THE SEA BOTTOM 83

the ocean depths — that in this region we see the " debris
of a drowned continental elevation " and so also, perhaps,
with the other great coral sea, that between l^Iadagascar^
and India, Clear evidence is, however, still wanting

^EEafsuch is actually the case. In the Fiji Islands of
the Pacific it is, however, probable that extensive submarine

Tiats on which coral reefs are situated have actually been
formed by a downward tilting of the ocean floor.

The Semper-Murray hypothesis dispenses with the
assumption of subsidence of the ocean floor in the
regions where reefs are being formed. Let there be an
elevation of the latter to begin with : the dead skeletons
of pelagic organisms rain down on this and, in time,
raise its level near to the surface. This assumes, be it
lioted, a general elevation of an ocean floor on which the
skeletons of pelagic organisms subside but, given a local
elevated region there will be a tendency for such remains
to accumulate there more rapidly than in the adjacent
deeper water, where they will dissolve to a greater extent
than in the relatively shallow water over the elevation.
But an apparently fatal objection to the hypothesis is
contained in the now established theory of isostatic
equilibrium of oceanic depressions and continental
elevations. The weighting of the ocean floor (whether

"over a local elevation or elsewhere) by the accumulation
of pelagic deposits ought to lead to subsidence. (See
the following chapter).

The theory of isostasy, however, was not formulated
when the Semper-Murray hypothesis of coral reefs was
in general acceptance. Let, then, the accumulation of
pelagic deposits gradually raise the level of a submarine
elevation : by and by the latter will come so near to the
surface that coral polyps become able to establish
themselves and build encircling or fringing reefs. To
account for the existence of the lagoon, or shallow channel,
it is assumed that solution of the lime of the reefs occurs ;
that there is less food and oxygen in these still enclosed

84 AN INTRODUCTION TO OCEANOGRAPHY

waters and that abrasion and removal of disintegrated
material proceeds. Here again the results of recent
research cause difficulties: in such enclosed waters as
those inside a fringing reef or atoj'l deposition of lime,
rather than solution and removal, ought to occur.

Are Coral reefs formed on subsiding regions ? This is
the main problem and it is one for geological investigation.
Where such investigation has been most successful it seems
to be established that reef areas are also, in general, areas
of subsidence of the ocean floor.- In the West Indies, for
instance, there are satisfactory evidences of depression
or downward tilting of a large area of sea bottom in the
seaward direction : there are deeply indented bays and
valleys, steeply sloping sea bottoms, drowned escarpments
and submerged peat deposits. Most important of all,
there is geological evidence (from the West Indies and
Southern United States) that fossil reefs occur and that
these rest unconformahly on eroded submarine platforms.
There is also evidence that this is the case with living
reefs. Coral reefs appear, therefore, to become established
on submerged rock platforms. This is the case with
fringing and barrier reefs and also with atolls. There is
no evidence that these grow up round subsiding cones, but
rather that they are based on submarine flat summits.

The Glacial Control hypothesis of Coral Reefs. There
ought, therefore, to be a flat platform underneath reefs,
whether barriers, fringing reefs or atolls and this should
show signs of erosion. How to account for the formation
of these submerged platforms which existed prior to the
deposition of reefs upon them, is a problem that must be
solved before constructing a general theory. According
to Daly, Andrews, Humphreys and others the formation
of the platforms, and the subsequent growth of reefs
on them, are due to changes in water level rather than
actual movements of the earth crust relative to the earth
as a whole. During periods of extensive glaciation
enormous volumes of water have been withdrawn from

THE SEA BOTTOM 85

the ocean and deposited on the continental elevations as
the great ice-caps. This, and the consequent gravitative
effect of the increased continental masses, has been estim-
ated to lower the ocean level by as much as 36 fathoms. At
the same time the ocean temperature became lowered
and the conditions for coral growth may have become
unfavourable except in a narrow equatorial zone. Land
surfaces unprotected by growing reefs were thus exposed
to wave action and wide and fiat terraces were cut out
round continental lands and large islands, while the tops
of smaller islands were planed down to a general level. On
the melting of the ice-caps the sea temperature was
probably raised as the result of the reversal of operation
of whatever causes led to the glaciation and, at the same
time, the water level became raised by the melting of the
accumulated ice. Corals then began to build on the
edges of the terraces and flat submerged island summits.
In the former cases fringing and barrier reefs became
formed and in the latter, atolls. In the channels between
the fringes and barriers and the land, and in the spaces
encircled by the atolls, conditions for coral growth must
have been rather less favourable, nevertheless it goes
on there, and both channels and lagoons are assumed to
be filling up rather than being deepened by scouring and
solution.

Ecology of Coral organisms. The problem is, therefore,
one for geology on the one hand, and marine biology on
the other. Coral polyps are plankton-feeders, capturing
and ingesting'minute pelagic organisms in the usual way,
but they have also the holophytic mode of nutrition.
That is, they contain in their fleshy tissues green cells,
which are really symbiotic algae with which the polyps
become infected at certain stages in their development.
These symbiotic algae are able, by reason of the chlorophyll
that they contain, to take up carbon dioxide from its
solution in the sea water and then to synthesise
carbohydrate from this in essentially the same way as

86 AN INTRODUCTION TO OCEANOGRAPHY

the ordinary green plant does. Obviously this manner
of feeding must be reckoned with in any discussion as
to the conditions in which corals thrive best.
;- As to the general depths at which the polyps live,
reproduce and build : we must distinguish between the
deep and shallow species. The former may be found in
water of 50 fathoms or over, but the massive reef builders
live best in water of 27 fathoms or less. They ^prefer
water which is free from sediment although they have
limited powers of removing sediment from their exposed
parts. They can only settle on a hard sea bottom.
The most favourable temperature is about 18° C. and
the salinity that appears to be most suitable is about
,27 to 30 per mille (see Chap. VI). This is a fairly wide
range and must include a variety of hydrographic
conditions.

Ajrather brisk circulation of the sea water is favourable,
as in the case of all sessile, marine animals. A fairly
strong light is favourable and in the complete absence of
sunlight many, but not all, corals die. The importance
of light is obvious when we consider the role of the symbiotic
algae in the tissues of the polyps, for the chlorophyll of
these can only function in the presence of light above a
minimum intensity. At a depth of 50 fathoms the
lighting is favourable. The red is still present, though
weak, and the blue and ultraviolet constituents are still
powerful.

In the lagoons and shallow channels within fringing
reefs, the conditions must be rather less favourable than
on the seaward sides of the reefs. The water circulation
is less brisk so that nutritive matter may be more scanty
than on the outside margins. There must be a tendency
for the withdrawal of carbon dioxide from solution, not
only by the green plants present but also by the algal
cells of the coral polyps. There may be increased
evaporation where the water is still. There will be a
Mgher temperature.

THE SEA BOTTOM 87

Now the shallow water in such regions is usually
saturated with hme. This does not exist in the form of
calcium carbonate (which is sparingly soluble) but in the
form of the hydrogen calcium carbonate (which is much
more soluble). There is an equilibrium between theJIJOg in
the atmosphere and that in solution in the sea water.
The latter is not present so much in ordinary solution
as in combination with calcium carbonate to form the
acid-salt, CaH2(C0g)2, or some such compound. If the
sea takes up CO 2 from the atmosphere, there will be a
tendency for solution of ordinary limestone so as to form
more of the hydrogen calcium salt. If, on the other
hand, CO 2 is withdrawn from the water in any way some
of^tjie^bicarbonate will dissociate and normal calcium
carbonate will be precipitated. Also if the alkalinity
of the sea water (see p. 14^) is increased, the same
precipitation must occur. In the lagoons and shaUow
channels there may be enormous numbera^q^f denitrifying
bacteria. Certain ordinary bacteria can reduce proteid
matter (arising from decomposed animal and plant
substance) to nitrate and nitrite, and it is in such forms
that inorganic nitrogen usually exists in solution in sea
water. The denitrifying bacteria are able to reduce
nitrates to nitrites, nitrites to ammonia and ammonia to
elementary nitrogen which then returns to the atmosphere.
But there will be a tendency for the ammonia, as soon
as it is formed, to combine with CO 2 in solution in the
sea water. Then the calcium bicarbonate will dissociate
into COg and calcium carbonate (CaCOg) which will be
precipitated as minute balls of aragonite.

In lagoons and on the leeward sides of reefs there will
therefore, be a tendency to the deposition of calcium
carbonate precipitates. This will cause sediments and
lead to the result that reproduction and growth of the
reef-building corals will be less active there than on the
outside margins of channels and lagoons. The net
effect will be — not solution and deepening of the

88 AN INTRODUCTION TO OCEANOGRAPHY

lagoons and channels — but deposition of sediments and
filling up. "

The processes in operation are, however, very numerous
and complex, and far too little is known about them to
utilise in the elaboration of a satisfactory theory of coral
formation.

The Pelagic Deposits. — Categories of marine organisms.
From our present point of view it is convenient to arrange
all marine organisms — plants and animals — in three
groups : the Benthos, Nekton and Plankton. To the
Benthos belong all rooted, sessile, sedentary or semi-
sedentary organisms. These include the rooted shore
and sea bottom algae, the zoophytes, corals (massive
reef -building and solitary species), the alcyonarians,
sea-anemones, sponges, polyzoa, all starfishes, sea-urchins
and crinoids, nearly all the molluscan shellfish, most of
the larger Crustacea, all the barnacles and very many of
the marine worms. These animals live, either attached
to hard objects on the sea bottom, or burrowing on the
sand and mud, or moving freely on the sea bottom though
incapable of making long journeys. Many of them, as
we have seen, have calcareous or siliceous skeletons which
accumulate to form the neritic deposits.

To the Nekton belong the whales, other marine mammals,
all the adult fishes, some of the cephalopods (the larger
squids) and some of the Crustacea. They are animals that
have well-developed locomotory organs and they can
make long migrations. They mostly have calcareous
skeletons and they contribute both to the neritic and
pelagic deposits.

To the Plankton belong nearly all the unicellular plants
and animals (the Infusoria, Flagellate Protozoa, Peridinians,
Diatoms, etc.), all the smaller Crustacea (Copepods,
Ostracods, Schizopods, Amphipods, etc.), many marine
worms, some of the Coelenterates (the Medusae,
Siphonophores, etc.), many molluscs (the Heteropods,
Pteropods and some of the Cephalopods) and nearly all

THE SEA BOTTOM 89

the eggs and larvae of the Benthos and Nekton. These
organisms contribute to the pelagic sea bottom deposits.

Density of life in the Sea. The most prolific region is
that of the shallow water just beyond the littoral zone
and extending out to about 50 fathoms. The littoral
zone itself teems with life (sand and mud-living molluscs,
sea-weeds, barnacles, etc.). The littoral and shallow
water regions contain most of the algae and it
is exceptional for these to inhabit deeper water.
In the case of the greater Laminarians, however, marine
algae may grow up towards the surface from water that is
as deep as 50 fathoms. The characteristic region of
the Benthos is the sea between the foreshore and the
50 fathom contour line, but the general abundance of
life there depends largely on the nature of the sea bottom
and that of the adjacent land ; upon the sea temperature ;
upon the nature and origin of the neighbouring ocean
currents and tidal streams and upon the presence of fresh
water coming down from the land. This region is that
of the greatest abundance of fishes and also of the
planktonic organisms.

Beyond the 50 fathom contour-line demersal (or bottom-
living) algae rapidly diminish in abundance because of
the great diminution in the intensity of light which reaches
the sea bottom. Many benthonic and nektonic animals
are herbivorous and so depend directly upon plant organisms
for their nutrition. Carnivorous animals feed on other
animals, whether herbivores or carnivores, but obviously
all animal life must depend directly or indirectly on
plant substance. There is, then, a poverty of plant life
on the sea bottom which is deeper than 50 fathoms, and
this density of plant life becomes less as the water becomes
deeper and has a lower degree of illumination. Benthonic
animal life therefore decreases in much the same ratio.

The Abysmal life. In the greatest depths (or, in general,
everywhere outside the continental shelves) Lfe of all
kinds becomes very scanty on the sea bottom. There is

90 AN INTRODUCTION TO OCEANOGRAPHY

no plant life at all, because of the darkness. All the
principal groups of animal life are represented in the
abysmal fauna, but the prevalent species of fishes,
Crustacea, echinoderms, molluscs, worms, and sponges
are different from those that inhabit shallow water. So
far, however, as their evolution can be traced they are
closely related to the shallow- water species and have, in
all probability, come from the shore areas originally and
have become adapted to the abysmal conditions.

In all cases the mode of nutrition differs from that of
the shallow-water species. Abysmal animals subsist
largely by eating the bottom deposits ; at the very low
temperature (see p. 191) of the water on the oceanic
floors putrefaction becoming greatly retarded. Pelagic
animals living in the surface layers of the ocean die and
fall down to the sea bottom. There the fleshy parts
decompose slowly — so slowly that they form the only
renewable source of food for the abysmal animals. This
poverty of food material is the principal reason for
postulating a very low density of demersal life in the
great ocean.

Categories of planktonic life. A description of the
organisms that make up the oceanic plankton cannot be
given here and only the main groups that contribute
to the sea-bottom deposits can be mentioned. The
planktonic plants are, of course, holo phytic in their mode
of nutrition : they synthesise starch and sugar from the
CO 2 in solution in the water. Their nitrogen is obtained
from nitrates, nitrites and ammonia (also in solution).
Lime and silica and other inorganic constituents have
the same source. In all respects their condition of life
is pelagic, appertaining to oceanic conditions and not
depending, except remotely, on the land.

The Heteropods and Pteropods are small, pelagic molluscs
which drift about with oceanic and wind currents at the
surface of the sea. They have delicate calcareous shells.
They exi3t in enormous shoals and furnish one of the

THE SEA BOTTOM 91

principal sources of food of the whalebone whales. The
Foraminifera are protozoa, the protoplasmic cell bodies
of which are enclosed in thin calcareous " tests " or
chambered cells. They are holozoic in their mode of
nutrition, that is they ingest the organic substances of
other planktonic animals and plants, or they may perhaps
be saprozoic or saprophytic, that is, they may
absorb organic food materials from solution in the sea
water.

The Peridinians are unicellular organisms which are
animal-like in their structure. But they contain
chlorophyll and have the same mode of nutrition
(holophytic) as have the typical green plants. They
have cellulose tests impregnated with silica.

The Diatoms are unicellular plants which are, of course,
holophytic as regards their nutrition. They have siliceous
shells, or frustules.

The Copepods are minute Crustacea which are (as are
all the above groups of organisms) extraordinarily abundant
in the sea.

Density and Distribution of the Marine plankton. In
general all the planktonic groups are more abundant
in shallow water near the land than in the truly oceanic
regions far out at sea, and this is because of the more
abundant food materials brought down into the sea by
rivers. It is because of this wealth of planktonic life
in shallow water and the nutritive influence of the rivers,
that the ordinary life of the shallow seas is more abundant
than that of the deep oceans. The plankton is much
more abundant in cold polar, subpolar and temperate
sea zones than it is in the warmer tropical and subtropical
oceanic zone. This is because the denitrifying bacteria
are more abundant, and function more rapidly in warmer
than in colder seas. The effect of these organisms is to
break down nitrates, nitrites and ammonia into elementary
nitrogen. Thus these materials, which are the
indispensable sources of nutriment for all marine plants

92 AN INTRODUCTION TO OCEANOGRAPHY

and animals, are destroyed to a much greater extent in
the warmer than in the colder seas.

Finally the plankton inhabits the upper, illuminated
strata of the ocean.

The reader is now prepared to consider the various
deep sea oozes.

Pteropod Ooze. — This material is characterised by the
presence of the entire and broken shells of Pteropod
molluscs. Of all the deep-sea oozes it is the least abundant.
It exists in two or three small patches in the Western and
Central Pacific ; in a few larger (but still relatively small)
patches in the Atlantic, mainly on or near to the Central
Rise, and in the Mediterranean. Generally it is found in
relatively shallow water of about 400 to 1,500 fathoms in
depth far from the land, and therefore on low elevations
of the sea bottom. On these places the temperature
of the water is fairly high and has a restricted annual
range. The deposit is not an individualised one and
easily passes into some form of globigerina ooze. In
it about 35 species of Pteropods and 32 species of
Heteropods have been found. When dried it is a coarse
white powder containing about 90% of calcium carbonate.
See Fig. 19 for the appearance of this and other
deposits and Figs. 17 and 18 for their approximate regions
of distribution.

Globigerina Ooze. — This deposit is, as a rule, a dirty
white, rather coherent powder when dried. About a
half to two-thirds of its weight is due to calcium
carbonate, mainly present in the form of detritus. When
the finer materials are washed away a coarsely granular
residue is left, and this is the " ooze " that is usually
represented in figures (as, for instance, in our Fig. 19).
From one half to a third of the substance is insoluble
in acid and one or two per cent, of this consists of the
siliceous skeletons of radiolarians and diatoms.
Recognisable minerals, such as quartz particles, volcanic
glass, plaglioclase, augite, magnetite, mica, etc., are

THE SEA BOTTOM

93

\gg \60

Wo /so

Fig. 17. The deposits on the bottom of the Atlantic Opean. The area
of terrigenous deposits is stippled. Elsewhere G=globigerina
ooze; P=pteropod ooze ; R = red clay; Ra = radiolarian
ooze ; D= diatom ooze and C= coral formations.

94 AN INTRODUCTION TO OCEANOGRAPHY

usually present in very small quantity. The rest of the
material is fine amorphous clay.

Globigerina ooze has a very wide distribution, being
found in all the oceans but particularly in the Atlantic.
Pelagic Foraminifera are, in fact, of universal occurrence,
though they are least abundant, and the number of species
is also least, in polar waters. In equatorial zones about
a couple of dozen species have been recognised. The
ooze occurs all over the Atlantic, close up to the Eastern
and Western continental shelf deposits of mud. It
extends, in the Atlantic region to about 72° N. lat. and
to about 60° S. lat. a difference which is due to the
European Stream circulation and the consequent higher
sea temperature in the North. It occurs to a limited
extent in the deposits of the deeper part of the Norwegian
Sea, It is, however, nearly absent in the Antarctic south
of 50° to 60°. It occurs all over the Indian Ocean except
on the eastern margin opposite to the Asiatic- AustraHan
continental shelf. It is present in the Pacific but again
mainly on the eastern side. The southern limit of its
distribution in the Pacific is about 60° but in the Indian
Ocean it does not occur in quantity much below about
43° to 45°.

Its vertical range extends from about 400 to about 3,000
fathoms. In the Atlantic it is even found at 3,500 fathoms,
but 2,800 to 2,900 represents the extreme limits in
the Indian and Pacific Oceans. Typically, however,
Globigerina ooze is a deposit characteristic of sea bottoms
that lie between about 1,200 to 2,200 fathoms in depth.

Radiolarian Ooze. — This deposit is characterised by
the presence of the siliceous skeletons of the protozoan
organisms called Radiolarians. It is a dirty grey powder
drying to a very coherent clay-like substance. It always
contains much clay and the siliceous remains, even when
the diatom and sponge skeletons are considered, are not
so predominant as are the foraminiferal tests in Globigerina
ooze. It contains silica and calcium carbonate, but the

THE SEA BOTTOM

95

96 AN INTRODUCTION TO OCEANOGRAPHY

proportions vary remarkably in samples taken from
various ocean bottoms. No typical Radiolarian oozes
occur in the Atlantic and there is only a small patch on
the eastern side of the Indian Ocean. Even in the
Pacific, where it is most abundant, it only occurs centrally
and as a belt on the eastern side between about 5° and 15°
N. lat.

It is not known to occur in sea bottoms of less than
about 2,000 fathoms in depth and it may go down to 5,000
fathoms.

Diatom Ooze. — The characteristic of Diatom ooze
is the presence, in great abundance, of the siliceous
frustules of pelagic diatoms. It is a fine, white, coherent
powder when dried. With the diatom remains are
numerous sponge spicules and some radiolarian skeletons
and the inevitable, amorphous, clayey constituents.

It does not occur at all in the Atlantic and Indian
Oceans, nor in the North Polar basin. Along the extreme
north of the Pacific, between latitude about 40° to about
55°, there is a broad band of diatom ooze sloping northerly
from West to East and just outside the zone of continental
muds. It is highly characteristic of the Antarctic Ocean,
where a broad band encircles the earth. This band has
a maximum breadth of about 20° in longitude 20° E. and
it is estimated to have an area of about 10 J millions of
square miles.

The extreme vertical range of diatom ooze is about
600 to 4,000 fathoms, the lower limit being found in the
Antarctic and the higher one in the North Pacific. The
usual range of depths is 600 — 2,000 fathoms.

Red Clay. — Red Clay is, when dried, a firm coherent
powder which is usually red-brown in the Atlantic and
chocolate-brown in the Pacific. It is, chemically, a
hvdrated aluminium silicate and its origin is mainly the
decomposition of pumice and other volcanic minerals.
Lime is always very scarce, and in red clays derived from
the deepest sea bottoms lime may be totally absent.

Nerific -. Shelly
and Null!fyore^ra\/el

W^-W%

and GlohiCerina cxQR

1^

Diatom oose

Man0onesp nodule

Neritic:
Nu/lipo re gravel

Pelagic:
Globi^erina ooje

Farbone or w/ja le

Shark's tooth

Aferitic -
Coral Sand

'Pelagic:
J^adiolarian ooje

Cosmic sl:>herulp

Fig. 19. Typical deep-sea and shallow-water deposits. The shelly and
nulliijore gravels are reduced ; the coral sand, pteropod and
globigerina oozes are moderately magnified ; the radiolarian
and diatom oozes are highly magnified. The cosmic spherule
is highly magnified while the manganese nodule, the whale
earbone and the shark's tooth are slightly reduced.

THE SEA BOTTOM 97

Red clay is essentially a residue — the end product of a
series of solutions and decompositions.

It is characteristic in that it contains the rare sea bottom
materials — the earbones of whales, the teeth of sharks,
synthetic minerals like phillipsite, manganese-iron nodules
and the very peculiar cosmic spherules. The earbones of
whales, like the teeth of sharks, consist of very hard,
highly resistant materials and they are " common " in
red clay (relatively to the other deep sea oozes) because
the rate of formation of red clay is so much slower than that
of any other deposit. This is also the case with regard
to the cosmic spherules : these can hardly be found in
any of the other oozes but may he expected in the red
clay. Phillipsite and the other synthetic products (the
peculiar manganese nodules, for instance) are probably
to be associated with the long series of decompositions
of which red clay itself is a terminus.

It is the most widely distributed of all the deep sea
bottom deposits, being found in all the oceans. It occurs
on both sides of the Atlantic between latitudes of 40° N.
and S. It is widely distributed over the Pacific and
Indian Oceans but mainly on the eastern sides. Something
like it occurs in the North Polar Basin but it is shut off
from the Antarctic Continental area by the broad band
of Diatom ooze which extends close up to the outer
margin of the continental terrigenous deposits. It occurs,
in general, everywhere at depths greater than about 2,700
fathoms and it is present at the bottoms of the "deeps."
To this statement there are, of course, exceptions and it
is often difficult to distinguish between a Radiolarian
ooze and a Red clay.

The Pelagic Oozes in general. — The two sketch charts
Figs. 17 and 18 give only the very broadest idea of the
distribution of the deep sea oozes. Of necessity the latter
are regarded as being quite distinct from each other, but
this is not really the case. Pteropod ooze passes gradually
into the surrounding Globigerina ooze and samples may

98 AN INTRODUCTION TO OCEANOGRAPHY

he examined which it is impossible to place with certainty
in either category. Radiolarian ooze shades off into
Red Clay and so on. The hard contours drawn in Figs.
17 and 18 are therefore quite artificial and are intended to
represent only the general distributional features.

The vertical range is equally vague. In general terms
the serial arrangement is Pteropod ooze (on the shallower
bottoms), Diatom ooze, Globigerina ooze, Radiolarian ooze
and Red Clay on the deepest bottoms). To some extent the
vertical' distribution depends on solution of the materials.
If the ratios to each other of Pteropods, Foraminfera,
Diatoms, Radiolarian and volcanic debris at the superficial
strata of the ocean were everywhere the same, then a
sorting out of these materials would occur as they sank
slowly to the bottom. The fragile shells of the Pteropods
would dissolve first of all, then those of the Foraminifera,
then the Diatom frustules and lastly the Radiolarian
skeletons. That means that lime becomes the less abundant
the deeper is the sea bottom, for its solution is apparently
accelerated by increasing water pressure. At a certain
rough limit of depth, then, no lime will be found and this
is actually the case. Then the siliceous skeletons of the
Radiolaria, Diatoms and Sponge also dissolve with
great depth and so they disappear completely, as
organically formed structures, in the very deep ocean
basins. In the Red Clay we have the insoluble residues
of the calcareous and siliceous skeletons, as well as the
end-products of the volcanic materials and the synthetic
products for which a low temperature, a high pressure
and, no doubt, some other conditions are essential.

The distribution at the surface of the ocean, of
Pteropods, Foraminifera, Diatoms, Radiolaria, etc., is
not, however, everywhere the same for the ecological con-
ditions there vary from region to region. So also the
differences that we know do obtain in the ocean with respect
to the vertical and horizontal water movements affect
the paths along which these organic remains fall towards

THE SEA BOTTOM 99

the bottom. The result is irregularity of distribution
such as is not representable on small-scale charts. Also
the observations at our disposal are far too few to
justify us in drawing the contours limiting the regions
of deposition in any other way than very roughly.

Finally we may again remind the reader that the
figures of deep sea oozes given in the books are usually
representations of the characteristic organic remains
rather than of the oozes themselves as they come from
the sounding tubes. What one sees in the latter is a mass
of general detritus resulting from the attrition of the
characteristic shells, etc. This contains fragmented shells,
tests, frustules, spicules, etc., some of them entire and it
is from the presence of these that the deposit is identified.

CHAPTER V
THE OCEANIC MARGINS

On any ordinary small-scale map there appears to be
a very distinct boundary line between the land and the
sea. On a large-scale chart, however, this line of demarca-
tion may disappear completely, revealing a region of
transition which is sea at one time and land at another.

On p. 101 we have three representations of the same
area, Morecambe Bay, on the West Coast of England, but on
different scales. (I) is a reproduction in line of the ordinary
small-scale map and we see that a quite distinct boundary
line apparently separates the water from the adjacent
land area. The Bay is here depicted as a sea-area.
(II), however, represents a small part of the same region
on a mu6h bigger scale and the markings are those that
we find on an Admiralty Chart. The approximate
boundary is the line which is marked " L.W.O.S.," that
is, low water of ordinary spring tides, and this is the
average limit of the sea during one day or so in each fort-
night. A little distance landward of this line would be
another (not usually marked on the Admiralty Charts)
giving the limit, low water of ordinary neap tides, and
this would be the average limit of the sea on one day
(or thereabout) every fortnight and intermediate between
the days of highest spring tides (see p. 101).

Near the arbitrary line representing the " shore " on
the Chart ought to be another pair of boundary lines —
the high water marks of ordinary spring and neap tides.
The water of the sea, then, may be anywhere between
the high and low water tide marks in normal conditions.
At certain times, twice or so during the year, however,
the high water marks are higher than usual while the

100

THE OCEANIC MARGINS

101

low water marks are lower than usual. This occurs
regularly during the periods of " equinoctial tides," but
it also occurs exceptionally at other times when unusual
gales of wind may raise or lower the expected heights
of the tides.

Fig. 20. I is a small-scale map of the West Coast of England ; II is a
sketch copy of the Admiralty chart representing the part
of I marked by circle 2 ; III is also a copy of the Admiralty
large-scale chart marked in I by the circle 1.

In addition to this broader region of shifting of the
water level, the coast line may still be more indefinite

102 AN INTRODUCTION TO OCEANOGRAPHY

Fig. 20 (III) represents a " salt-marsh " from the same
general area. We see here that the shore is cut up in
a most complex manner by a multitude of small, sinuous
channels up which ordinary flood tides penetrate. At
very high spring tides most of these channels are obliterated
and the sea covers much of the area represented by them
and the miniature islands, capes, straits, etc., of the
vegetation-covered part of the marsh.

This indefiniteness of the land boundary is characteristic
of a large part of the world : wherever there is a significant
rise and fall of the tides. In some parts (Liverpool and
in the Bristol Channel, for instance) this extreme rise and
fall may be about 30 to 40 feet. In other places (as in the
Firth of Clyde) the rise and fall may not exceed 10 feet.
In the Mediterranean it is only two or three feet.

Further, a very little observation will show that on
many parts of the coast the average high water tide
markings are not constant — even in the space of time
throughout which one person may observe them. That
is, there is significant erosion of the coast line in some
places or filling up by the deposition of material elsewhere.
Such changes are quite noticeable when maps, made at
various times during a century or two, are compared :
they are due to erosion, or deposition of land materials.
Channels, ports and river courses may become silted up,
or new channels may be eroded out. Such changes may
be slow, progressive ones, or they may be catastrophic
(when, for instance, they are caused by very exceptional
tides, or floods in rivers entering estuaries). They may
be variable, reversing themselves because of conditions
that are exceedingly difficult to trace.

Finally, when we consider the changes of land and sea
that occur in those periods of time which we call geological
ones we see that they occur on a great scale, altering
the outlines of the continents themselves. We have now
to do with earth movements of enormous magnitude
and acting very slowly but with huge effects.

THE OCEANIC MARGINS 103

The Foreshore. — On most parts of the oceanic margins
there is, then, a zone situated between the tide-marks.
The latter are conventional, being taken to be the
junctions of land and sea at the moments of mean high and
mean low water at intervals of approximately 15 days.
The levels to which the tides rise and fall at these fortnightly
intervals are variable and so the marks " L.W.O.S.T." on
the Admiralty charts represent the average water levels.
(To this matter we return in Chapter VIII). The zone
between high and low water marks is called the Foreshore,
or sometimes the Strand. Below it, in this country, is
the legally conventional " sea." Above the average level
to which the tide rises once a fortnight, at springs, is the
legally conventional " land." The foreshore is a kind
of no-man's land regarded as the property of the Crown,
unless there are immemorial, prescriptive, or manorial
titles to its partial monopoly for some purpose or another.

The area of the foreshore depends not only on the
vertical range of the tides but also on the shore gradients.
The vertical range of the tides can only be given an average
value for it changes from day to day. The area of fore-
shore, therefore, undergoes a continuous change : (1) daily,
being greatest at low water ; (2) fortnightly, being greater
still at low water of spring tides ; and (3) six-monthly, being
greatest at low water of the highest equinoctial spring
tides. It is least at the corresponding high waters of the
above tidal conditions. It depends also on the shore
gradients, vanishing altogether on those coasts formed
by rocky cliffs. There the vertical change of tide is
marked in various ways on the face of the cliff but there
may be no change at all in the sea area of such coasts.
On those coasts, then, where the land is flat and low and
easily eroded, and where there is a large tidal range, the
extent of the foreshore may be very great. In Morecambe
Bay it amounts to about 100 square miles.

High water tide-marks are nearly always recognisable
on such flat coasts by lines of debris (sea-weeds, etc.) on

104 AN INTRODUCTION TO OCEANOGRAPHY

the foreshore. On falling tides, that is, during the few
days following the highest springs, several such temporary
tide-marks can be seen, each showing the level to which
the sea rose on a particular day. The levels of the highest
tides of all are usually indicated by some fairly permanent
markings, such as low sand-hills, deposits of shingle, low
earthy cliffs, etc.

Low water tide-marks are not so easily recognised, though
they can often be indicated by the presence of typical
animals and plants. Thus the large Laminarian sea-weeds
mark the approximate levels (on suitable shores, of course)
of the lowest tides.

The Foreshore Gradients. — On a rocky coast the land
surface immediately adjacent to the sea is usually a low
plateau bounded seaward by a cliff. The latter may be
the typical, precipitous wall of rock, usually inclined
backward at a small angle to the perpendicular ;
rarely overhanging or undercut but often with caves due
to wave action. At its base, and exposed by low tides,
there is often a rocky shelf (though as often, perhaps,
the cliff may be continued below the sea level), or there
may be a terrace of shingle or boulders or gravel. The
cliff may be of softer material than rock : boulder clay
deposits, for instance, and then there is often a terrace
at its foot, partly above and partly below high water
level. This terrace corresponds to the talus at the foot
of an escarpment on the land, but here its materials
have been disintegrated and spread out horizontally by
waves and tidal streams. We may find, then, on coasts
of these, or similar materials a rather steep, or even an
approximately vertical slope from the land plateau to
the upper limit of the foreshore.

Even where the land is flat and low and composed of
easily eroded materials, something similar to this rather
steep gradient may exist. The coastal substance is
disintegrated by wave action, but the stones, shingle, etc.,
which are separated out tend to become piled up on the

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Upper Figure— A vertical section of the Lancashire coast near Formby. The

dark part represents the foreshore and adjacent land. The shore is a very flat

one, composed of drift materials. There are low irregular sandhills.

Lower Figure— A vertical section passing out nearly due west from Bradda

Head m Isle of Man. The dark part represents the cliff and sea-bottom. The

coast IS rocky, composed of slate.

The scale is the same in both Figures and is identical for horizontal and vertical

dimensions. It is 1 millimetre for every 10 feet.

106 AN INTRODUCTION TO OCEANOGRAPHY

upper margin of the foreshore as a low rise, rather than
a cliff. The rest of the shore materials become abraded ;
muds are removed by the tidal streams and deposited
elsewhere, and quartz sand is spread out on the lower
levels of the foreshore or on the sea bottom adjacent to
the latter. But even when the land is exceedingly easily
eroded and stones and boulders resulting therefrom are
rare, the loose sand at the upper margin of the foreshore
becomes blown by winds on to the land and then compacted
together here and there by vegetation. Such wind-blown
sand may even accumulate behind a foreshore consisting
of shingle or boulders, the latter being left behind after
the separation of the sand by wind action. Thus there
arise the low sand-hills so characteristic of many coasts.
These give to the shore-line a fairly large gradient down
to the foreshore.

The latter has, itself, a gradient which is always slight
but is variable with a host of conditions. It will depend,
for instance, on the nature of the land, the tidal range,
the velocity of the tidal streams, the amount of shelter
from heavy seas, etc. We must, however, remember that
it is an average gradient. There are " features " on the
foreshore : shallow channels and gutters through which
land water drains ; " lakes " which depend in some way
upon scouring eddies ; patches of raised stones or boulders
covered by sea- weed. There are soft parts or " quick-
sands " : these are generally found near to the land and
not often far down the foreshore, and they appear to
depend on land water oozing up through the sand on to
the surface of the latter. Thus there is considerable
diversity of feature on the foreshore as we see from Fig. 20,
or almost any large-scale Admiralty chart of a coastal
area where there is a considerable rise and fall of the tide.

The Shallow-Water Bottom Gradient. — At the lower
margin of the foreshore the gradient again increases
slightly : that is, the sea usually deepens rather rapidly
at first and then more slowly. Then there is the gradient

THE OCEANIC MARGINS 107

of the upper part of the continental shelf : this upper
part may be taken as the sea bottom where the water
is less than 100 fathoms in depth. Beyond that it is
usually said that the gradient again increases, though
there are remarkably few good series of soundings that
illustrate this point. We must note particularly now that
the gradient of the upper, shallow part of the continental
shelf must also be thought about as an average one. In
reality there is, here again, very considerable diversity
of bottom relief. Particularly in shallow bays and estu-
aries do we find this variety : sinuous channels, sand-banks,
bars, ridges, etc. It is all on a very small scale when
compared with those features of ocean bottom which we
considered in Chapter III, but it is interesting, and of
immense practical importance for navigation and fishery.
It may be illustrated by almost any Admiralty chart of
the sea in the immediate neighbourhood of an important
harbour. There the soundings are usually very numerous
so that the natural detail is all represented.

This variety of feature that the shallow-water seas
exhibit contrasts strongly with the bottom relief of the
ocean basins. But we must always remember that the
soundings on the latter regions are very few indeed when
compared with those that have been made on the sea
bottom within the 100 fathom line. Even, however, when
we bear this in mind, and entertain the possibility that
more detailed examination of the oceanic abysses might
reveal a greater variety than is known so far, it is, neverthe-
less, probable that the latter regions are monotonous in
their flatness.

The Marginal Seas. — All the region of sea bottom within
the 1,000-fathom, or the 2,000-metre contour lines (for the
limit of depth cannot be precisely stated) we are regarding
as the continental shelf. This corresponds roughly with the
zone of continental sea-bottom deposits and there are
structural reasons why we regard it as belonging rather to
the areas of earth-elevation than to those of earth-depres-

108 AN INTRODUCTION TO OCEANOGRAPHY

sion on the big scale. On this continental shelf margin,
then, are situated a number of smaller seas. Some of these
are partially land-encircled ; others are partially separated
from the oceanic basins by chains of continental islands
while others again are depressions of the shelf itself, areas
of the latter where the depths exceed those roughly limiting
ones that we have mentioned. These water areas have
been called the Marginal Seas and there are several
categories of them.

The Epeiric Seas. These are the smaller sea-areas
that have, as a rule, very limited depths (not much
exceeding 100 fathoms and usually a great deal less) and
which present the characters of great gulfs or bays with
relatively narrow communications with the ocean. Ex-
amples from the Atlantic region are Hudson's Bay, the
Gulf of St. Lawrence, Baffins Bay, the Baltic, the North
Sea and the White Sea. It is difficult to differentiate
them always from the great bays or straits such as, for
instances, the Bay of Biscay or the Irish Sea, and some
of their individuality must be traced to economic or
historical reasons. They are mostly very shallow sea-
areas with depths of 50 to 100 fathoms. They are
also characterised by a low salinity and large temperature
ranges, on the physical side, and with marked faunal
features, from the biological side. Their presence is
rather characteristic of the North Atlantic region.

The Epi-Continental Seas. Just as the Epeiric seas
characterise the Atlantic margins so do the Epi-continental
seas seem to belong typically to the Pacific. They are
the Behring Sea, partially enclosed by the Aleutian
Islands and the Kamschatka Peninsula ; the Sea of
Okhotsk, between Siberia, Kamschatka and Sakhalin
and bounded towards the Pacific by the Kurile Islands;
the Sea of Japan ; the Yellow Sea, which is a great
gulf with a shoaling of the bottom emerging to form
the arc of the Lu Chu Islands, across its opening ; the
China Sea, partially enclosed towards the Pacific by

THE OCEANIC MARGINS 109

the Philippines and Formosa ; the Celebes Sea, between
Borneo, Celebes and the Philippines, and the Banda Sea,
between Celebes, New Guinea and Australia (See Figs.
58-9, Chap. X). Fairly well-marked characters distinguish
the Epi-continental Seas : depths which, in places, exceed
1,000 fathoms and boundaries towards the ocean formed
by chains of islands arranged in arcs with their con-
vexities outwards from the continental land. Doubtless
there are physical and biological characters, but as to
these much detail is not yet available.

The Epi-continental seas are rather characteristic of
the Western sides of the Pacific and Atlantic Oceans.
On the Eastern side of the former there is no example
except the untypical one of the Gulf of California. This
Eastern side of the Pacific, has, we shall see, an altogether
different structure. Nor is there anything in the Indian
Ocean quite like them, and the only examples in the Atlantic
are the Caribbean Sea and the Gulf of Mexico. The
Norwegian Basin may, perhaps, be regarded as such a sea
bounded to the south by Iceland, the Faeroes and Northern
British Islands, but the pecuUar arcuate boundary with
its concavity facing continental land is wanting in this
case.

The Mediterraneans and Relict Seas. — These may be
mentioned here although they do not belong to the general
category of marginal seas.

The Mediterraneans are the Roman Mediterranean and
the North Polar Basin. The Classical Mediterranean-
Black Sea region is almost completely enclosed, for the
communication through the Straits of Gibraltar is very
restricted and permits of only a very peculiar water cir-
culation. The Caribbean and Gulf of Mexico basins have
been called a Mediterranean Sea but, obviously, they are
separated from the Atlantic in the same way as are the
Epi-continental Pacific Seas — ^that is by an arc of numerous
islands, the Greater and Lesser Antilles. The North
Polar Mediterranean has been called the " Arctic Ocean "

110 AN INTRODUCTION TO OCEANOGRAPHY

for so long that it is difficult to think about it by another
name : it is, however, separated from the Pacific in the
same way as the Roman Mediterranean is separated from
the Atlantic and it is separated from the Atlantic by an
extensive region of continental shelf. The Mediterraneans
have depths that belong to the order of the truly oceanic
abysses and their bottom deposits may have the pelagic
character. Further, there are indications that they have
belonged in the past, to the oceanic earth-depression
regions rather than to those of elevation — which latter
character distinguishes the Epeiric and Epi-continental
Seas. The latter we consider to be of the same nature
as the seas that have transgressed on the land during the
geological periods : their loci have alternately been land
and sea. The Mediterraneans, on the other hand, share
with the great oceans the character of relative permanence.

The Relict Seas are vestigial sea-areas that have lost
their connection with the ocean and have become land-
locked, or nearly so. The Epeirics, Epi-continentals and
Mediterraneans which have been mentioned are, it should
be noted, in working communication with the ocean
and have some importance when the general circulation
, of the latter is concerned : Relicts have no such influence
on the water circulation of the ocean. They are the
remains of seas that have transgressed upon the continental
land during periods of depression of the latter and have
then become isolated from the ocean as the result of
subsequent continental elevation.

They are the Black Sea, Caspian, Sea of Aral, Lake
Ontario, Lake Champlain, probably Lake Tanganyika,
Lough Neagh (in Ireland) and doubtless many other
smaller lake areas. In all these cases the water is fresh
because the salt has been removed by drainage. In the
Northern part of the Baltic, for instance, the water has
a very low salinity : there is little evaporation, much
inflow from rivers and a deficient inflow from the North
Sea area. Isolation of the Baltic by elevation across the

THE OCEANIC MARGINS 111

Sound and Belts would therefore leave a relict sea that
would speedily become fresh. Salt water land-locked
areas — as for instance, the American Great Salt Lake,
the Bitter Lake in the Suez Canal, the Dead Sea, etc., are
not necessarily relicts because of their high salinity but
simply terminal basins suffering removal of water by
evaporation and continuous addition of saline materials
in solution.

Indications of the original nature of the Relicts are the
geological history of their general land areas, as in the
case of the Black Sea ; elevated beaches with marine
fossils (Lake Champlain) ; the rehct marine fauna (squids
in Lake Onondaga, which was a tributary to Ontario) ;
the fishes of the Caspian (sturgeon, salmon, herring) ;
sponges in Lake Tanganyika, etc.

The Marginal Sea-bottom Deposits. — In general these
are the sea-bottom deposits of the Continental shelf, that
is, Murray's terrigenous deposits and the shallow-water
neritic materials. This region of sea-bottom is that on
which the greater mass of the sedimentary rocks of the
earth's crust has been deposited, and on which future
sedimentaries are, of course*, now forming. The general
limits of depth of the marginal deposits we may take
to be those of the Continental shelf, but this is necessarily
a rough approximation. Muds of land origin may be
carried much further out to sea before they subside to
the bottom : thus blue muds may be found at 2,000-
3,000 fathoms. In general, however, the two limits —
that of the Continental shelf in about 1,000 fathoms,
and that of the oceanic distribution of continental sea-
bottom deposits will be much the same.

The Foreshore Terrigenous Deposits. For the most
part the material lying on the foreshore is quartz sand,
but even a very limited degree of observation will show
many other kinds of deposit. In the neighbourhood of
rock shores the intertidal zone may be clean, or nearly
clean rock covered with weed ; off boulder-clay cliffs

112 AN INTRODUCTION TO OCEANOGRAPHY

there will be rounded water-worn stones of all sizes
passing into quartz sand ; elsewhere there may be sand
and mud, or mud alone. The conditions are obviously
the nature of the adjacent land, the strength and direction
of the tidal streams, the volumes of rivers and streams
entering the sea, the configuration of the coast line — as
in bays of difTerent forms, estuaries, straits, etc. Obviously
the variety of these conditions will be very great.

The Foreshore Neritic Deposits. Quite different materials
may, however, be seen in the deposits due to molluscan
shells, nuUipores, the sandy tubes of marine worms ; remains
of polyzoa, etc. '{See Fig. 19). Coral materials driven on
to the intertidal zone, or beyond this, by wave action we
have already considered. Shelly gravels and sands are very
common, but such accumulations on the foreshore, or on the
adjacent shallow sea bottom are, as a rule, not the results
of the growth of mollusca in situ. Wave action, assisted
by peculiarities in the local tidal streams and in the form
of the coast line, drive the dead shells of molluscs living
beyond the low water marks on to the foreshore (as one
sees by examining the debris of the tide marks). Even
in a clean sandy foreshore thickly populated by cockles
we do not usually find many dead shells in the sand. As
a rule foreshore shell deposits, then, have originated
elsewhere in the neighbourhood of the locality where they
are seen. On the other hand, highly characteristic fore-
shore neritic deposits formed in situ are the sandy tubes
of the worm Sabellaria : here we have true " Annelid
Reefs " of quite a massive kind.

Shallow Water Neritic Deposits. It would more com-
monly be the case that the extensive deposits of molluscan
shells found on sea bottoms less than 50 fathoms in depth
had originated in situ from animals living and dying there.
Possibly examination of the shell fragments from a
neritic deposit would indicate whether the accumulation
had been made on the region inhabited by the living
animals or had been transported on the sea bottom, or

THE OCEANIC MARGINS 113

driven by wave action on to the foreshore. Evidence
of wear by rubbing among stones and sand on the beach,
or rolling gently on the sea bottom, the marks of corrosion
by solution, or those of boring by marine molluscs or
sponges might indicate deposition in situ. But there
is need for prolonged and careful observations on these
matters.

Neritic shallow- water deposits will, of course, vary in
nature according to the local faunas. Thus the gravels
and sands composed characteristically of the tests of
Foraminifera {Orbulites) or of calcareous algae {Halimeda^
for instance) are typical of some tropical sea bottoms — as
off Ceylon. Here too, we may mention such limy deposits
as may be found on the continental shelf in the West
Indian Seas. These may be very largely the result of
precipitation of calcium carbonate from solution in the
sea water, and they are neritic only in the sense that they
are, to a great extent, due to the action of certain species
of marine bacteria.

The deposits of the foreshore and the shallow water
immediately adjacent to this are, then, exceedingly varied
— far more so than those laid down in the oceanic abysses.
But terrigenous materials of continental origin must
always predominate except, of course, on those marginal
areas (as on the North-East coast of Australia) where
coral formations on the great scale occur. By studying
the ways in which shallow water neritic deposits form and
are transported, much useful data might become available
for elucidating the conditions in which many marine
sedimentary rocks have been formed.

The Continental Margins. — We may now neglect the
great variety of feature which is exhibited by the
bottom deposits in the transitional zone between the
continental elevations and the oceanic depressions. On
a chart showing whole oceans and continents this
transitional zone is, after all, a very narrow one, and what
one looks at are the great continental and oceanic regions

114 AN INTRODUCTION TO OCEANOGRAPHY

of the earth's surface. Something, then, must be said
as to the general nature of this continental-oceanic
margin on the great scale and from the point of view
taken in Chapter II — the origin of the oceanic abysses
and their assumed permanence as great earth features.
It is assumed that the general positions on the earth of
the oceanic depressions and continental elevations were
marked out quite early in the history of the planet and
that the continental margins represent zones of weakness
in the superficial earth layers. These margins have
always been, and still remain zones of weakness because
movements of the earth body as a whole still continue.
There are causes that lead to instability of these surface
layers : strains set up by the tide-generating force in an
earth body which is not perfectly rigid ; loss of heat which
has been locally generated ; erosion of the continental
areas ; deposition on the sea bottom and possibly other
factors. Therefore,* there is instability and this has led,
and still leads to alterations in the shapes of the continents
as marked out by the land-sea margin, by the surface
relief of the land and by its elevation above sea level.
This means that extensive areas of the submarine contin-
ental shelf have, at various times in the past, been dry land,
while far more extensive areas of the present continental
regions have been shallow sea bottoms. It does not
follow, however, that correspondingly large areas of the
present oceanic regions have ever been dry land. There
is geological evidence that the greater part of the North
American Continent, for instance, has been submerged
beneath shallow seas, but evidence of the uplifting of a
truly oceanic basin is not nearly so strong.

The physical condition of the interior of the Earth. —
As a whole the .evidence indicates that the interior of
the earth, generally, is not highly heated. There are
local regions of high temperature but these are probably
superficial in their situation and the heat is locally
generated. The temperature gradients are very irregular

THE OCEANIC MARGINS 115

and the observations apply only to a very restricted
depth. The conclusion that has been made from these
observations : that a high temperature exists towards
the earth's centre is unjustifiable because it involves
extrapolation from the data of the surface temperature
gradients and this extrapolation itself involves an
hypothesis — that the rate of increase of temperature
continues. The main reason for the assumption of a
very high internal temperature is, of course, the postulation
of a gaseous-molten origin for the earth, and this we have
no good reasons for accepting. We take it, then, that
the whole interior mass of the earth body, down to the
centre, is not, as a whole, highly heated.

The Density of the Earth. The mean density of the earth
as a whole is about 5' 6 times that of water — this result
depends on gravity measurements. But the most superfi-
cial layer of earth substance — the atmosphere — is very
much less in density than water. The next layer — the ocean
(or hydrosphere) — is very little greater in density than water
(the ratio is about r03 to 1*00). Then comes the rocky
part of the earth (the lithosphere) and the density of this
is about 2*7 times that of water. Since the density of the
earth as a whole is about 5'6, it follows that the nuclear part
(the centrosphere) must have a density that is greater
than the mean : it has been taken to be anything from
about 7 to 11.

The Rigidity of the Earth. The rigidity is the measure
of the resistance which the earth body opposes to external
causes which would lead to its deformation. Obviously the
atmosphere opposes very little such resistance since it is
greatly deformed (in changes of barometric pressure and
winds) by relatively small causes. Nor is the ocean rigid
in a marked degree, for tidal deformations are conspicuous
features in its form. The lithosphere is obviously deformed
in the processes by which great masses of stratified rocks
are crumpled, or even folded into mountain ranges. There
remains the (presumably metallic) centrosphere, and this

116 AN INTRODUCTION TO OCEANOGRAPHY

is also susceptible of deformation but to a degree which is
almost infinitesimal when compared with that of the
lithosphere — to say nothing of the hydrosphere.

That the earth body (that is the metallic centrosphere
as distinguished from its envelopes, the rocky lithosphere,
the watery hydrosphere and the gaseous atmosphere) is
actually deformed is proved by (1) the wandering of the
axis of rotation. The poles trace roughly circular paths
on the earth's surface, with a period of about 14 months
and a range of about 10 metres. If the earth body were
perfectly rigid the period can be shown to be 10 months.
(2) The experiments of Hecker, OrlofE and Michelson.
Hecker and OrlofE used horizontal pendulums, and Michelson
made use of horizontal water columns 500 feet long and
sunk to a depth of 6 feet in the ground. The descriptions
and interpretations of these experiments are difficult but
their results are clear. The earth body does respond by
changes in shape to the tide-generating force but to an
excessively small degree. It responds to a significantly
greater extent in a North-South direction than in an
East- West one. But the amount of deformation produced
by the tide-generating force is no greater than if the earth
were composed of tempered steel.

The Changes towards the Earth^s Centre. The outer
envelope — the atmosphere — is highly compressible. The
hydrosphere is very much less compressible (see p. 166)
but is viscous in that it flows. The character of the
lithosphere is a certain " viscosity " — not, however, quite
the same thing as physical viscosity. There is " flowage,"
or " rock-creeping " rather than semi-liquid " flow."
Compressibility and viscosity are infinitesimal in the
centrosphere but this is, to some extent, elastic as is
shown by the propagation of earthquake waves. The
latter can be thought about as set up in the earth in much
the same way as vibrations would be set up in, say, an
anvil when the latter is given a sharp tap with a hammer.
When an earthquake shock is made, three series of waves

THE OCEANIC MARGINS 117

are established and are propagated outward through the
earth body.

(1) The first precursors, or first preliminary tremors :
these are long waves of compression (longitudinal, like
sound waves) and they travel through the earth in paths
which are chords to the surface. (2) The second precur-
sors, or 2nd preliminary tremors : these also travel through
the earth body in chordal or curved paths. They are
transverse waves and they cannot exist in a liquid medium.
(3) The main waves, that is the destructive ones. These
are rocking waves that travel on the earth's surface.

The velocity of the preliminary waves is greater than
that of the superficial ones. Since they take approximately
the shortest paths through the earth body, they may pass
through any parts from the surface layer down to the
centre. Their paths can be deduced from a knowledge
of their points of origin and receipt, because they go
straight, or nearly so, through the earth from the one
point to the other. Now the deeper is their path (down
to a certain distance from the centre) the greater is their
velocity. Down to about 2,450 kilometres from the
surface the velocity increases then it decreases rather
suddenly at a depth of about 2,900 kilometres and finally
increases again towards the centre. The velocity depends
on the ratio of elasticity of earth material to density, so
that either or both of these properties change down to
a certain depth, then remam approximately constant
and finally change again towards the centre. But every-
where below about 1,200 kilometres the ratio is high
relatively to its value above that level.

The Theory of Isostasy. — The intensity of gravity is not
the same everywhere on the surface of the earth, and this
is due to the spheroidal shape of the latter : a point near
the pole is about 13| miles nearer to the earth's centre
than is a point on the equator. The gravitational field
is therefore more intense at the latter point. This
variability in the intensity of gravity is measured, on the

118 AN INTRODUCTION TO OCEANOGRAPHY

land, by observing the times of swing of a pendulum of
invariable length and, on the sea, by measuring the
atmospheric pressure by a barometer and, at the same
time, finding the temperature at which water boils. The
height of the barometer depends not only on the air
pressure but also on the intensity of gravity, and the boil-
ing point depends only on the air pressure. The two
observations, taken simultaneously, therefore give us
the intensity of gravity, which depends on the earth's
mass beneath the point where the observations are made.
This value depends again, on the density of the subjacent
earth substance and on its quantity, that is the distance
from the centre of earth to the place where the experiments
are made.

Over the deep oceans the intensity of gravity is every-
where the same though the depth may vary, say between
three miles and two miles. In the latter case we have
an earth column which is greater in depth by one mile
and a water column which is less in depth by one mile,
but the intensity of gravity is the same as at the place
where the earth column is one mile less in depth, and the
water column one mile greater. The density of the earth
column cannot, therefore, be the same at the two places
and must be less where the water is less deep than where
it is more deep.

The intensity of gravity is measured on the continents
by observing the time of swing of a seconds pendulum.
On the top of a mountain the intensity will be rather
greater than at the base, but we can imagine the inequalities
of a continental land area to be smoothed down, that is,
the materials that constitute the mountains can be imagined
as being removed to the extent that they are necessary
to fill up the valleys. Then we have the continental
plateau, or mean plane. The latter can be calculated
from the data of surveys so that an estimate can be always
made as to the vertical distance of any place above or
below the mean plane. Corrections to the observed

THE OCEANIC MARGINS 119

values of the intensity of gravity are then made so that
the latter is reduced to what it would be at the mean
plane level. If the station of observation is above the
latter a certain deduction has to be made, if below an
additive correction is made.

Then it is found that the intensity of gravity is the
same for places on a continental plateau as it is for places
on an oceanic depression (in the same latitude of course).
This means that the total mass of a column of earth beneath
a certain area on a continental plateau is the same as that
beneath the same area on an ocean bed. Yet the difference
in the heights of the two columns may be (say) four miles
The conclusion is that the earth material beneath the
continental area is less dense than that beneath the oceanic
area. There is other evidence leading to the same con-
clusion : thus the Himalayas, and other mountain ranges,
are less attractive than they ought to be if they were
composed of the material that is found at their margins.
So their interior parts must be formed of less dense rock
than that which is observable.

The Isostatic Earth Layer. Rock materials have certain
^' strengths," varying between about J ton and 4 tons per
square inch : when the strain exceeds the limit of strength
of any particular rock material the latter is crushed or
disintegrated. Now the general density of the substance
of the continental plateaus is such that the weight of a
column of rock three to five miles high would crush the
materials at the base of the column. In general, then,
the continental elevations are so high above the oceanic
■depressions that at the level of the latter the rocks
beneath the continents must be self-crushed.

Somewhere beneath the ocean beds and continental
plateaus, then, there must be an earth layer which is
unstable because it consists of crushed materials. This
is, nevertheless, a zone of very rigid rock and it is only
potentially unstable or crushable. It will yield easily
to a change of stress which would be resisted by an un-

120 AN INTRODUCTION TO OCEANOGRAPHY

crushed rock. In this zone there will be " rock flow " —
not the flow of a viscous material such as solid pitch but
a transference of material by solid solution, particle by
particle, by shearing of crystals, molecular re- arrangements,
etc. The materials will, in general, be in such a state
owing to the superincumbent weight, that they will slowly
yield to stresses in appropriate directions.

This zone of yield is the isostatic earth layer. It has
been estimated as being placed at about a depth of about
122 kilometres beneath the mean earth level, but recent
observations tend to reduce this depth by about one-half.
Below it the earth material is rigid in the geological sense ^
but in it the material is " plastic " — when sufficient
length of time is allowed for the process of rock-flowage
indicated above.

Above the isostatic level there is " isostatic equilibrium,"
that is, higher earth regions, such as the continental
plateaus, are balanced against the lower earth regions,
such as the oceanic depressions. The equilibrium will
be essentially the same as that to be seen in the two limbs
of a U-tube, one of which contains sea water and the
other distilled water. The liquids will stand at different
levels in the two limbs and the difference of level will
depend on that of their densities.

The great features of the earth's surface are balanced
in this way, that is, the lighter materials of the higher-
standing continents are balanced against the heavier
materials of the lower-standing ocean beds, the balancing
occurring in the layer of earth about 29 to 122 kilometres
in depth. Small features, say mountains and valleys of
magnitude equivalent to mountains, may exist in virtue
of the rigidity of the earth's crust, and need not be balanced
in the isostatic way. Such features as are, perhaps, over
one square mile in area but less than one square degree,
may exist unbalanced just because of their own rigidity
and that of the supporting earth crust. The picture of
the earth that we now make is as follows : —

THE OCEANIC MARGINS 121

(1) A nearly rigid centrosphere occupying at least
fths of the earth's diameter. Stresses due to tide-
generating force distort this centrosphere in a very
small degree.

(2) An isostatic layer some 100 kilometres thick. This
yields slowly to stresses due to the tide-generating
force or to the increased weight of materials deposited
on it, or to lightening due to the removal of materials
from it. " Materials " here are rock removed by
erosion, or sands, gravels, muds and oozes deposited
by geological transporting agencies, or snow or ice
deposited or removed by meteorological agencies.

(3) A superficial layer some 30 to 130 kilometres in
thickness. This is the part of the lithosphere
represented by the ocean beds and continental
elevations. It may be deformed by tidal action.
It is the locus of subaerial and submarine denudation.

(4) The envelopes — ocean and atmosphere. These are
mobile and easily deformed by tidal action.

Movements of the Continental masses with respect to the
theory of Isostasy. — We have now to consider in what
ways these notions of earth-structure on the great scale
enable us to understand the relations of continental and
oceanic regions — premising that the theory is still too
incomplete, and the data of observation too meagre to
do more than join together descriptions of earth-features
which would otherwise have little scientific interest.

The Shield-lands. First then, it is to be noted that each
of the continental elevations contains several ancient^
nuclear, land-masse^ called " shields." The situations of
of these are roughly indicated in Fig. 64. In the Eastern
Hemisphere there are (1) Baltica, the Scandinavian-
Russian region ; (2) Angara, Northern Asia and Siberia ;
(3) Ethiopia, the greater part of Africa ; (4) and (5)
Lemuria, Madagascar across to India ; (6) Australia, the
western part of that continent. In the Western Hemi-
sphere there are (1) Canadia, Canada and Greenland ;

122 AN INTRODUCTION TO OCEANOGRAPHY

(2) Columbia, Mexico ; (3) Antillia, the West Indian Islands
and Central America ; (4) Atnazonia, Brazil and N.W.
North America ; (5) Archiplata, Patagonia and possibly
across the Southern Ocean to Antarctica. See further,
in Chapter X.

The Oceanic Abysses. Opposed to the shield-lands are
their " opposite numbers," the oceanic abysses. These
great areas of depression have been supposed to have
originated (like the original elevations now represented
by the shields) in the primitive movements of depression
and elevation following the maturing of the earth after
its period of growth by the accretion of planetesimal
matter. Thus they may have existed since Archean
times.

The General Continental Regions. These (including the
nuclear lands to some extent) we know to have had a
degree of elevation above the mean earth level that has
certainly been variable : that is, the continental land has
alternately been elevated and then lowered by denudation
and erosion from the shore inwards. They may have
always been elevated relatively to the oceanic abysses.

The Oceanic-Continental Margins. Here are the loci of
greatest geological change. Both erosion and deposition
proceed at the maximal rates in these zones.

Interchange between Continental and Continental Shelf
Regions. We can attempt now to trace in what ways the
interchange of earth materials goes on between land and
sea. First of all, it may be noted that the interchange
between the land and the sea bottom under the great oceans
is indirect and one sided. Lime, silica, and (to a much less
degree) iron, manganese and potassium are taken from
the land and pass into solution in sea water. Then these
materials are separated from the sea by pelagic plants
and animals and are finally deposited on the ocean bottom
as the calcareous and sihceous oozes, as glauconite (con-
taining the potassium) and as concretions which contain
the iron and manganese. On the whole, the specific

THE OCEANIC MARGINS 123

gravity of such deposits is greater than that of the siliceous
sands and clays which are laid down on the continental
shelf ; also there is no return of these deep-sea deposits
to the continental regions. With the exception of a very
few cases (which may be accounted for quite satisfactorily)
there is no evidence of sedimentary rocks of such a nature
that they can be supposed to have been laid down at the
bottom of a deep ocean. Lastly the deposition of material
on the oceanic bottoms is exceedingly slow, when compared
with the corresponding process on the bottom of the
marginal seas — but it has been continuous throughout very
protracted periods. The conclusion is that the sea bottom
outside the limit of the continental sea-bottom deposits
is heavier than the remainder of the earth's crust. It
spreads out, flattening itself, so to speak, under its own
weight on the plastic isostatic layer underneath. Hence
the flat oceanic floors.

Between the continents and the zone of continental
shelf, however, the interchange is much greater and more
rapid. It must be regarded as a highly significant fact
for structural geology, that by far the greater part of the
material derived from the waste of the land is deposited
on the continental shelf in relatively shallow water as
sandstones, mud-stones and limestones. Further, a glance
at Figs. 13 to 16 will show how very restricted, in com-
parison with the areas of the continents and oceans, this
region of greatest deposition is. When one remembers,
then, how great a thickness of strata (25,000 feet or more'
is represented in some series of sedimentary rocks it is
easy to see that space for such accumulated material on
the continental shelf can only be obtained in a vertical
direction. That is to say, as these deposits accumulate —
in shallow water, it must be remembered — they must
subside, pressing down into the plastic isostatic layer of
the earth's crust.

But beneath this again, and not deeper than about
30 to 60 miles, lies the rigid unyielding centrosphere.

124 AN INTRODUCTION TO OCEANOGRAPHY

The material on the shelf region, then, must spread out
laterally as it accumulates from above. The flatness
of the ocean floor, seawards from the continental slope,
suggests (but see pp. 128-9) that it does not spread out
laterally in a seaward direction and so we conclude that
the materials deposited on the shelf sink down deeply, are
metamorphosed by pressure and ''flow " inwards beneath
the continental elevations. This is how isostatic balancing
of heavier ocean beds and lighter continental core-materials
occur. On the ocean beds there is continual slow deposition
and so there is time for vertical subsidence into the plastic
isostatic layer. The latter, but not the upper, relatively
rigid ocean floor, spreads out laterally in the only possible
direction, that is towards the region of continental shelf.
But there a much more rapid deposition of material
occurs and there is still greater loading on the isostatic
layer. The only region of yield, then, is underneath the
shelf itself, or underneath the continental plateaus.
Material " flows," in the ways suggested already, and so
there is uplift of the oceanic-continental margins, or of
the continental plateaus themselves.

The great features of the Oceanic-Continental Margins. —
Next we may consider (but quite shortly, in the mean-
time) how the present forms of the oceanic-continental
margins fit in with the ideas that come from the theory
of isostasy, leaving a more detailed discussion for the
last chapter. Fig. 60 represents the best known region —
that of the Atlantic and the Eastern side of the Pacific.
Now, in many ways the Atlantic differs strikingly from
the Pacific, and some of these we notice now, deferring
further consideration of the facts until Chapter X.

Two main kinds of margins are indicated. The longitu-
dinal coasts and the transverse coasts. The former are
represented in the most typical manner by the Pacific
Coast of South America. Here the great ranges of the
Andes run near the ocean and everywhere parallel to the
shore. This is a longitudinal coast in the most obvious

THE OCEANIC MARGINS 125

form. So also the Pacific Coast of North America shows
the same great features in the Sierras and Rocky moun-
tains, also bordering the land and forming the land rampart
of the continental slope. All these ranges of mountains
are recent earth folds ; great masses of strata crumpled
and bent in complicated ways but in general with the
folds lying parallel to the oceanic margins. There are
no recent longitudinal earth- folds on the Atlantic Margins,
but inland from the coast of Brazil there are ancient
foldings. Also on the Scandinavian Coast there are
similar old earth-folds and these are continued outwards
across the North Sea as the Scottish mountains striking to
the S.E. On the Scandinavian- Atlantic Margin, then, we
have an old longitudinal coast. This is also the case on
the Western North Atlantic Margin : here we have the
old Appalachian earth-folds which run longitudinally
on the North American Atlantic coast between about
latitudes 30° to 45° N.

Elsewhere on the Atlantic Margin we have transverse
coasts, rias and fiords with " fractured margins of horsts
and fractured table lands." Thus in New Brunswick and
Cape Breton Island folded Archean and Paleozoic rocks
run about N.E. out into the Atlantic and are continued
over to Newfoundland. These folds slope down seawards
and are finally submerged, leaving the drowned valleys
sloping down into the sea as the rias. They also occur
on the North and South sides of the Bay of Biscay and
elsewhere.

Fiords occur in typical form on the Norwegian coast,
and in less typical forms on the West Coast of Scotland,
in Greenland and elswhere. These features, rias and
fiords, characterise the greater part of the European
Atlantic Margin, the Newfoundland and Canadian coasts
and the Eastern Coast of Greenland. Here old earth-
folds, partially submerged and much eroded, run into
the ocean either perpendicularly, or at some other angle
to the general direction of the coasts. Between them are

126 AN INTRODUCTION TO OCEANOGRAPHY

submerged valleys. In other places the continental
land inwards from the Atlantic Coast tends to form horsts
(which are great blocks of land, left elevated while the
adjacent land has sunk down by systems of faultings).
These are broken and eroded. The shields we may regard
as worn and fractured table-lands.

All this suggests that the Atlantic oceanic-continental
margins represent a phase of decadence. Turn now to
the Pacific margins and on the eastern side of that ocean
we see a young formation — a series of recent folds rising
high above sea level, continuing parallel to the oceanic
margin over immense distances and lying landward to
relatively steep continental slopes down into deep water.
This represents the continental margin in its growing,
or mature phase.

The normal condition of a continental region we may
take to be one of moderate elevation, a low table-land
(when its average features are considered) rather than
a great region bearing extensive and high mountain
systems. It is the locus of extensive denudation — atmos-
pheric and fluviatile. The result is the transport of eroded
material into the sea on a great scale and the deposition
of this on the continental shelf. Remembering the limited
area of the latter, we see that long continued continental
erosion and deposition must ultimately lead to the accumu-
lation of immense thicknesses of strata on the shelf region.
Seaward from this is the ocean bed beneath deep water.
Here also there is long continued deposition of material,
and our hypothesis is that the oceanic region is a relatively
stable one. It has been there for a relatively long time
as an area of depression ; it has thus received pelagic
deposits for periods that are much longer than those
during which the continental regions suffer the change
of which we are about to speak ; these pelagic deposits
are relatively heavy and, in addition, the ocean bed is
loaded by the two to three miles of water standing on it.
It is, therefore, an area of stability and beneath it the

THE OCEANIC MARGINS 127

yielding isostatic earth layer must also be in a condition
of relative stability.

Loading, then, of the restricted region of the shelf
must cause yielding of the isostatic layer and then rock
flowage. What direction will the latter take ? Not
underneath the ocean bed, we may expect, for that is
loaded to a greater degree, as we have seen. Further,
the deposition thins out on the oceanic margin of the
continental slope, so that there must be a region there
where the underlying strata are not self-crushed and are
relatively rigid. The shelf deposits are, in a way,
buttressed towards the oceanic margin, and the flowage
due to self-crushing will probably tend to occur on the
continental margin. After long continued erosion and
shelf-deposition, therefore, there will be a tendency for
the transference of material, from underneath the continental
shelf region to underneath the continental region. By
this time the general elevation of the latter has been
reduced ; the load on its underlying isostatic layer has also
been reduced, and so the lateral pressure from the margins
towards the central parts of the continental regions
must increase. The result will be renewed elevation.

But that again must lead to lateral or horizontal thrust
towards the oceanic margins. The outer regions of the
shelf are, we have seen, buttressed and resist the thrust
from the continental interior. So the shelf strata are
subjected to lateral compression and are folded. Such
would appear to be the origin of the great Pacific geo-
synchne formed by the Andes and Rockies. With this
process of continental uplift and marginal folding, the
conditions again become favourable for renewed erosion
on a great scale.

Quite other phases are represented by the Atlantic
margins and that of the Western Pacific, but these we
consider in Chapter X.

The Continental displacement hypothesis. — The hypo-
thesis of displacement formulated during recent years

128 AN INTRODUCTION TO OCEANOGRAPHY

by Wegener states that the great continental plateaus
are moveable on the earth's surface. The equatorial
bulging and the polar flattenings are features of such
magnitude that, relative to them and to the centrosphere,
the plane of the equator, and the axis of the earth are
invariable except for the very small cyclic movements
mentioned on pp. 13, 14. The lithosphere itself is, however,
to be regarded as a film on the centrosphere, and between
it and the latter is the yielding isostatic layer. Represent
the continental plateaus by paper shapes fastened on to
the surface of a small sphere with wet glue : we might
then easily shift each paper continent bodily on the globe,
sliding it on its wet glue surface (which would then represent
the yielding isostatic layer). Thus the symmetry of the
globe — its axis of rotation, its equatorial plane, its lines
of latitude and longitude — would remain invariant (or
nearly so) but the continents themselves would change
relatively to this fixed frame of reference.

The shapes of the continental elevations lend themselves
to this hypothesis (just as they do — in a way — to the
tetrahedral one). The Atlantic margins of the old and
new world give the most plausible case. Translate the
Eur- African land mass across the ocean and it will nearly
fit into the American one : the Cape Verde protuberance
of Africa fitting into the great Central American Bight ;
the Cape San Roque American protuberance fitting into
the Gulf of Guinea Bight and so on. On the north there
will be a gap which can be filled up by Greenland. On
the south there will also be a gap to be filled in, perhaps,
by Antarctica. If, further, Antarctica and Australia are
bent up to the north, being pivoted on Cape Horn, the
Indian Ocean will be filled up. Then we should have one
great continental land mass completely surrounded by
ocean. It is assumed that such a single land mass did
originally exist and that it broke apart, so to speak. The
Americas drifted away westward from Eur-Africa, and
the Antarctic- Australian lands drifted southerlv from

THE OCEANIC MARGINS 129

Asia. These movements led to crumpling and folding
of strata. The Andes rolled up, so to speak, in front
of the sliding American continent. There would have
been a general movement from North towards the Equator,
and so we can account for the tendency to form the great
transverse foldings — the Himalaya-Alps one. On each
side of the regions of rupture (the Atlantic, for instance)
there ought to be corresponding features. Thus we can
suppose the old Appalachian and Caledonian foldings
(in North America and Northern Europe respectively)
to have been continuous before the two Americas broke
away from Eur-Africa — and so on.

Thus the hypothesis claims to have support in the facts
of structural geology, but, without considerable space,
the latter cannot be summarised here. It also claims
support from the records of paleontology. The present
distribution of many species of animals is undoubtedly
puzzling, and to account for it land " bridges," or communi-
cations between the continents, are assumed to have
existed in the past — Gondwana, for instance, an ancient
continent joining Africa and America. Some of these
assumed land connections are made probable by the facts
of structural geology, but others seem to be ad hoc hypo-
theses designed to account for the observed distribution
of species of animals. Possibly as much is to be said
for the Wegener ad hoc hypothesis, in so far as it accounts
for the facts of biological distribution, as is to be said for
the others which had the same goal. Their weakness is
their special object — one which biological research may
yet render unnecessary, for it does not yet seem certain
that convergence of animal characters may not explain
some, at all events, of the problems of distribution.

So far as the displacement hypothesis is understood at
present (for it is in process of revision) it would appear
that the continental plateaus "float " on the present sea
beds. But the limit of isostatic compensation must be
put at a depth much greater than this — at 122 kilometres.

130 AN INTRODUCTION TO OCEANOGRAPHY

Below this level the centrosphere is rigid and unyielding;
except in so far as it " trembles " to earthquake shocks
and to the changes in the tide-generating force. Thus
hoih the ocean-bed and the continental plateaus rest on a
yielding stratum. It must, then, be shown that the latter
do actually yield, or tend to yield, more to forces of trans-
lation than do the oceanic plateaus — which must have
a considerable thickness. Forces, or stresses, competent
to set up such movements must be found. Finally, astro-
nomical evidence is quite essential to the hypothesis : that
is, non-cyclical variations of latitude and longitude must
be traced.

CHAPTER VI
THE CHEMISTRY OF SEA WATER

The volume of the oceans and seas of the earth is about
324,000,000 cubic miles, which is about 15 times the
volume of the part of the earth above sea level. If surface
inequalities of the earth were smoothed out, the higher
parts of the continental elevations being deposited in
the lower parts of the oceanic depressions, water would
cover the entire globe to a depth of nearly two miles.

Such figures are impressive but they must not give the
reader a false impression of the mass of the oceanic water
relatively to that of the earth body. A two-mile deep
ocean is only a sheet of water about 1 /4000th part of the
earth's diameter in thickness. Represent the globe by
a rubber ball of 4 inches in diameter : then the ocean
would be represented by a film of water 1/1000 inch in
thickness. Such a film would cling to the rubber ball
by capillary attraction. It might, quite easily, be sweated
out from a ball of this size composed of suitable material.
The analogy has some importance.

The origin of oceanic water. — We considered the
possible modes of origin of the oceanic earth-depressions
in Chapter II, and found that these depended upon
the particular hypothesis of the origin of the earth
itself that is assumed. This must also be the case with
regard to the origin of the water, and the saline matter
which this holds in solution. On the gaseous-molten
hypothesis the problem is quite a simple one : the
original earth-nebula contained the materials of the
present gaseous and liquid envelopes, which formed part
of an original atmosphere. When the solid earth-body
had cooled sufficiently the water of the primitive atmos-

131

132 AN INTRODUCTION TO OCEANOGRAPHY

phere condensed on it to form the ocean, accumulating
in the depressions of the crust.

A difficulty is now encountered : there is apparently
too little water on the earth. It is known that the molten
lavas, or volcanic magmas of the present earth's crust
contain considerable quantities of water and even crystal-
lised rock materials are known to contain water inclusions.
If, then, the rocky earth crust, or lithosphere, resulting
from the solidification of an originally molten earth had
a considerable thickness it must have contained quite a
large quantity of water. In addition to this absorbed
water present in the rocky lithosphere there was that
assumed by the hypothesis, to be present in the primitive
atmosphere. Now during the great crustal movements
that have always proceeded, most (or at least very much)
of the original crust must have been brought to the surface
from time to time and have suffered disintegration. In
the course of this the contained water — or very much of
it — must have been sweated out, so to speak. If we assume
reasonable data for the amount of this included water, it
would appear that the total volume that has come from an
earth solidifying from a gaseous-molten stage would
probably be greater than the volume of the earth-film
now represented by the ocean. It may be, of course,
that the process of " sweating-out " is still going on and
that the volume of the ocean is increasing and has always
been increasing throughout geological time. And, indeed,
there are suggestions in geological history that this is so —
that, for instance, the ocean was much less in volume
in Permian time, than it is now.

Far less difficulty is encountered if we adopt the planet-
esimal hypothesis of the origin of the earth. We may
admit that the original bolt of gaseous-molten matter
ejected by the eruptive sun contained a very large quantity
of water — as much as one likes to assume. But the
earth, in this embryo stage was small and so its gravita-
tional field was, for a long time, insufficient to retain water-

THE CHEMISTRY OF SEA WATER 133

vapour as an atmosphere ; and it is doubtful whether
Mars can do so. Therefore, as the juvenile earth solidified
and squeezed out water the latter would escape from its
atmosphere, and not until it had grown to near its present
size, by planetesimal accretions, would its gravitational
field be intense enough to prevent water from evaporating
and escaping from the atmosphere into outer space.

Probably, then, the ocean has not had its origin in a
primitive atmosphere surrounding a hot earth body slowly
solidifying from a gaseous-molten phase. It is far more
likely that it has come from the water absorbed by the
planetesimal dust, or chemically combined in the materials
of the latter. As this solidified and then became dis-
integrated, the water would be liberated. At first it would
escape, but when the earth grew big enough by the accretion
of new planetesimal material the water would be retained.
With its present mass it is even possible that the earth
continually captures water molecules from outer space, and
it is also probable that the water included in the last
planetesimal accretions is still being sweated out. On
this view, then, the ocean is very slowly increasing in
volume.

The Chemistry of Sea Water. — Sea water is a complex
solution of very many substances and the origin of this
saline material must also be considered. On the gaseous-
molten hypothesis, the question is a very simple one.
The earth solidified and water which was approximately
" fresh " accumulated in the depressions, or on those
areas where the gravitational force was greatest.
Evaporation, and subsequent precipitation as rain on
the dry land, led to denudation of the materials of
the latter. Soluble constituents, sodium chloride, for
instance, were thus " leached " out and the originally
fresh ocean became salt by degrees. On the planetesimal
hypothesis it is difficult to see why it was not salt from the
beginning since water which was squeezed out from a
loosely compacted porous material must have exercised

134 AN INTRODUCTION TO OCEANOGRAPHY

its solvent action all the time. To this question we return
later in the present chapter, for it will be more convenient
to deal now with the actual composition of sea water.

Water is almost an universal solvent, and there are few
substances which are not soluble in it to some extent.
In the conditions that have existed — prolonged periods
of time ; occasional high temperature and pressure and
favourable conditions of percolation through, and over
rock materials in all stages of disintegration — the solvent
action of water upon the earth-materials has been
facilitated. Therefore, a great number of elements can
be found in solution in the sea.

This solution, even in the case of the most abundant
substance sodium chloride, is a dilute one and for most
of the detectable substances it is very dilute. What
exists in sea water are not, then, simply the salts, sodium
chloride, calcium sulphate, sodium carbonate, etc., but
the dissociation products of these. In most cases the
dissociation will be nearly complete ; that is, for instance,
it is not sodium chloride that is present but sodium and
chlorine ions, with sodium chloride molecules, the whole
forming an equilibrium which varies with the temperature.
Even the water itself is dissociated to some extent, there
being hydrogen and hydroxyl ions in the solution (see
p. 149). What we regard as the substances present in
sea water are, therefore, the ions mentioned in the following
pages. It is true that we can isolate common salt, as a
pure chemical, in the dry, crystalline form, /rom the sea,
but in the sea it exists mostly as sodium and chlorine ions.

The substances jpresent. These are, roughly in order of
abundance. Chlorine, Sodium, Sulphur (as SO^), Mag-
nesium, Calcium, Bromine, Carbon (as COg), Silicon (as
SiOg), Phosphorus (as PO^), Nitrogen (as NH^ and
NO3), Iron, Aluminium and Rhubidium. All these
elements can easily be found, in weighable quantities,
either in the sea water, or in the dry salts obtained by
evaporating the latter.

THE CHEMISTRY OF SEA WATER 135

In addition to them the following elements have been
detected by chemical and spectroscopic methods even if
their amounts, in some cases, cannot be precisely esti-
mated : — Iodine, Fluorine, Arsenic, Boron, Lithium,
Caesium, Barium, Strontium, Manganese, Nickel, Cobalt,
€opper. Zinc, Lead, Silver, Gold and Radium. Some of
these elements can be traced in the dried salts, others in
the ash of various plants or animals and others again in
concretionary bodies on the sea bottom. " Radium " is,
of course, the assumed cause of the observed radio-activity
of oceanic salts and sediments, but it may be due to other
elements. In the case of most of the substances present
the concentration is too small for precise measurement,
but the total quantities present in the ocean must be very
great. It is curious to note, for instance, that there are
certainly thousands of millions of tons of gold in solution
and even a minimum estimate of the quantity of Radium
present is taken to be about 1,400 tons.

The composition of Sea-Salt. Some of the rarer sub-
stances have extraordinary importance and we consider
them later (pp. 156-60). " Sea-Salt " is the saline
residue obtained when sea water is slowly evaporated
to dryness, and this is the substance of which the
composition is given below. A very great number of
analyses have been made but only the later ones are
considered here, and of them only a few are quoted. Sea-
salt, then, has the typical compositions given on
p. 136.

The numbers given are percentages of the weight of
the dry salts taken.

The localities are : —

(1) All the oceans : means of 77 samples taken by the

" Challenger." The salinity was 33-01 to 37-37.

(2) The Atlantic : mean of 22 samples. Salinity = 36-31.

(3) The Baltic : Salinity = 7-21.

(4) The Mediterranean : Salinity = 38-97.

(5) The Red Sea : Salinity = 39-76.

136 AN INTRODUCTION TO OCEANOGRAPHY

(6) The Indian Ocean : Salinity = 35-53 to 36-68.

(See pp. 137-146 for " Salinity ").

A glance over these analyses will show how very uniform
in composition is sea-salts taken from the most varied
localities. Some of the differences in the results quoted
are not much greater than the magnitude of the errors
to be expected. Even in the cases of the Baltic (with
only about 0-7% of dissolved soUds) and the Red Sea
(with nearly 4%) the ratios to each other of the ions
present are nearly the same. Where we do get significant

Locality

.

.

(1)

(2)

(3)

(4)

(5)

(6)

CI .

. 55^29

55-19

5501

55-53

55-60

55-41

Br .

•19

•18

•13

•18

•13

•13

SO4.

7-69

791

8-00

774

7-65

7^79

CO3.

•21

•21

•14

•19

•02

•05

Na .

. 30-59

30-26

3047

3037

3081

30-89

K .

Ml

Ml

•96

109

•97

-85

Rb .

—

—

•04

—

•04

-03

Ca .

. i \-20

1-24

1-&1

1^26

•89

1-16

Mg .

. 3-73

3-90

353

3-64

387

3-67

Fci, S]

0,.P0, .

. 1 —

—

•05

—

•02

•02

differences is in the neighbourhood of great rivers, and this
is because the saline matter in solution in river water is
rather different from that in the sea. Differences in the
ratio of lime to sodium are also noticeable where there is
a rapid removal of this element by marine organisms.

In general, however, the movements of the sea due to
wave action, currents and tidal streams lead to a very
thorough mixture of the water of different origins and this
cause, which is in constant operation, has resulted in the

THE CHEMISTRY OF SEA WATER 137

very remarkable uniformity of composition indicated in
the analytical results.

The Salinity. — What varies in a notable degree is the
actual quantity of solid matter present per unit volume of
water. This is the result of several causes : dilution by
river water in the neighbourhood of continental lands, or
by rainfall in the tropical zones of the oceans, or by the
melting of ice, or concentration in the regions of high
temperature and strong winds. Thus the salinity varies
from one or two parts per thousand (in the Baltic) to
nearly 40 per 1000 (in the Red Sea).

Salinity is (arbitrarily) defined as the total weight in
grams of solid matter present in 1,000 grams of sea water
where (1) the Bromine and Iodine are replaced by Chlorine ;
(2) the carbonates are reduced to oxides, and (3) the organic
matter is ignited. It is estimated in various ways.

(1) By titration with standard silver nitrate solution.
This method depends on the condition that the ratio of
chlorine to the total salts present is so very nearly constant
that the error involved in assuming that it is absolutely
constant may be neglected. An empirical formula
(obtained by Martin Knudsen) shows that Salinity — 0-03
+ 1'805 Chlorine so that the estimation of the latter
element enables us to calculate the salinity. Special
burettes and pipettes are used so as to make the estimation
rapid and accurate. The graduations on the stem of the
burette are so arranged as to give the actual quantities
of chlorine contained in 1,000 c.cs. of the water, of which
15 c.cs. are taken, as a sample for the titration and the
concentration of the silver nitrate solution is made such
that this is nearly the case. The sample is contained in
a porcelain evaporating basin ; a few drops of sodium
chromate are added to it. Then the silver nitrate solution
is added slowly from the burette until a very faint red
colour appears : this happens when all the chlorine is pre-
cipitated as silver chloride and the next drop of solution
throws down a ruby-red precipitate of silver chromate.

138 AN INTRODUCTION TO OCEANOGRAPHY

There are of course many precautions which are not
mentioned here. It is generally impossible to make, and
preserve, a solution of silver nitrate of precisely the strength
theoretically required, and so the approximate solution made
is titrated against a sample of " standard sea water," the
chlorine concentration of which is known accurately.
Tables are then used to obtain the correction which is to
be made to the burette reading.

(2) The electrical resistance of the sea- water sample
is determined. Reference to tables then gives the salinity.

(3) An " interference method " has lately been developed.
In practical work the chlorine titration method is used.

It is easy and accurate enough for most purposes, giving
results correct to about 0*02 per mille. Usually this will
suffice, but in the cases of samples taken from mid-ocean
and at considerable depths the salinity variations are so
very small that a higher degree of accuracy may be neces-
sary. Then the interference method seems to be the only
one that is refined enough for the purpose.

The variations in salinity. — Movements of the ocean
which we shall consider in greater detail in the
following chapters lead to a very thorough mixture of
the water. These movements are the tidal streams
which everywhere surge backwards and forwards in the
neighbourhood of the land ; slow currents and drifts which
transport water from equatorial to polar regions and vice
versa, and vertical movements of the ocean which lift
up water from the bottom to the surface in some regions
and cause surface water to sink to the bottom elsewhere.
The effect of this mixing is to give the water a nearly
uniform composition in respect of the ratios to one another
of the various substances in solution, and the data quoted
on pp. 136 will show how very nearly constant are these
ratios. Long-continued mixing, due to the movements of
the ocean would lead, of course, to an absolute uniformity
in composition were not other factors in operation.
Everywhere the salinity would be the same.

THE CHEMISTRY OF SEA WATER 139

Then oth^r factors lead to diversity. In the equatorial
regions there are zones of heavy rainfall which dilutes the
surface water and so leads to an appreciable reduction of
salinity. North and south of this intra-tropical zone of
calms, high temperature and heavy rainfall are the zones of
high temperature and strong N.W. and S.E. trade winds.
These blow strongly throughout the year and lead to great
evaporation of the surface water, raising the salinity of
the latter to a marked degree. Movements of this surface
water then lead to its transport into high northerly and
southerly latitudes, and so the salinity which is normal to
the temperate zones is increased by the entrance of denser
water. At the same time an upward movement occurs :
the surface water in the tropical zone becomes expanded
by the high temperature and stands at a higher level
than the normal one. Therefore it flows away to north
and south and colder and lighter water rises up from the
lower strata of ocean to take its place.

In the polar and subpolar regions there is an excessive
amount of precipitation of water from the atmosphere
in the form of snow. This accumulates on the land and
soUdifies to the form of ice. The ice is pressed down
towards the sea as glaciers and the latter break away as
icebergs which drift to south and north and slowly melt
as they move towards the temperate latitudes. Melting
there, they dilute the water of the ocean and so reduce
the salinity.

There are smaller regions of high temperature and
increased evaporation. The Mediterranean, for instance,
is such a region and so are the Gulf of Mexico and the
Red Sea. The salinity in these areas rises to much above
the average value and the temperature also increases.
The effect is an outflow of highly saline water which
raises the salinity of the ocean in the regions into which
such an outflow occurs. Thus the sea in the Bay of
Biscay, and even at the entrance to the English Channel,
is denser than normal because of the highly saline water

r

140 AN INTRODUCTION TO OCEANOGRAPHY

flowing out into the Atlantic from the Mediterranean Sea.
So also the South- West Atlantic, in the neighbourhood of
the Gulf Stream outflow from the Straits of Florida, has
an increased salinity.

In other places just the reverse change occurs. Thus
the sea in the neighbourhood of the Amazon, Congo,
Mississippi and other great rivers is much less saline than
normal because of the entrance into it of great volumes
of fresh water, and the reduction of salinity is a general
feature of the ocean in the vicinity of continental land.

General variation of Salinity. Thus there is a general
variation of salinity according to latitude and this is
approximately as follows : —

Zone of latitude

70-55 N.

55-40 N.

40-16 N.

15 N.-IO S.

Salinity . .

. 30-31

33-34

35-36

34-5-35

Zone of latitude

. 10-30 S.

30-50 S.

50-70 S.

Salinity . .

35-36

34-35

33-34

That is, roughly, low salinity in the equatorial zone ;
high salinity in two belts north and south ; low salinity
in the temperate zones ; minimal salinity in the sub-
polar zones.

The Salinity Charts. Let a great number of observations
of the value of the salinity of the surface water be made
simultaneously : by " simultaneously " is meant, say,
during the same month, for the values will not change
greatly during that period and it is impracticable to
attempt to deal with a shorter interval. The values are
written down on a chart at the places where the obser-
vations were made and then a smoothly running line is
drawn through all the points which have the same
numerical value, say 35'5%o.* This is done for each
grade, say 31, 32, 33, 34, 35, 36, 37 and 38. The 35-5
contour line will not necessarily pass through all the points
35*5, say, but will go as nearly as possible to all of them
and will change in curvature as gradually as possible.
These lines are called IsohaUnes, and they are contours
which are to be regarded as the average boimdaries of

* The sign, °l^^, means " per thousand."

THE CHEMISTRY OF SEA WATER 141

areas of water of the same ranges of salinity : thus the
contours 35 and 36 enclose an area within which the water
varies in salinity between those limiting values.

On large-scale charts, and when the observations are
numerous, the isohalines represent very closely the distri-
bution of the salinity. Such will be the case for such
small areas as the North Sea, where there have been many
investigations in progress. For much larger areas, say,
the Atlantic Ocean, the courses of the isohalines must be
much less precise because the observations must necessarily
be made at places much further apart. In the case of
the small areas the courses of the isohalines will generally
be rather complex when compared with those of the large
areas. Further, there are rather considerable changes in
the contours from month to month, and even from day to
day in the case of a small sea-area, and so the chart of
isohaline contours must always be taken to represent the
approximate conditions only. Remembering, then, that
the actual distribution of salinity at any moment must
always be more complex than that represented by the
average distribution for a period of several weeks, and
that it will be more complex on the small, than on the
large scale, we may consider Figs. 22 and 23.

Far more attention has been paid to the Atlantic than
to any of the other oceans, and the distribution of water
of different degrees of salinity is fairly well known, both
on the surface and in the lower strata. The chart shows,
first of all, the general variation summarised on p. 140.
Rather to the north of the equator, that is roughly in the
zone 0° to 10°N., the salinity is low varying between 33 and
36. Then there are two regions of higher salinity north
and south of the 0° to 10°N. zone but these are not so much
belts as large, roughly elliptical areas centred round
smaller regions where the salinity rises to over 37'5°/oo-
The northern region has its centre roughly about 25°N.Lat.
and 40°W.Long. while the southern one is about 15°N.Lat.
and 25°W.Long. Round these central areas the salinity

142 AN INTRODUCTION TO OCEANOGRAPHY

decreases, in the ways represented by the contours, towards
the value 35°/oo, which we may regard as characteristic
of the higher temperate latitudes, both North and South.

Fig. 22. The average distribution of salinity in the surface water of
the Atlantic Ocean.

There is, therefore, an evident asymmetry in the distri-
bution of the salinity of the Atlantic, inasmuch as the

THE CHEMISTRY OF SEA WATER 145

values do not decrease at the same rate to north and south
of the equator, and the latter does not represent the
central part of the zone of low salinity but lies about 10°
to the south of the latter. Further, we see that there
is a tendency in the South Atlantic for the isohalines
37, 36, 35 and 34, to run roughly parallel to the equator
while this is not the case in the North. There the isohalines
are pressed up to the north-west, so to speak, against the
coast of North America. Also the extension to the north
of the 35 contour is very noticeable. We find water of
35 to 36°/oo salinity round the S. and E. of Iceland, up
close to Spitzbergen and well to the north of North Cape,
thus higher up than the 70th parallel of latitude. In the
South Atlantic, on the other hand, the 40th parallel is,,
roughly speaking, the southern boundary of water which
has a salinity of 35°/oo or more. Again, we see that the
more highly saline water is to be found in the north, on
the eastern side of the Atlantic : thus the British Islands
in latitudes 50° to 55°N. are bathed by water of 35 to
35'5°/oo, while on the western side the Labrador Peninsula
is fringed by relatively fresh water between 31 and33%o.
In the South Atlantic, on the other hand, the heavier
water is found on the west, and the fresher water on the
east borders of the ocean. Close up to Brazil we find
the 37°/oo contour line while, nearly opposite, in the Gulf
of Guinea we get water which is only from 32 to 35%o-
Finally, the toe of South America is surrounded by a
rather wide zone of water which is also relatively fresh,
that is, from 33 to 34%o.

Next look at the " seas " that are situated on the contin-
ental margins. The two true " Mediterraneans," that is,
the classical Mediterranean and the Gulf of Mexico, show
high salinity values. In the former the water may attain
a salinity of over 39°/oo while in the latter the corresponding
value is over 36'5°/oo. The shallow epi-continental seas,
the Baltic, North Sea, and Gulf of St. Lawrence and
Hudson's Bay, on the other hand, are filled with water

144 AN INTRODUCTION TO OCEANOGRAPHY

which is relatively fresh, although this is much more
noticeable on the West than on the East.

The figure is on far too small a scale to show the varia-
tions in salinity due to fresh water entering the ocean
from the great rivers. Thus one could, by inspection of
this chart, hardly deduce the situation of the openings
of the Amazon, Rio Grande or Mississippi, although these
would be indicated by the course of the isohalines in large
scale charts. The low salinities in the Gulf of Guinea are
evidently not due to the presence of water poured into
the Atlantic from the Congo — we find just the same
presence of low salinity water, and the great extension of
this to the west as a long narrow tongue, in the Pacific
Ocean.

The main features that Fig. 22 exhibits, with respect
to the distribution of salinity, are due primarily to the
equatorial dilution referred to on p. 140, and to the concen-
trating effect of the North-East and South- West trade
winds. These causes would lead to the arrangement of
the isohalines parallel to the equator. But the general
trend of the great currents leads to the distortion of the
isohalines in a way that we may the better study in
Chapter IX. The low salinity on the extreme N.W.
is due to the inflow of water from the Arctic regions, and
that in the region situated off the Gulf of Guinea is the
result of the upwelling of relatively cold and light water
from the deeper parts of the ocean.

The conditions in the Indian and Pacific Oceans are
represented in Fig. 23, which is drawn on a much smaller
scale because the observations there are much less numerous
than they are in the Atlantic. But, considering the ways
in which these oceans are bounded, it is evident that we
have to deal with essentially a similar kind of distribution.

Thus the equatorial zone of water, that is, the ocean
between about 0° and 10°N. has lower values for the
salinity than the zones to the north and south. There
are two roughly elliptical regions, in about 20° N. and

THE CHEMISTRY OF SEA WATER 145

S. Lats. where the salinity is high, over 36%o, though it
is not so high as in the corresponding Atlantic areas.
Smaller " islands " of water which is over 36 %o are situated
to the north-east, east and west of Australia.

On the whole the tendency of the isohalines is to run
more nearly parallel to the equator than in the Atlantic.
The great diversity of land and sea, and of oceanic depths
in the Australasian area, however, introduces a host of com-
plications which are still more apparent when the contours
are plotted on a larger scale chart. The epi-continental

1^ |60 \80 \/oo pgQ \/*C [60

[^ \bO \80 yOO |/gO |/*0 \lbO |/80 |/60 \jlM |go \loo \Qo

Fig. 23. The variation of salinity in the surface waters of the Indian
and Pacific Oceans.

seas are, as in the Atlantic area, distinguished by their
lower salinity. The influence of the Red Sea and Persian
Gulf areas is indicated in the region of high salinity (over
36°/oo) in the North-Easterly part of the Indian Ocean,
and the evident influx of water from the Arctic is shown
by the low salinity south of the Behring Sea. Just the
same easterly projecting tongue of low-salinity water is
shown in the eastern side in the equatorial latitudes and
this, as in the case of the Atlantic, is due to the up welling

146 AN INTRODUCTION TO OCEANOGRAPHY

of cold and relatively light water from the lower levels
of the Pacific.

We return to a consideration of the distribution of
water of varying salinity in Chapter IX, for the origin
and courses of ocean currents and drifts are, to a very
great extent, traced by a consideration of the course of
the isohalines.

The Gases of Sea Water. — In addition to the saline
substances mentioned on pp. 134-6, the water of the ocean
contains gases in solution. These are nitrogen (with
argon), oxygen, carbonic acid gas, and, in exceptional
places, sulphuretted hydrogen. The nitrogen and oxy-
gen are mainly taken up from the atmosphere either by
sea water itself, or by fresh water entering the ocean
from rivers. This, to a great extent, is also the source
of the carbon dioxide. In other ways, however, the sea
receives gases : carbon dioxide may be liberated from
solution in the water of submarine volcanic springs and
as the result of putrefactive processes in the sea ;
oxygen is formed in the process of carbon-assimilation
by plants ; and nitrogen may be set free from organic
matter as the result of the activity of certain species of
marine bacteria. In other ways the sea loses the gases
held in solution : oxygen is removed as the result of the
respiration of marine animals and in processes of putre-
faction of organic matter ; carbon dioxide is removed in
the process of carbon-assimilation ; and nitrogen is certainly
removed by the nitrogen-fixing bacteria and locked up
as nitrites and nitrates. These gains and losses probably
balance each other during long periods of time.

For the greater part, however, the interchanges of
nitrogen, oxygen and carbon dioxide take place between
the sea and the atmosphere. Distilled water dissolves
aU the gases mentioned, and we shall consider the solution
to be " dissolved air." The quantity of any gas that can
be taken up by water depends on the temperature, on
the partial pressure of the gas and on a chemical constant.

THE CHEMISTRY OF SEA WATER 147

Thus Q = T . k/p, k being a constant depending on the
chemical nature of the gas, and on the units adopted, p
being the partial pressure of the gas in the atmospheric
mixture and T being the absolute temperature of the
water.

In the case of sea water the expression given above will
contain an additional constant which depends on the
salinity. At 0°C. and 760 mm. pressure, a litre of distilled
water will take up about 19'5 c.cs. of nitrogen and about
10 c.cs. of oxygen — thus this " dissolved air " will contain
about 34% of oxygen whereas the atmosphere contains
only about 21%. The gases mentioned are, however, less
soluble in sea water then in distilled water : thus sea
water of ordinary (35 to 36°/oo) salinity will dissolve about
10"5 c.cs. of nitrogen and about 5*3 c.cs. of oxygen at the
temperature of 20°C. and so this " dissolved air " will
contain a little more oxygen than does the mixture dissolved
in pure water. This percentage will remain nearly the
same even if the temperature varies.

There is a continual tendency towards equilibrium
between the gases dissolved in the sea and those contained
in the atmosphere. This equilibrium would exist in the
case of a dish of sterilised sea water exposed to the air.
If the temperature decreases more gases will be dissolved
by the water and if it rises gases will be liberated from
solution into the air. Any change of atmospheric pressure
will also affect the condition of equilibrium. To some
extent this is the case with the ocean and atmosphere, but
the equilibrium will be an average one which is continually
being disturbed. The facilities for solution of air by the
ocean are evidently very great. Winds blowing strongly
bring repeated quantities of air in intimate contact with
the water surface and the latter becomes enormously
increased by breaking wave and spray. The tidal streams,
currents and vertical movements of the sea continually
change the water that is exposed at the surface. Thus
the interchange is facilitated.

148 AN INTRODUCTION TO OCEANOGRAPHY

Nevertheless the equilibrium changes from time to
time and place to place. A cold current flowing into a
place where the temperature of sea and atmosphere is
higher must also rise in temperature and, the solubility
of gases being diminished, some of these dissolved must
be liberated into the air. 'The reverse reaction will occur
when a warm current flows into a colder region, or when
cold bottom water wells up to the surface of the sea.
Change of winds will act in the same ways : a cold wind
blowing off the land will chill the surface layers of water
and then the latter will take up more air : on the other
hand a warm wind will heat the surface and deprive it
of some gas. Changes in atmospheric pressure will
obviously produce similar effects. Finally, the activities
of animals and plants in the sea lead to very marked
local changes in the concentrations of all the dissolved
gases.

The dissolved carbonic acid gas. The conditions
regulating the interchange of CO 2 between sea and
atmosphere are much more complex than in the case of
oxygen and nitrogen. First, the partial pressure of
carbon dioxide in the air is much less than that of
the other principal gases : its proportion is only about
0"04% as compared with about 79 and 21 in the case
of nitrogen and oxygen respectively. If, then, we
calculate the quantity contained in a litre of sea water
from the expression on p. 147 we shall find it to be about
0*5 CCS. whereas sea water usually contains about 50 c.cs.
Thus sea water dissolves far more CO 2 than if it were a
saline solution strictly neutral in its reaction.

The Alkalinity of Sea Water. — Here we must consider
why sea water can take up this relatively enormous
quantity of CO 2 and, first, we must make it clear what
is meant by the " alkalinity." When an alcoholic solution
of phenol-phthalein is added to sea water a pink color-
ation may be produced, and this is the indication of
an alkaline reaction. To destroy the pink colour, acid

THE CHEMISTRY OF SEA WATER 149

must be added, and one or two c.cs. of centi-normal hydro-
chloric acid are usually required. This quantity of
standard acid that is necessary in order to make sea
water neutral to phenol-phthalein as an indicator is a
convenient, though rough measure of the alkalinity.*

The hydrogen-ion-concentration is, however, a much more
precise measurement of the reaction of the water. The
purest water obtainable by repeated distillation in insoluble
glass vessels may be regarded as neutral in reaction. It
is dissociated to a very small degree, that is some water
molecules, H^O, are split up into their ions, hydrogen-ion,
or H*, and hydroxyl-ion, or HO', and the symbol ^h is
used to represent the potential, or concentration of the
hydrogen-ion. ^h for pure water has the value 10-^, or
1/10^, and this represents the quantity of hydrogen-ion,
measured in grams, contained in a Htre of water : obviously
it is exceedingly small. Since every hydrogen-ion corres-
ponds to an hydroxyl-ion the concentration of the latter
is represented by the same value : that is ^oh = 10"'^.

Pure water, then, contains as many H-ions as it does
OH-ions and so its reaction is neutral. If there is an
excess of H-ions the water becomes acid and if OH-ions
are in excess the reaction is alkaline. Now, since sea
water usually has a distinctly alkaline reaction the
OH-ions must be in excess.

To what substances in the state of dissociation is the
excess of OH-ions due ? This is a very difficult problem
and the matter is not yet fully understood, yet the
discussion turns on the dissociation of the carbonates
present in solution and we conveniently consider it in
connection with the question of the COg in solution. Now
although sea water contains some 40 to 60 c.cs. of CO 2 per
litre (that is, that volume of the gas can be extracted from
a litre of sea water, the quantity of COg dissolved,

* The alkalinity may be negative. One or more c.cs. of N/10 alkali
may have to be added to sea-water in order to produce the colour
change in phenol-phthalein.

150 AN INTRODUCTION TO OCEANOGRAPHY

in the " free " condition (like the nitrogen is dissolved,)
i s only a few tenths of a c.c. The rest exists in combination
with normal carbonates of magnesium and calcium,
forming the hydrogen, or acid carbonates MgH2(C03)2
and CaHy (003)2. These are much more soluble in sea
water than are the normal salts, MgCOg and CaCOg, yet
the solution is not so simple as the above sentences indicate :
in the water the various substances are ionised to an
extent that depends on the dilution. Not only have we
(say) the hydrogen calcium salt, but also the normal salt,
the Ca-ions, the bicarbonate-ion, HCO3, hydrogen-ion
and the free CO 3. The state of equilibrium, that is the
relative quantities of the various undissociated salts and
ions present, varies with the physical and chemical con-
ditions.

If the equilibrium is disturbed, the reaction of the solution
will change also. If, for any reason, lime is removed, say
by absorption by calcareous organisms, such as corals,
or molluscs, or algae, then HCOg-ion will be increased and
the alkalinity will decrease (or the acidity increase). If
the CO 2 be removed — as by the carbon-assimilation of
marine plants — then OH-ion will be increased. The
plant cannot usually get enough CO 2 simply from that
which exists in simple solution as such, but as fast as this
is removed HCOg dissociates into CO 2 and OH-ion and
the increase of the latter is the reason for the increased
alkalinity. As fast as the HCOg dissociates CaH2(C0g)2
will also dissociate into CaCOg and HCOg in the effort to
maintain the equilibrium. Thus carbon-assimilation by
green plants first removes the free CO 2, then leads to
dissociation of HCOg and lastly to dissociation of the
hydrogen calcium salt. The end phase must be CaCOg
and OH-ions. The water becomes the more strongly
alkaline. Now the equilibrium is never attained in
actuality. Pure water, we have seen has a ^h value =10""'^
and ordinary sea water has the values 10'"^ to 10~^'° As
the dissociations due to carbon-assimilation proceed the

THE CHEMISTRY OF SEA WATER 151

reaction tends towards the value about 10~^. (Note that

these also mean -,-ir ^tk,, and ,~- in terms of H-ions.)
10^ 10^ 10**

Thus the proportion of H-ion in a volume of water decreases,

or the quantity of OH-ion increases and so the alkalinity,

as measured say by the number of c.cs. of centi-normal acid

necessary for neutralisation, increases.

Photosynthesis and Carbon dioxide. An enormous
quantity of COg is annually withdrawn from solution in
the sea. In the process of carbon-assimilation COg and
HgO are taken into the tissues of a green plant and
are synthesised to form a carbohydrate. The free energy
of the latter substance is much greater than that of the
CO 2 and OH 2 and so there must be an absorption
of energy during the synthesis. It is known now that
this is obtained from sunlight and that it is utilised to
activate the CO 2 and OH 2 molecules which therefore enter
into combination. The pigments of the green plant act
as catalysts, intercepting and transforming the incident
sunlight. For some reasons not yet understood, either
the sunlight has a greater energising power during the
months March to May, or there are necessary substances
in sea water, other than CO2, which are more abundant
in those months than at other seasons, or there is a rhythm
of constructive activity inherent in the plant organisms.
Anyhow, photosynthesis — that is the production of
carbohydrate by green plants — is far more active, in the
sea, during the spring months than it is at any other time
during the year.

During those months, then, there will be a tendency to
upset the average CO 2 equilibrium. As the activity of
the marine plants — the Algae, Diatoms and Peridinians —
becomes the more intense, first the " free " CO 2 will be
used up, then HCOg-ions will dissociate and lastly the
acid magnesium and calcium salts will also dissociate,
approaching the phase in which all the magnesium and
calcium are contained in the form of the normal salts.

152 AN INTRODUCTION TO OCEANOGRAPHY

But before that occurs the conditions for continued intense
photosynthesis will have become rather unfavourable,
or the other necessary substances will be used up to a
significant extent and will not have been regenerated.
The process of intense reproduction of the green plants
will thus tend to its normal rate. But the alkalinity of
the sea will have increased. It is difficult to believe that
this increase of alkalinity is the reason why the rate of
reproduction of the Diatoms (say) falls off so rapidly
during the late spring, or early summer months and it is
very probable that we must assume a rhythm of reproduc-
tion which is nearly independent of external conditions.
There are arguments for this conclusion.

Experimental work shows that photosynthesis by
marine algae tends towards cessation as the ^H-value of
the sea water approached 10"". Take the normal value
as about 10~^ and there are data for rough calculations as
to the quantity of CO 2 that may be withdrawn from solu-
tion in the sea and incorporated into the living tissues of
marine organisms. Even a reasonably minimal estimate
gives about 8'8 milligrams of carbon taken from the water,
per litre of volume, during the months December to May.
Expanding this we find a total production of carbohydrate
of the order of magnitude of 10 tons of wet vegetable tissue
per acre of sea surface. The complexity introduced into
the conditions of chemical and biological equilibria in the
sea by this change are easy to indicate but quite impossible
to specify, even approximately.

Variations in H-ion Concentration in the Sea. Lately
the estimation of the ^h values of sea water has
been practised in a routine way as part of systematic
hydrographic investigations. So far the results are not
of great importance either from the biological or physical
side and the method promises more when applied to very
localised problems than to the study of a large sea area.
Nevertheless some of the more recent results may be
quoted here. The variable that is estimated is ^h and

THE CHEMISTRY OF SEA WATER 153

for convenience we may express it as the logarithm of the
reciprocal of the H-ion concentration. The latter is, we

have seen — -^ gram per litre for pure water and so ^h

is the index of 10, that is 7. The following data are
those obtained in a study of the conditions in the neigh-
bouraood of the Plymouth Laboratory of the Marine
Biological Association.

(1) In the open sea at the mouth of the English Channel
^H had the mean value, 8 "20 (the full expression

is .^q). The variations were roughly, 8'27 to 8"14.

(2) Round Lands End, into the mouth of the Bristol
Channel, the values ranged from 8*18 to 8*14. The
mean was about 8' 16. Here we see the influence
of proximity to the land.

(3) In Plymouth Soimd ^h varied between about
8 and 8'2 the mean may be taken as 8' 10. There
was a decrease between high water and low water
of about 0*05. The conditions on the foreshore
and in shallow water — that is, a greater quantity
of marine life and an amount of contamination due
to decomposing organic matter increases the CO 2,
and therefore the acidity — or conversely the alkal-
inity is diminished.

(4) In the Aquarium tanks. Here the water is much less
alkaline. The ordinary -^h value is 7 '6. Plant
activity is decreased while respiration by animals
tends to increase of CO 2 — there is relatively much
organic matter in solution and this decomposes by
the activity 01 micro-organisms, again increasing
the acidity — the quantity of COg-ion. At Ph=7'6
CO^ is abnormally in excess. At Ph=7'3 fishes
show signs of respiratory distress, for CO.^ is much
in excess.

At 7*1 the water is foul and smells badly.

154 AN INTRODUCTION TO OCEANOGRAPHY

(5) In a jar of sea water without circulation and renewal,

decomposition processes proceed to their limit — that

is, to the resolution, by bacterial activity, of the

carbohydrates into CO.^ and OH.^. The % values

approach the extreme figure of about 6'4. The

water is definitely unfit for animal life. Nevertheless

strong agitation by a current of air will remove the

excess of CO.^ and raise the ^h- value to about 8"0.

Variations in ^h indicate, then, the varying balance

between the processes of photosynthesis by plant life,

respiration by animal life and decomposition of organic

matter by micro-organisms. Add to these renewal of

the CO.^ or its removal by agitation of the sea water.

Distribution of dissolved gases in the ocean. — The
general distribution only is known and that alone with
respect to oxygen (which is more easily estimated then
CO^ or nitrogen). From what has been said above,
the reader will be prepared for the following rough state-
ment, which refers to the Atlantic Ocean. It is not very
precise but the local variations are probably so great that
no precise statement is possible.

The average quantities of oxygen are given for the
various zones of latitude and depth as cubic centimetres
per litre of sea water in situ.

0-500 m. 500-1000 m. 1000-5000 m.
N. lat. 60°-30° . . 4 to 7 c.cs. 3 to 6 c.cs. 4 to 6 c.cs.
N. lat. 30°-00° . . 1 to 5 c.cs. 1 to 4 c.cs. 2 to 5 c.cs.
S. lat. 00°-30° . . 1 to 6 c.cs. 1 to 5 c.cs. 3 to 5 c.cs.
S. lat. 30°-50° . . 4 to 8 c.cs. 4 to 6 c.cs. 4 to 5 cos.

Thus the water at the surface generally contains more
oxygen than the water at the bottom. This is because
it is exposed to the air. If, however, the water at
the bottom were within access of oxygen it would
contain more because, at a lower temperature, it
could hold more in solution.

Also the water near the poles contains more oxygen than
that near the equator. This is because of the much
lower temperature of the sub-polar seas.

THE CHEMISTRY OF SEA WATER 155

In general, sea water contains less oxygen than it might
hold in solution in the given conditions — that is, it is
under saturated. The deficiency from saturation is, in
geneial, less at the surface than at the bottom. But it
may easily become saturated in experimental conditions,
or n locaUsed regions where intense photosynthesis by
ma.'ine plants goes on. It does actually become super-
sa^rated in the sea, exceptionally, it is true. Super-
saburation with oxygen is known to give rise to a definite
fo-m of disease in fishes inhabiting fresh water. There
tie water receives so much oxygen from plants that,
gtiould a sudden rise of temperature occur, the state of
upersaturation would follow and the gas would tend to
oecome liberated from solution. A fish has the same
temperature as that of the water which it inhabits, and
its intake of oxygen will depend to some extent on the
concentration of the gas in the water. With super-
saturated water an unusually large quantity of oxygen
may be absorbed by the blood and then the latter, itself,
may become saturated at a given temperature. Should this
rise significantly, oxygen may be liberated from the blood,
forming visible bubbles, or gas-emboUsms in the vessels.
Deficiency of oxygen, that is, a defect well below the
quantity to be expected from such a general statement
as that of p. 154, may easily occur as the result of a lack
of circulation in t\ie water. Such a defective circulation
occurs, on a small scale, in some of the Norwegian fiords
and, on a large sca'ie, in the Black Sea. In both cases
the communication \tith the ocean is restricted by sub-
marine elevations reseiabling dock sills. At the entrance
to tbe fiords the water is often very shallow, while it is
relaiively deep at the 'jiner parts. The Black Sea is
moderately deep but its entrance (the Sea of Marmora)
is shallow. In both cases, then, there is stagnant water
near the bottom. The oxygen is used up in a variety of
ways and is not renewed, and sulphuretted hydrogen,
resuling from organic decoraposition, takes its place.

156 AN INTRODUCTION TO OCEANOGRAPHY

Nitrogen will be distributed in the same general way as
oxygen, that is, the quantity in solution is a function of
the temperature and pressure, and will, on the vhole,
be less at the surface and in the equatorial regions, than
at the bottom and in the sub-polar and polar regons.
The gas, however, is chemically inert and it can onl} be
withdrawn from solution by the activity of nitrogen-fixing
bacteria. We have no good data as to the distributon
and intensity of action of these organisms. Nitrog»n
is present in solution in something like the order ♦f
10 to 12 c.cs. per litre of sea water. The latter is nearly
always saturated with nitrogen at the surface and, therefore,
a large mass of water falling from the surface to a botton
stratum of much colder water must become super-saturated
with this gas.

The indispensable food substances ; vital production in the
sea. — Certain chemical compounds in solution in sea water
are of extraordinary significance with respect to marine
life. These are carbon dioxide, salts of ammonia, salts
of nitrous and nitric acid, " albuminoid substances " (that
is amino-acids), organic carbon compounds (volatile and
other acids and basic substances), sihcic acid, and salts
of phosphoric acid. All these are indispensible to life.
All of them, except CO^, occur in extremely small quantities
and are variable in abundance from place to place and
season to season. Their estimation in sea water is extra-
ordinarily difficult, and far too little work, in this direction,
has yet been done. It is, howevei', essential that the
general nature of the results so fgr obtained should be
indicated here.

Vital 'production in the sea. Animal fife in the sea
and on the land depends absolutely on the supplj of
food substances in the forms oi proteids, carbohydiates
and fats. Such materials cin only be obtained :'rom
the bodies of plants and animals. Animal fife, therefore,
subsists on animal and plant life. The hmit to vhich
animal processes tend is the utiHsation of the avalable

1

THE CHEMISTRY OF SEA WATER 157

energy of the food substances mentioned, and the
ultimate reduction of these materials to the stages
of carbon dioxide, water and amino-acids. The amino-
acids are then further reduced, by micro-organisms, to
the stage of nitrates. When this limit (organic matter
to CO2, OH^ and nitrates) is attained the food-materials
can no longer be utilised by animals and are thrown out
of circulation, so far as these organisms are concerned.
The general process of animal metabolism is one of energy-
dissipation. The substances involved are chemically
degraded, and pass into their most stable conditions,
while the energy involved becomes unavailable. Physically
the processes lead to augmentation of entropy. The
animals are, therefore, the " consumers " of the sea.

Plant life in the sea tends to reverse the processes indi-
cated above, since it can utilise the chemical substances
(CO.^, OH.^, nitrates, etc.) that are no longer available as
food for the animals. So far as we know the typical, green,
unicellular or multicellular plant cannot utilise energy in
the form in which it is unavailable for the animals.
Possibly micro-organisms may do so by making use of
the Brownian movement of small particles. Thus syn-
thesis of carbohydrates can be effected, in the absence of
visible radiation^ by certain bacteria. The plant can,
however, arrest the process of dissipation of solar energy
by using the latter to activate the CO.^ and OH^ molecules.
Otherwise inert, these molecules can now enter into com-
bination, with the result that formaldehyde and then
carbohydrates are synthesised. Somewhat similar syn-
theses must occur so as to form amino-acids from nitrates
and other mineral nitrogen compounds, together with
the synthetic carbohydrates.

In the ordinary course of events, then, marine plants
(that is, the large algae and the minute Diatoms, Peri-
dinians, etc.) arrest the process of dissipation of solar
energy. The latter tends, in the absence of light, to
transform into low temperature heat, which becomes

158 AN INTRODUCTION TO OCEANOGRAPHY

unavailable inasmuch as it is uniformly distributed. In
photosynthetic plant metabolism, the dissipating solar
energy becomes stored, in the potential form, as the
carbohydrates and proteids of plant tissues. This is
vital production.

For its continuance there must be, in the sea, the various
substances mentioned at the head of this section.

We have already considered the carbon dioxide. Its
abundance in the sea is limited by an equilibrium between
sea, and air. The equilibrium is always being locally
and temporarily disturbed. When CO.^ is in excess (as in
sea water containing decomposing organic matter) it
becomes returned to the atmosphere. When it is deficient
(as when photosynthesis, in the spring and in shallow
shore pools proceeds at an unusally rapid rate) CO^ is
absorbed from the atmosphere. Now the supply in the
latter and in the open sea is practically unlimited when
all the conditions are taken into account. It must vary
throughout geological periods. Excessive emergence of
the land means a more rapid rate of weathering of rocks,
the decomposition of silicates and the removal of CO^
from the air. Converse conditions will occur when the
land is, on the average, low and when subaerial denudation
is minimal. Such conditions have alternated in the past
and so has the quantity of CO,, in the air (and by conse-
quence, in the sea). Upon these alternations in quantity
of CO,^ may depend climatic changes, because of the
heat-absorbing property of the gas.

The Nitrogen compounds. These are salts of ammonia
and salts of nitrous and nitric acids, with " albuminoid
ammonia." Their origin is partly the decomposition of
proteid matter arising from dead bodies, or the excretions
of marine organisms, and partly substances of the same
nature draining down into the sea from the land. In addi-
tion to these sources of mineral nitrogen, some comes from
the atmosphere where it is formed from elementary
nitrogen and oxygen combined by electric discharges.

THE CHEMISTRY OF SEA WATER ]59

Some also comes from the activity of nitrogen-fixing
bacteria on the shallow sea bottoms. These organisms
are able to oxidise elementary nitrogen, first to nitrous,
and then to nitric acids. As the result of all these
processes there is an excessively small quantity of
mineral nitrogen compounds in any volume of sea water,
say a litre, that can conveniently be submitted to analysi?.
The nitrates are the stable forms and are rather more
abundant than the albuminoid ammonia, which is a
temporary stage in the breaking down of proteid matter.
Ammonia, as such, is much less abundant than either
of the other two kinds of nitrogenous material.

The quantities are very small and are expressed as
parts per million (milligrams per litre) of sea water.
The following data are rough means from many
estimations : —

Water from the Antarctic contained about 0"5 part

per million ;
Water from the Indian Ocean contained about 005 —

0.02 part per million ;
Water from the Equatorial Seas contained about

0*1 part per million ;
Water from the Mediterranean region contained about

0"2 part per million ;
Water from the North Atlantic contained about 015

part per million ;

of nitrogen in the form of nitrite, nitrate, ammonia and
albuminoid ammonia. On the whole it appears to be
greatest in the Antarctic Seas, where the temperature is
minimal ; more abundant in the North Atlantic, where
the temperature varied between about 5° to 10°C. and was
least in the equatorial seas where the temperature was
over 28°C. To this relation between temperature and
quantity of inorganic nitrogen, we return later.

There is also a seasonal variation which, however, is
not very distinct. The results of a great number of

Mar.

April

May

June

July

200

125

136

160

60

92

71

134

161

112

34

21

66

42

50

Aug.

Sept.

Oct.

Nov.

Dec.

84

166

105

84

138

_

156

118

110

130

63

63

43

60

43

160 AN INTRODUCTION TO OCEANOGRAPHY

analyses made in the Baltic and North Seas by the German

fishery investigators give the following results : —

Feb.

Nitrate 135

Albuminoid Ammonia . . —

Ammonia . . . . . . 62

Nitrate

Albuminoid Ammonia

Ammonia

The numbers represent parts of the substances in 1000
million parts of sea water, or grams per cubic metre of
water. The range of variation of the total nitrogen
represented in the same way as is the salinity of sea water
is from 0-000387oo to 0-00015%o. Upon these excessively
small quantities of combined nitrogen depends all vegetable
life in the sea. Looking at the results in the most general
way we see a tendency for the quantity of combined N
to attain a maximum in the winter and spring and to fall
to a minimum in the summer and autumn. The seasonal
variation is, however, indicated rather than definitely
demonstrated by the data at our disposal.

The Silicic Acid. Sea water contains exceedingly small
quantities of colloidal silica in solution. The origin of
this substance is the silica brought down from the land
in the water of rivers, but it may also result from
bacterial action on the fine particles of silicates suspended
in the water. A series of analyses carried out by the
German oceanographers give the following results : —
Feb. Mar. April May June Aug. Sept. Oct. Nov.
796 683 650 659 915 794 905 1000 942

Those are parts per 1000 million parts of sea water.
Expressed in the same way as the salinity of the water
we get the range of variation, 0"001 to 0'00065 %o.

The origin of the Oceanic Salts. — Why has the ocean its
present saline composition and concentration ? To this
question no satisfactory answer can be given so long as
the problem of the origin of the earth itself remains
unsolved. Obviously the assumption of either of the two

THE CHEMISTRY OF SEA WATER 161

main alternative hypotheses — the gaseous- molten and the
planetesimal ones — will lead to modes of treatment of
our question that must be very different.

Further, the problem is bound up with that of the age
of the ocean-carrying earth itself and that is a question
that seems to become more difficult as the investigation
is pressed. The moderate estimate of 40 to 100 millions
of years, based by Sir William Thomson on an assumed
original temperature for a molten earth crust and a
certain mean rate of cooling, has now been definitely
abandoned. Recent methods based on the study of the
residues of radio-active substance in the earth's crust
promised well for a time, but the serious discrepancies
between the estimates of the quantities of helium and
lead in the rocks make this method also an unsatisfactory
one. No value more definite than a " small multiple of
a thousand million years " seems to have been obtained
from the line of investigation.

Joly's estimate,, based on the quantity of sodium in
the ocean is of interest in relation to the subject of this
book. The total quantity of sodium in the ocean can
be found approximately : it is about 15,000 X 10^ '^ tons.
The quantity of sodium entering the sea via the rivers
can also be estimated : this is about 157 millions of tons
annually. Joly assumed that the primitive ocean contained
about 14% of the sodium now present in it and that this
was originally precipitated with the water of the first
atmosphere. Some of the salt entering the ocean from
the rivers is cyclical : that is, it is removed from the sea
by winds carrying surface spray on to the land, or it may
become imprisoned in sediments forming on the sea bottom.
When these become continental land and suffer denudation,
the salt is returned to the ocean.

Assuming, then, that the ocean obtained its salt (with
the above qualifications) from sodium chloride dissolved
out from the rocks of an original earth-crust formed by
solidification of an originally liquid earth-body, we get

162 AN INTRODUCTION TO OCEANOGRAPHY

an age for the ocean of about 90 millions of years. There
are various sources of error which may make it necessary
to quadruple this estimate, but these need not be considered.
The much greater (though strikingly divergent) values
deduced from the study of radio-activity puts the whole
matter on another plane. In any case our cosmogony
is still too undeveloped to permit of a discussion that
would be very useful.

Obviously there are two main hypotheses of the origin
of the salts of the ocean : (1) that these were there from
the beginning, being dissolved out from the planetesimal
materials that cohered to form the nuclear-earth by the
first accumulations of water. It is known that meteoritic
materials do contain chlorides and it is possible, then that,
we have here an origin for the saline matter in solution
in the sea. (2) The salts came partly from condensation
of saline materials in the original heavy atmosphere that
remained behind after solidification of earth-crust from
a molten condition and partly from the slow addition of
salts dissolved out from the land surface by rain and then
brought down to the sea by rivers.

But either the materials thus reaching the sea were
different in the past than they now are or there have been
processes at work that have made the composition of
ocean salts different from that of river salts. At present
the two dissolved mixtures are not the same : in the
ocean we have

CI ) SO^ ) CO3 and Na > Mg > Ca

while just the opposite distribution exists with regard to
the salts of river water, that is,

CO3 ) SO^ ) CI and Ca ) Mg ) Na
Something, then happens in the sea to change the ratios
of abundance of the various substances that are added
to it by river water.

Certainly calcium is, and has been, removed from
solution in all parts of the ocean except at the greatest

THE CHEMISTRY OF SEA WATER 163

depths. It accumulates as deep-sea oozes formed by
the tests of Foraminifera and the shells of molluscs ; as
massive coral reefs ; as the very abundant neritic deposits
of the past and present (molluscan shells, Echinoderm exo-
skeletons, polyzoan skeletons, etc.) ; and as chemical
precipitates. Magnesium is removed from solution in much
the same way. Sihca goes out of solution to form radio-
larian and diatom tests and the spicules of sponges, and
accumulates as bottom-oozes, flints, etc. Iron and man-
ganese go to form concretions, and iron becomes locked
up as a coating over quartz-sand particles. Potassium has
perhaps accumulated in this way. Potassium has also since
it goes out of solution as the synthetic mineral glauconite
and it appears also to be removed from the sea in certain
clayey sediments. In the course of time enormous quanti-
ties of nitrogen and carbon compounds must have entered
the sea in the form of organic matter resulting from the
decomposition of the tissues of plants and animals, and
yet the quantity of these substances in solution is ex-
ceedingly small while they do not accumulate as insoluble
deposits. The nitrogenous organic matter is sooner or
later oxidised by bacterial agents to the form of nitrate,
but some part of this is always being reduced to nitrates,
ammonia and finally elementary nitrogen by denitrifying
organisms. The gas then escapes into the atmosphere.
Organic carbon compounds are also broken down by
bacteria, with the appearence of CO^ as the end product.
This also can escape into the atmosphere. Sulphur can
also reduce to SH^ which can escape from solution. Thus
we can account for most of the elements present in solution
in the sea : they either pass into insoluble compounds and
so form deposits, or they may assume the gaseous state
and be liberated into the air. They are continually being
added to the sea, but it seems probable that a condition
of equilibrium has been established or at all events, that
the composition of sea-salts change, very slowly indeed.
Only the sodium is difficult to account for, and it seems

164 AN INTRODUCTION TO OCEANOGRAPHY

likely that this element is also being removed from solution
in some way not yet known, for the tendency of modern
investigation is greatly to lengthen the period estimated
by Joly as that of the duration of the ocean. If, as seems
probable, these increased estimates of the age of the sea
will be confirmed by future work, it can hardly be doubted
that sodium is being removed in some way for we have
no reason to doubt that the present rate of addition of
salt to the sea by river water has been maintained during
past geological time.

CHAPTER VII
THE PHYSICAL CHARACTERS OF SEA WATER

The movements of the ocean, that is, the tides and tidal
streams ; convection currents ; surface currents and
drifts due to heating, concentration and dilution of the
sea water, etc., are considered in Chapters VIII and IX
and here we have to deal with those other physical char-
acters with regard to which the sea differs from ordinary-
water examined in such masses as can be handled in the
usual laboratory operations. In considering any volume
of sea water in situ we have to take account of several
variables : the hydrostatic pressure, the temperature and
the salinity. The pressure may be that due to the weight
of a column of saline solution varying between 0 and about
10,000 metres in height ; the temperature may be any
between 0°C. and 28°C. and the saline solution may be
one containing from 0 to 3^% of dissolved substance.
Any physical characteristic of water, when applied to the
sea, is in general, a function of these three variables. Its
pressure, viscosity, electrical conductivity, sound con-
ductivity, density, colour, surface tension, vapour pressure,
osmotic pressure, freezing point, expansion, etc., depend,
in general, upon the hydrostatic pressure, the temperature
and the salinity. The whole subject is exceedingly
complex and the experimental research necessary is by no
means complete. As a general rule empirical, or interpol-
ation formulae, based on values of the various functions
obtained experimentally, have been devised and these,
with tables, are given in the literature of oceanography.

The Pressure. — At any point in the sea the pressure
is that due to the weight of the column resting on the
surface of unit area, say a square cm. To this must be

166

166 AN INTRODUCTION TO OCEANOGRAPHY

added the weight of the column of air of the same cross
section resting on the surface of the sea. The weight of
a water column of the same height and cross section
will vary according to the place on the earth's surface
considered, being, in general, greatest at the equator
and least at the poles. We assume that this water column
has everywhere the same salinity and temperature but
this is, of course, not the case. The salinity may be
approximately 0 or 38 or any intermediate value and the
temperature may be anything between 0°C, and 28°C.
The mass, then, of a column of sea water of certain
dimensions varies with its chemical and physical con-
figurations, and the weight of this column of specified
dimensions and physico-chemical configuration varies with
its position in latitude on the earth's surface.

Given a mean salinity and temperature and a specified
latitude the pressure at any required depth cannot be
calculated simply from the depth itself. Distilled water
is compressible, though the amount of contraction of
volume with increased pressure is very small. It is,
nevertheless, great enough to be of much significance in
hydrographic investigations and it becomes a factor of
importance when such questions as the conductivity of
sound waves, or the density of water at any specified
depth are considered. How significant it is when we take
account of the actual depth of the ocean may be grasped
when we note that the sudden annihilation of the property
of slight compressibility of sea water would increase the
volume of the sea by about 11 millions of cubic kilometres
and raise its level by about 30 metres. Such a rise of
mean sea level of 15 fathoms would alter the outlines of
the land surface in a noticeable degree.

The compressibility coefficient of sea water is a number
showing what is the diminution of volume when the
hydrostatic pressure has a certain value. For a mean
salinity and temperature the contraction of volume is
given, for the pressure due to a water-column of one metre

PHYSICAL CHARACTERS OF SEA WATER 167

height, by the fraction 0"00000466. That is to say, the
volume of, say, 1 Utre of water at the depth m metres
would be 1-0-0000466 m.

The pressures at the bottoms of deep oceans are therefore
very great. Roughly they are given by rather over a ton,
per square inch of surface exposed, for each 1000 fathoms.

0

I

looo
Sooo
Zooo

IfOOO
f)000

looo

\

^

tooo

\

\,

'3000

IhOOO

\

\

5ooo

tooo

£

\

\

looo

8ooo

\

s

90C0

loooo

V

re s

su r

e

\

/\00 £pO 5

a? 4^ Sfo 6

oo jpo 6^0 9

oo jdfx

^BS^V

/I 2

s\ ^

5\ 6

7

Tons

Fig. 24. Graph of the hydrostatic pressure of sea water at the depths
0 to 10,000 metres.

The Temperature. — The temperature of the sea is due
almost entirely to solar radiation. Even if we assume a
very high temperature in the deep interior of the earth,
it appears to be the case that whatever quantity of heat
would be transmitted from this store, through the
lithosphere, to the overlying ocean is negligible. It is
known that the bottom deposits are slightly radio-active
and must, therefore, generate heat, but this also may
be neglected. In the absence of solar radiation the ocean

168 AN INTRODUCTION TO OCEANOGRAPHY

would freeze and its temperature every where- on the earth
would be that of cosmic space — only a degree or two
above absolute zero. As it is the temperature of the
unfrozen water varies between about — r3°C., at the
bottom of polar seas, to about 28°C. in the tropics.

Estimation of the oceanic temperature. At the
" surface " there is no difficulty, for a sample of water
is simply withdrawn by a bucket and its temperature
is read by a thermometer graduated in tenths of a degree
centigrade. By the " surface " is meant the stratum of
water to a depth of about a foot : we shall see later that
there may be a considerable temperature-difference even
in the upper stratum of sea to the depth of one foot.

In the deep sea the estimation of temperature is a matter
of much greater difficulty and many forms of apparatus
have been devised for this purpose. Nowadays the
estimation is always made either by a reversing thermo-
meter, or by an insulated water-bottle. In the reversing
thermometer there is an S-shaped bend in the capillary
tube just above the bulb and somewhere in the neighbour-
hood of this bend there is a constriction. The instrument
is enclosed vertically in a frame, with its bulb downward,
and when the frame has been lowered to the required
depth a weight is allowed to slide down the wire rope and
release a catch, whereupon the frame turns upside down.
The mercury column in the thermometer has meanwhile
contracted or expanded, as the case may be, but when it is
turned upside down the thread breaks away from the
mercury in the bulb at the constriction in the stem. The
latter is graduated from the top downwards, and from the
volume of the column broken off the temperature is
dftHuced. Several corrections have to be made.

The insulating water bottle now generally used consists
essentially of a central chamber of about 300 c.cs in capacity.
The walls of this chamber are formed by several concentric
cylinders of zinc and ebonite with annular spaces of about
^ cm. in thickness. The vessel is represented in diagram,

PHYSICAL CHARACTERS OF SEA WATER 169

in Fig. 9, (3), and it will be seen that when it is close
the central chamber containing the water sample is insu-
lated by three concentric shells of water of the same
temperature and by three water partitions at top and
bottom. The thermometer is so placed that its bulb is
immersed in the water of the central chamber so that
it assumes the tempi jrature of the latter. The apparatus
is lowered to the required depth, and then a weight is
allowed to slide down the wire and on reaching the bottle
a catch is released and the ends shut down into the top
and bottom of the multiple cylinder, closing the water
sample chamber and the insulating water- shells. During
the time when the apparatus is being hauled up to the
surface there is no significant conduction of heat into the
water sample from a warmer sea outside, nor any con-
duction of heat from the sample to a colder sea outside.
The temperature is read on the thermometer and then
the water contained in the central chamber is run down
into a bottle and is preserved for analysis.

At considerable depths the water which is enclosed
within the chambers of the bottle is under pressure and
is compressed. When the apparatus is lifted to the surface
the pressure diminishes from, it may be, several hundreds
to one atmosphere and the water in the central and insu-
lating chambers expands. So also does the material of
the apparatus. This expansion involves cooling of the
expanding materials of the bottle and of the water sample
itself, but the latter is heat-isolated. The expansion is
therefore adiahatic and so the temperature read on the
thermometer is lower than that of the water sample when
it was in situ. At moderate depths, down to, say, 1000
metres the reduction in temperature due to adiabatic
expansion of the water sample may usually be neglected.
In the case, however, of water brought up from the greatest
depths the reduction may be as much as r3° to r4°C.

At Ordinary depths the pressure is so great that the
apparatus used must be constructed so as to resist it.

170 AN INTRODUCTION TO OCEANOGRAPHY

Ordinary thermometers, such as are used for taking surface
temperatures, may be crushed to fine powder when lowered
to depths of much over 1,000 fathoms. In any case the
thin-walled bulbs must be protected. The thermometer
itself is always enclosed in a glass tube which has thick
and strong walls and hermetically sealed ends. This is
evacuated of air so that the insulation of the stem may
be as complete as possible. The section of the enclosing
tube containing the thermometer bulb is shut off from the
rest and contains some mercury which conducts heat
through the space between jacket and bulb.

The temperature variations. Suppose that the earth
were to revolve round the sun in the plane of its
equator, that is, suppose the ecliptic and equinoctial
planes were coincident. Suppose also that the ocean
were to cover the earth everywhere to an uniform
depth. The precession of the poles may be neglected
for we can assume that the earth's axis remains
parallel to itself throughout any period of time that we
need consider. In these very simple conditions the
temperature of the ocean would vary in a very regular
way : it would be greatest at the equator, where the
sun's rays fall perpendicularly on the surface of the sea,
and it would be least at the poles, where they would fail
most obliquely. At the equator all the solar radiation
would either penetrate, or be absorbed by the atmosphere,
but somewhere between equator and poles a certain fraction
would be reflected by the upper layers of the atmosphere.
This fraction would be greatest at the poles. Everywhere,
then, the temperature of the ocean would be a function
of the latitude decreasing regularly from equator to poles.
The isotherms would be parallel to the lines of latitude.

In fact the equinoctial is inclined at an angle of 23|°
to the plane of the ecliptic. That means that the sun's
declination varies throughout the year so that he may be
overhead at the equator, or at latitude 23|° S. or N. At
any one place, then, the declination varies and so the

PHYSICAL CHARACTERS OF SEA WATER 171

amount of solar radiation received per unit of area of the
ocean also varies. This means that there will be an
annual period of temperature variations, the maximum
occurring about the time when the sun's declination is
greatest and the minimum occurring about the time when
it is least. There would be, of course, a lag, the change
of temperature following the change of declination.

There would be also a variation in the temperature
in the course of the year at any particular place beneath
the surface : as the temperature at the surface varies so
do those at various depths beneath — though the range
of variation there may be very much smaller than it is
on the surface. Water is a very bad conductor of heat,
but differences of temperature between the equatorial, and
the zones of higher latitude set up convection currents and
the intensity of these at any one zone would undergo an
annual variation. To this matter of convection currents
we return in the next chapter.

So far we have assumed that the depth of the ocean is
uniform and that it covers the earth everywhere. In
fact the highly irregular distribution of land and water
introduces irregularities of a very complicated nature.
So does the variable depth of the ocean. The result is
that the isothermal lines nowhere run parallel to those
of latitude though we can see an approximation to this
postulated parallelism in the Pacific and Southern Oceans.
Neither is there any very regular variation of temperature
according to depth, because of the way in which the con-
vection currents are modified by the irregular depths and
the distribution of the land masses.

Thus there are periodic annual changes of temperature
both at the surface and in the depths and these are to be
traced to the variations in declination of the sun, with,
of course, changes in the distance of the earth from the sun.
At present the earth is in perihelion (is nearest to the sun)
in the northern winter and ought to receive more heat
then. The effect of varying declination at any one place

172 AN INTRODUCTION TO OCEANOGRAPHY

is, however, greater than that of varying distance between
earth and sun.

That there are periodic changes of greater frequency
than one year is evident from the common experience
that the minima and maxima of sea temperature are not
of the same value from year to year nor do they occur
at the same times in every year. As to the causes of these
long periodic variations there is much difficulty. It
appears to be certain that the sun is a variable star, and
it is made probable, by a study of the frequency of occur-
rence of sunspots, that the period is somewhere in the
neighbourhood of eleven years. If the same causes that
lead to the outbreak of sunspots lead also to changes in
the intensity of solar radiation, it would follow that there
would be an eleven-year periodicity in ocean temperature.

The establishment of such a periodicity is very difficult
because the effect of change in the intensity of solar radia-
tion is not exhibited directly in change of ocean
temperature. Of the total heat incident on the earth a
certain fraction (about 40%) is reflected away into outer
space from the atmosphere and of the remainder only
about 33% penetrates to the ocean or land^ — the rest is
absorbed by the atmosphere. This heat taken up by the
air sets the latter in motion, and the cyclonic storms and
winds so caused then lead to variations in ocean temper-
ature. Thus the connection between changes in solar
radiation and changes in ocean temperature are of a very
indirect kind and are to be traced only with great
difficulty.

Other changes are competent to set up long periodic
variations in ocean temperature. Thus the presence of
dust in the atmosphere, as the result of long continued
periods of volcanic eruptions, must by its absorptive
effects reduce the quantity of heat directly received by
the sea. The same effect would follow a significant increase
in the quantity of carbon dioxide contained in the atmos-
phere since this also has a heat-absorbing effect. Such

PHYSICAL CHARACTERS OF SEA WATER 173

an increase in the CO.^ of the atmosphere would result
from the transgression of the sea on the land, thus leading
to a smaller surface of rocks subject to subaerial denu-
dation : there would be less decomposition of alkaline
siUcates by the CO.^ of the air and so the proportion of
the latter would increase. If the latter proportion were
largely to increase, would it be absorbed by the ocean ?
It is doubtful, because the latter is already saturated, in
the ordinary physical sense, by CO.^ and a larger quantity
could only be held were the carbonates of calcium and
magnesium in solution in sea water to increase.

If some such cause, an increase in the quantity of heat-
absorbing material in the atmosphere, were to continue
to operate over a considerable period of years, such con-
ditions as were represented in the various ice-ages would
be set up. Even a fall of only a very few degrees in the
average temperature of the atmosphere overlying the
ocean would have momentous effects in that it would lead
to a great increase in the precipitation of water, as rain
and snow, on the land, or frozen sea, in higher latitudes.
Thus the sea level would be lowered to a significant extent
and the outlines of the continental land would be changed.
The courses and velocities of oceanic currents would
probably be changed and these changes would react on
the meteorological conditions of the lands that come under
their influence. The effects we have indicated would,
most probably, be cumulative, leading to all the conditions
that characterise periods of extensive glaciation. They
might co-operate with, or tend to cancel the effects of
astronomical changes.

Thus the well-known hypothesis of the cause of the
ice-ages, due to Croll and Geikie, makes use of the periodic
change known as the precession of the equinoxes. The
planes of the ecliptic and equinoctial are not absolutely
fixed in relation to each other. They cut each other at
two points such that the earth is at one of them at the
time of the vernal equinox and at the other at the time

174 AN INTRODUCTION TO OCEANOGRAPHY

of the autumnal equinox. These points are not fixed
with relation to the stars but move round the earth's
orbit in a direction retrograde to that of the earth's path.
At present they are so placed that the winter solstice
(position of least solar declination in the northern hemi-
sphere) occurs when the earth is nearest to the sun. In
the course of a period of about 30,000 years the vernal
equinox will move completely round the ecliptic, and so
such a configuration of earth and sun must occur that the
winter solstice will occur when the earth is furthest from
the sun. Then the conditions must be such as will lead
to a reduction of temperature in the northern hemisphere
and an increase in the south. The hypothesis would be
competent to explain the succession of ice-ages, if it could
be shown that the latter occurred strictly periodically and
about the time when the astronomical events referred to
above had happened. Now the ice-ages are not periodic
in this sense. It will be seen, however, that the other
cause of variation in the average quantity of heat received
upon the sea during the year may so operate as sometimes
to co-operate with, and at other times to counteract the
effects of the astronomical causes.

The oceanic Isotherms. Let a great number of
observations of the temperature of the sea, at the
surface, be made simultaneously over a large area, say
the Atlantic Ocean : by " simultaneously " we may
mean during the same month. These observations are
marked on a chart and then all the points on the latter,
where the temperature was, say, 10°C. are joined by
straight lines. The latter are next replaced by a curved
line drawn freehand as near as possible to all the points
and having as few changes of curvature as possible. It
would be allowable to take points of (say) 9*5° or 10 "5°
as representing 10°. (But this process of " smoothing "
would depend on the scales adopted, the number of obser-
vations and their probable accuracy). In this way a mean
curve is obtained. The same process is repeated for the

PHYSICAL CHARACTERS OF SEA WATER 175

other temperatures, say 9°, 8°, etc. Such lines are iso-
therms, or lines of equal temperature.

•50 60 lO

70 60 SO

SO bo 10

TO bO SO

Fig. 25. Chart of the Atlantic Ocean showing the summer isotherms
at the surface.

Premising that we can only deal with them in a very
general way and that any amount of detail could be given
for various seas, we may now consider how they exhibit

176 AN INTRODUCTION TO OCEANOGRAPHY

the variations of sea temperature on the surface of the
ocean.

First we notice that the extreme range of temperature
is about — 1°C. to about 27°C. ; that the maximum is in
a zone lying from about 5° to 10° N. lat. ; that the minima
are in the Arctic and Antarctic Oceans, where the tem-
perature at the surface approximates to zero on the centi-
grade scale and may sink to -1°C. below the surface and,
lastly, that the isotherms do not run parallel to the lines
of latitude, though there is a very rough parallelism at
about 40° to 50° S. latitudes.

Each isotherm is a boundary, delimiting an area where
the temperature has a certain mean range of values.
Within the curved line marked 27° and the coast of
Africa, on the East, and within that marked 27° and the
coasts of Central America and Brazil, on the West, the
temperature is 27° and over that value. The two isotherms
marked 26° and 25° bound an area where the temperature
varies between 25° and 26°. This is, of course, only an
approximation, or rather an average expression of the
range of temperature variation. In each of the areas
mentioned there are certainly small patches of sea where
the temperature, during the summer months may fall
below, or rise above the limits indicated by the isotherms.

Further, there is a certain trend of the isothermal lines
up to the North- West in the North Atlantic tropical and
sub-tropical zones as far as about 30° N. Lat. Higher
than that they trend in a moat marked way towards the
North-East. About 40° N. the isotherms 5° to 20° are
crowded together in a very remarkable way and then
they diverge outwards in a fan-like manner. This peculiar
arrangement, we shall see, is to be explained by the way
in which the relatively warm water of the Equatorial
Streams is deflected northerly as it approaches the American
coast, and is then dispersed to the North-East as the Gulf
Stream Current and Drift. Also, the isotherms in the
South Atlantic are deflected to the South- West in the zone

PHYSICAL CHARACTERS OF SEA WATER 177

0° to 30° S. Lat. and for the same reason — the direction of
the South Equatorial Stream. In the South, however,
there is nothing quite corresponding to the strongly marked
Gulf-Stream circulation of the North Atlantic although
the general features of the water movement, as a whole,
are the same in both oceans.

The asymmetry of the two oceans in regard to the
distribution of temperature is very noticeable. Thus the
5° isotherm reaches the latitude of 70° in the North but
it runs nearly along the 50th parallel in the South. Nearly
everywhere there is a considerable difference in temperature
on the East and West sides of the Atlantic : thus the sea
off Labrador, in latitudes 50° to 60° N, is at, or little above
0°C. while in latitudes 50° to 60° N. on the European side
the temperature of the sea in summer varies between about
9° and 13°C. This particular difference is due to the
Gulf Stream circulation which bathes the coasts of Europe
on the East, and to the Labrador Current which flows down
the coast of N. America, on the West. In general (with
the exceptions of the regions affected by the Gulf Stream
drift) and for the same range of latitudes, the sea temper-
ature is higher on the western side of all the oceans than
it is on the eastern sides, and this is because the directions
of the Equatorial Streams are as they are because the
Earth rotates from West to East. Because water flows,
on the surface, from East to West it must be replaced
by water which rises up from the lower strata and this,
we shall see, is always colder than that on the surface.

Finally the conditions in the Mediterraneans and marginal
seas may be noticed. In the Roman Mediterranean we
find temperatures of 18°C. to 21°C., while those of the
Atlantic in the same zone of latitude vary between 15°
and 18°. The Caribbean Sea and Gulf of Mexico exhibit
temperatures of 25°C. to 27°C. in contrast with a range
of about 23° to 27° outside. On the other hand the two
epi-continental seas, the Baltic and Hudson's Bay, are
colder than the adjacent Atlantic, the Baltic being about

178 AN INTRODUCTION TO OCEANOGRAPHY

2°C. to 10°C. ; Hudson's Bay being about zero while the
Atlantic is, in general, about 5° to 10°C. between the two
seas.

The general courses of the isotherms in the Pacific and
Indian Oceans are represented in Fig. 26,

In the above figures the isotherms are drawn for tem-
perature-intervals of 5°C. except in the equatorial zones
where the differences are more noticeable. Both in this
and the last chart only the most general representations
of the variation in temperature are attempted. Even

Fig. 26. The distribution of surface temperature in the Pacific and
Indian Oceans during the summer months.

with the data that we possess the amount of detail that
could be dealt with is enormous, and this increases still
further whenever we consider the conditions in the shallow
seas in the temperate zones, where variability both in
space and time is much greater than it is in the open
ocean. Looking at Fig. 26 we see, then, that there is a
closer approximation to paralleHsm of the isotherms with
the lines of latitude than can be observed in the Atlantic
regions : this is particularly the case in the southern
ocean South of Australia but it can also be seen in the

PHYSICAL CHARACTERS OF SEA WATER 179

North Pacific. Even here, however, there is a tendency
for the isotherms to become bent up from S. to N. and
from W. to E. and this is because the water circulation
of the North Pacific is fundamentally the same as that of
the North Atlantic, although the movements are less
exaggerated, so to speak, in the former ocean.

In general the temperature tends to be higher in the
West Pacific than in the East. Thus there is a large area
of sea in the Austral- Asiatic region where the temperature
rises to over 28°C.

We leave the more detailed study of the spatial variations
of temperature to the student who has access to large-
scale charts of the various oceans at different times through-
out the year. Without these, and some general idea in
mind, the subject is one that is full of interminable detail
of little interest. But the main conditions that lead to
temperature differences may be briefly noticed. First
of all there is the relation between such variations and the
currents and ocean drifts. If water flows from a region
of higher intensity of solar radiation to one of lower intensity
there will be a transference of heat : thus the Gulf Stream
drift in the North Atlantic carries water that has been
strongly heated in the region of the equatorial streams
up to temperate, and even Arctic latitudes. Therefore,
the temperature in the latter regions is higher than it
would be if there were no Gulf Stream circulation. So, also,
the Labrador Current carries water from regions where
the sea is cooled by melting ice down to latitudes where
the temperature normal to those zones would be much
higher were there no southerly-flowing cold Arctic streams.
Again, the westerly-flowing equatorial streams transfer
warm water towards the West and pile this up, so to speak
against the American shores of the mid-Atlantic, against
the Austral- Asiatic Islands in the Pacific, and against
the East coast of Africa.

Then we have the very strong effect of winds. These,
impinging on the surface of the sea actually propel the

180 AN INTRODUCTION TO OCEANOGRAPHY

superficial layers of water in the directions towards which
they blow : that, for instance, is why the temperature
in the Caribbean Sea and in the Gulf of Mexico is higher
than it is in the Mid- Atlantic of the same range of latitudes.
But here we have to do, really, with the condition mentioned
above, the transfer of heated water by ocean currents :
the North-East and South-East Trades are the propulsive
forces of the North and South Equatorial streams. Irregu-
larities in the distribution of temperature, that is the
local and seasonal deviations from the courses of the
isotherms of Figs. 25 and 26, which can always be seen
when we look at large-scale charts, are due to local winds
or to series of cyclonic storms.

Upwelling of water from the deeper parts of the sea
is a cause of variation in surface temperature. Thus
water is actually driven from East to West in the tropical
Atlantic by the trade winds, and it follows that this water
must be replaced. It is so replaced to a considerable
extent, by water which rises up from below, off the West
Coast of Africa. This water is colder than that which
would be present at the surface of the sea were there no
upward movement.

Obviously the lands present barriers to the progress of
currents or drifts of ocean water, and this is also the case
with shallow sea bottoms. Currents always tend to flow
along depressions on the floor of the ocean, and shoals
impede them to a marked extent, or even arrest them
entirely. So do straits which are relatively wide : thus
the Gulf Stream drift flows round to the North of the
Shetland Islands into the North Sea, but not through the
Pentland Firth, and only to a very small degree through
the Straits of Dover. One sees here how intimately the
phenomena of variations of temperature from place to
place are related to those of ocean drifts and currents : thus
those smaller local irregularities that are so apparent in charts
which represent the courses of the isotherms in shallow
inshore seas are due in a large measure to the tidal streams.

PHYSICAL CHARACTERS OF SEA WATER 181

Periodic temperature changes at the same place. First,
there is a small daily variation of temperature at a
fixed spot (say at a light vessel). This may be fairly
large in a shallow sea area near the land (say two or more
degrees centigrade) and it is due to the tidal streams.
If there are considerable areas of foreshore alternately
covered and laid bare by the tides, there is a periodic
heating effect in the summer and a cooling effect in the
winter months. The extensive areas of sand-banks become
hotter than the adjacent sea during the summer and so
the water that covers them when the tide flows is heated
to over the average temperature of the region. When
this water flows out to sea on the next ebb-tide it is warmer
than the water in situ would be were there no tidal streams.
If the in-going and out-going streams had exactly the
same directions the effect would not be so noticeable, but
it generally happens that the water flowing into a bay
or estuary from the open sea does not return to exactly
the same place when it ebbs out again. In the winter
months the sand-banks are cooled, on exposure, to below
the temperature normal for the sea-region off shore and
so the effects mentioned above are reversed.

Further, the in-going and out-going tidal streams are
more rapid at springs than at neaps and the areas of
foreshore exposed are also greater. There will, then, be
a fortnightly variation of temperature at stations chosen
so as to be affected by the tidal streams.

In the open ocean, tides can have no such effect, of course,
but there a small daily temperature variation can be
observed. This is usually less than about 0"5°C. It is
due to the heating effect of the sun on the superficial
stratum of water.

The annual temperature change is, of course, the most
noticeable variation and it is due to the change in the
sun's declination. That is to say, the quantity of heat
received by the ocean at any one place is variable, reaching
its maximum and minimum values some time after the

182 AN INTRODUCTION TO OCEANOGRAPHY

summer and winter solstices. This annual range of sea
temperature varies greatly from place to place. Looking
at the ocean as a whole, we have the following general
distribution of temperature ranges : —

(1) The polar regions : the mean temperature is lowest
and the annual temperature range is also low. In the
North this region of minimal mean temperature and
minimal range is, roughly speaking, the Arctic Ocean
(excepting the sea between Greenland, Spitzbergen and
Norway). The extreme range is from about 2°C. to 10°C.
but the mean annual range is only about 6°C. In the
South the region hes between the pole and about 50° S.
latitude.

(2) The tropical regions : these comprise the ocean
between about 20° N. and S. latitudes but extending to
about 30° S. lat. in the Central Pacific. The extreme
temperature range is from about 21°C. to 32°C. but the
mean range is only about 6°C. This is a region of maximal
temperature and minimal annual temperature range,

(3) The temperate regions : these lie round about 40° N.
and S. latitude but they extend along the North- West
Pacific and the North-East Atlantic to about 20° S. and
70° N. The extreme temperature ranges are very variable
but lie intermediate between those characterising polar
and equatorial regions. The mean annual ranges for
limited areas in the North- West Atlantic South of Nova
Scotia and in the North- West Pacific, in the Sea of Japan,
may exceed 28°C. and elsewhere it is, in general, between
about 11°C. and 22°C. Here, then, we have regions of
moderate temperature and maximal annual temperature
range.

In the shallow epi- continental seas the ranges may be
very variable, even in places which are near to each other.
In shallow bays and estuaries where there are extensive
areas of foreshore and much fresh water entering the sea
from rivers, the annual range may be exceptionally great.
Reversals in the directions of winds may also increase

PHYSICAL CHARACTERS OF SEA WATER 183

the range. In general the latter is greater close to the land
than it is at considerable distances out to sea.

If we observe the sea temperature taken at the same
place at regular intervals (say every day) throughout the
year, the form of the annual variation can be studied.
Neglecting the small diurnal variation, we find that the
graph of the observations (temperature plotted against
time) may be very irregular. At inshore stations the
disturbing effects of the tides may sometimes be recognised,
but more important are the effects of the winds which
not only heat or cool the surface waters above or below
the temperature that would be observed were there no
variable and violent winds, but also lead to actual transport
of water from regions of higher to regions of lower tem-
perature (and vice versa) and, further, to the upwelling of
water from lower strata, where the temperature may be
lower. If the observations taken daily be reduced to
means (say of 10-daily periods), the graph becomes more
regular but even then we can see the influence of the
" weather," that is of the succession of cyclonic and anti-
cyclonic disturbance of the atmosphere. If the observa-
tions be reduced to monthly averages and if the latter be
taken over (say) 10 to 20 years the graph of annual tem-
perature variation approaches a limiting form, that of
an harmonic curve. This expresses the effect of the
generalised heating effect on the sea of solar radiation, at
the particular place under observation, apart from the
other cause of variability mentioned above, and apart
also from the change, from year to year, in the intensity
of solar radiation.

Change of temperature is certainly the biggest factor
in leading to change in the nature and abundance of
marine life. Looking at this matter in a very general
way we see that the facts of temperature distribution
outlined on p. 182 can easily be correlated with biological
phenomena. The intra-tropical zone of high temperature
and minima] temperature range is, thus, the region

184 AN INTRODUCTION TO OCEANOGRAPHY

of great and characteristic coral-reef formation. Here
the secretion of lime from its solution in the sea is
displayed on the great scale, not only in the massive
reef deposits but also in the large-shelled molluscs and
the pelagic foraminifera and other calcareous organisms.
In the temperate zones secretion of lime by organisms is
far less pronoimced, and it is minimal in the polar regions.

In the intra-tropical seas there is a great wealth of species
of animals as compared with individuals. Animals are,
on the whole, bigger (though the whales are an exception
as they are more characteristic of polar than of equatorial
seas). Colouring is more vivid and patterns, both of form
and colour, are more diversified. On the other hand, the
polar and temperate seas are characterised by an extra-
ordinary wealth of individuals (both pelagic and benthonic).
Thus the great fisheries of the world are in the higher
temperate latitudes ; the abundance of plankton in the
Arctic and Antarctic Seas is surprising when the latter
are compared, in this respect, with the tropics. Even
the land fauna, as exemplified by the Antarctic penguin
rookeries, or the seal fisheries in the higher latitudes, is
seen to be very abundant. But in the polar and temperate
seas there is a poverty of species as compared with the
intra-tropical regions.

In localised regions, and in small ways, the same
importance of sea-temperature change, as an agency
leading to great changes in the distribution of marine
life, can be verified. But the subject cannot be discussed
here as it carries us too far away from the limits set out
in the planning of this book.

Ice in the Sea. When distilled water is progressively
cooled it contracts until the temperature reaches the
well-known limit, 4°C. At that point water has a
maximum density, and as it continues to cool to tem-
peratures lower than 4°C. it expands. At 0°C. it
freezes and the ice that is formed is, therefore, lighter
than the unmelted water in its neighbourhood of temper-

PHYSICAL CHARACTERS OF SEA WATER 185

ature and so floats at the surface. Fresh water at-
4°C. will, therefore, be denser than liquid water at any
other temperature and will sink from the surface where
it has been cooled, but newly-formed ice will, should
it form at the bottom, rise to the surface and float there.
It will, to some extent, insulate the remainder of the
body of cooling water from the atmosphere, not only
preventing the abstraction of heat by contact of water
with cold winds, but also hindering convection currents.
When sea water is progressively cooled it continues to
contract until the freezing point is attained, and at that
temperature sea water exhibits its maximum density. The
freezing point is not a constant one but depends on the
quantity of dissolved solids. In the case of a solution
of pure sodium chloride in distilled water there is a regular
series of phases of the system ice and solution at each
temperature. The quantity of ice formed increases, the
quantity of sodium chloride crystals formed also increases
and the concentration of the solution remaining unfrozen
increases. Finally, the whole mass of salts, ice-crystals
and solution, becomes solid : this is the eutectic phase.
The ice formed at each stage varies in composition, it may
be H^O, 2{}1^0), or ^Ii^O) and the solid crystals of sodium
chloride that are formed also vary with regard to the
quantities of water that they contain as water of crystal-
lisation.

The process, even when we start with a solution of
pure salt of known concentration, is thus very complex,
but it is much more so when we deal with sea water. This
is a solution of many substances and its concentration
varies from place to place in the ocean and from time
to time. The freezing-point varies with the salinity,
becoming lower as the latter increases, and the reduction
cannot be calculated but must be found empirically. It
is graphed in Fig. 27.

The freezing-point of sea water, then, is depressed from
0, when the salinity is 0 (or the water is " fresh ") to

186 AN INTRODUCTION TO OCEANOGRAPHY

about -2°C. when the sahnity is about 35%o. The be-
haviour on freezing is quite different from that of fresh
water inasmuch as the hquid continues to contract until
the freezing-point is attained. The ice formed from sea
water will, therefore, be slightly heavier than the unfrozen
water from which it has been formed and it would sink
below the surface (or place of lowest temperature) but

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for the condition that in the process of freezing the jpart
of the water that remains unfrozen increases in density. In
a pure solution of NaCl in distilled water this would
occur, as we have noted, and the solution would become
more and more concentrated as the eutectic point is
approached. In sea water, however, the conditions are
much more complex.

PHYSICAL CHARACTERS OF SEA WATER 187

The solution, we have seen, cannot be regarded as one
of NaCl, MgCl.,, CaSO^, etc. What we have is water which
is itself slightly dissociated and is so a mixture of H^O
molecules, 2(R,fi) molecules, H and OH-ions. Each of
the salts is dissociated, thus instead of NaCl, MgCl^ and
CaSO^ we have Na and Cl-ions, Mg and Cl-ions, Ca and
SO^-ions, with the undissociated salts, the concentrations
of all these substances depending on the total quantities
in solution and on the temperature. The smaller the
total quantity of Na and Cl-ions present the greater will
be the ratio Na-ion and CI- ion to NaCl, and so with each
of the other substances. The solubility of the various
salts, NaCl, MgCl^, CaSO^, etc., varies, and also varies in
a different way with change of temperature. Thus the
composition of the solid ice (that is the solid form of H.^0
and the solid crystals of the salts separated out on freezing)
wiU be different at each temperature-stage and so also
will be the composition of the remaining mother-liquor.
In ordinary sea water the solubility of calcium carbonate,
CaCO.^, is very small and so ordinary lime will separate out
at once when the temperature falls significantly. Calcium
sulphate, CaSO^, will not separate out as such but there
is a possible re-arrangement of the ions such that the
Na and SO^ can combine to form NaSO,. The eutectic
point of this salt is only about -0'7°C., that of KCl is
about -11-1°, NaCl is about -21-9°, MgCl^ is about -33-6°
and CaCl.^ is about -55°C. Thus the last substance to
separate out from the mother-liquor would be calcium
chloride at a very low temperature, seldom, or never
attained in the sea. Among the first substances to separate
out would be sodium sulphate, because of its high eutectic
temperature, and so we find that the unfrozen mother-
liquor in freezing sea water becomes less and less concen-
trated, as regards SO^, as the temperature falls. But the
behaviour of actual sea water in these respects is not
precisely the same as that of pure solutions of the various
salts since the presence of each in the mixed solution

188 AN INTRODUCTION TO OCEANOGRAPHY

affects all the others. Enough, with regard to this very
difficult subject, has been said to show the reader what
a very compHcated process, from the physico-chemical point
of view, the freezing of sea water is.

As a result, however, we have the formation of an
ice containing crystals of salts in its substance along
with polymerised water crystals, H^O, 2(H20), 3(Hj^0),
and floating upon a solution which has become rather
denser by reason of concentration, and rather different in
composition by reason of selective crystallisation.

The forms of Ice in the Sea. These are (1) land ice
brought down by rivers, that is ice resulting from
the freezing of fresh water in lakes and rivers and
subsequently dislodged and carried down into the sea.

(2) Land ice is broken away from glaciers as these
approach the sea, or broken off from the great ice
caps resting on Greenland, the Antarctic continent and
elsewhere. This is the mode of origin of icebergs.

(3) There is, lastly, the ice resulting from the freezing of
sea water in situ and this assumes a great variety of forms
according to the conditions under which the freezing
occurs, or the subsequent treatment to which the ice is
subjected. Descriptions of sea ice — pack ice, floes, bank-
ice, hummocks, pancake-ice, etc., etc. — are very common
and are probably sufficiently well known to most readers.

The vertical distribution of sea temperature. There is
usually a marked variation of sea temperature with
increasing depth such that, in general, the surface
waters are warmer than those at the bottom. We shall
have occasion to refer to many specific instances of this
temperature variation when we deal with ocean currents,
and so only the main features of the vertical distribution
of temperature need be mentioned here.

The surface water is heated in two ways : (1) by direct
solar radiation, and (2) by contact with warm winds. It
is cooled by direct radiation and by contact with cold
winds. In the latter cases — the heating or cooling effects

PHYSICAL CHARACTERS OF SEA WATER 189

of winds — the results are usually very complex since not
only does the wind warm or cool the surface water but it
also propels the latter in the direction towards which the
air current moves. On approaching land this warm
current becomes deflected in a variety of ways and is
often driven beneath the surface thus heating the lower
strata of water to an extent that would not be exhibited
in the absence of strong winds. Barriers to the flow of
surface water, whether these be lands or merely submarine
elevations of the ocean floor, thus lead to marked variations
in the temperature of the sea beneath the surface. The
winds themselves are variable, changing from season to
season, and the combinations of these conditions lead to
very complicated effects.

Generally, however, the sea is warmest at the surface
in the tropical and temperate regions and then the tem-
perature decreases with depth, attaining minimal values
on the ocean floors. To this general statement there are
some important exceptions which we shall notice in
Chapter IX.

In calm seas the heating effect of the sun may be very
marked indeed, but in the ocean absolute stillness of the
surface water is very exceptional. In small lakes the
condition of entire calm is more often observed, and there
the temperature gradient immediately beneath the surface
is quite steep : this is illustrated by the graph that follows.

Fig. 28, B, is a graph of the temperature of the surface
water of a lake as determined at intervals of depth of 1 cm.
or less beneath the surface. In this case the thermometer
bulb is a glass tube of 1 mm. in cross section and about
10 cms. in length. The stem is bent at right angles to
the bulb and so the latter can be lowered to depths varying
by a few millimetres, the elongated tubular bulb of the
thermometer being horizontal and the stem vertical.
From graduations on the stem, which is held perpendicular
to the surface of the water, the depth to which the horizontal
bulb has been lowered can be measured very exactly.

190 AN INTRODUCTION TO OCEANOGRAPHY

At the surface, then, the temperature was about 15°C.
and this decreased rather quickly throughout the upper
4 cms. of water, to about 10°C. — a range of 5°C. in 4 cms.
of depth. Then the temperature fell much more slowly-
down to about 9"5°, a range of about 1|° in the succeeding
8 cms. From that stratum the rate of decrease would
remain nearly the same down to the bottom of the lake,
or at all events, to relatively considerable depths. Thus

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Fig. 28. (A) The general temperature gradient in the ocean as deduced
from a very great number of observations in all parts.
(B) The temperature gradient in the upper 11 cms. of a
freshwater lake.

there is a very steep temperature gradient given the
favourable conditions, an absolutely calm surface and a
strong sun. In water which is disturbed, even to a slight
extent, mixing of the warm surface water and the cooler
strata beneath occurs very rapidly, for wave motion is
accompanied by both vertical and horizontal movements
of the water particles, even if the mean place of these
particles is constant. With wave motion there is,

PHYSICAL CHARACTERS OF SEA WATER 191

however, always some wind and tMs leads to a translation
of the surface water film in the direction towards which
the wind blows. At the same time a new surface is exposed
to the heating action and so mixing down to many cms.
of depth must be a very rapid process. The consequence
is that, in ordinary conditions the temperature in the
upper metre layer of the sea, or in a lake, is uniform, or
very nearly so. The observations referred to above are
interesting because they show how the surface layers of
the sea become heated and the heat distributed to lower
layers apart from the processes of condition and connection.

Fig. 28, A, is a graph of the average temperature of the
ocean in all latitudes, and at the depths recorded, as deduced
from the " Challenger " observations. The upper layer
(at 100 fathoms) has a temperature of about 16°C. and
then this falls relatively rapidly down to about 4°C., at
about 600 fathoms. From that depth downwards it falls
much more rapidly to about 2°C. at a depth of 2,000
fathoms.

In general, the bottom temperature everywhere in the
ocean abysses is little above freezing-point of fresh water.
At the bottoms of the icebound regions of the Arctic and
Antarctic Oceans the temperature falls to below -1°C.
Over the whole of the Antarctic and Southern Oceans,
most of the Indian Ocean and parts of the Atlantic and
Pacific the bottom temperature varies between about
-1°C. and 2°C. while over the bottom of most of the Atlantic
and Pacific the temperature lies between about 2° and
4*5C°. The North Atlantic floor, however, has a tem-
perature which is about 2°C. higher than that at the
bottom of the South Atlantic and Pacific and this is
because of the position of the thermal equator well to the
North of the geographical equator. Thus the South
Equatorial stream sends large volumes of warm water
into the North Atlantic, and sooner or later this becomes
denser than the water which would lie at the surface
were the conditions not asymmetrical in the way indicated.

192 AN INTRODUCTION TO OCEANOGRAPHY

So it sinks, and raises the average temperature of the
bottom water.

Conditions may be very different in the epi-continental
seas, but to these matters we return in Chapter IX.

Seasonal changes in the temperature at the loiver levels.
Those changes in the temperature of the surface water
that we noted on pp. 174-8 become less and less as the depth
increases, until at a level of about 100 fathoms they are
very difficult to observe with certainty. At about that
stratum of the ocean, then, the heating and cooling effects
traceable to the changes in the sun's declination become
nil, or nearly so, and the temperature there ma}' be regarded
as approximating to the mean annual surface temperature.
It is probable that some slight seasonal changes, due mainly
to seasonal changes in the circulation of water between
the surface and the lower strata, occur during the year
but these are very small.

The Density of the Water.— The density of a liquid
is, in general, the ratio of the weight of a certain
volume to that of a similar volume of distilled water,
both being at the same temperature. The latter is
usually taken as 4°C. for at that point distilled water
has its maximum density. In the case of sea water,
however, the expression is much less simple for it is really
the density at the temperature and pressure of the stratum
from which the sample was taken that is of importance
in oceanographical investigation. The actual estimation
must, however, usually be made at some convenient
arbitrary temperature, and this is generally that of the
atmosphere on the deck of a ship at sea, or in a laboratory
ashore. The pressure is always that of one atmosphere.
To obtain the density of the water sample in situ corrections
to the experimental value must, then, be made and these
corrections involve a knowledge of the temperature and
pressure in situ. The temperature will have been found
in the process of collecting the water sample, and the
pressure is calculated from the depth and the salinity.

PHYSICAL CHARACTERS OF SEA WATER 193

Determination of the Density. This can be obtained in
various ways : —

(1) By a pycnometer. This is a U-tube with capillary
side stems which have marks on them. The tube is
weighed empty and dry, then filled to the marks with
distilled water at a convenient temperature and lastly
with sea water from the sample, at the same temperature.
The ratio between the weight of the sea water and that
of the distilled water is the density at the temperature
of the experiment and at the pressure of one atmosphere.

(2) By a hydrometer. This method can give only a
rough result. The stem of the instrument clings to the
surface film of water and this effect varies with the condition
of the glass. It can be avoided by using a " total-immer-
sion " hydrometer but here again there are sources of
error. Further corrections must be applied according to
the kind of glass from which the instrument is made.

(3) By the Petterssen specific gravity balances. The
water sample is contained in a glass vessel consisting of
a narrow tube enlarged at its upper end. A closed silica
bulb floats in the enlarged end and the lower end of this
bulb has attached to it a very fine gold chain, which lies
partly heaped up at the bottom of the tube. The vessel
is filled with the water and then the bulb with its attached
chain is placed in it. The bulb rises in denser, and sinks
in lighter water dragging as much of the gold chain as will
keep it at a certain level. From the length of chain
uncoiled from the heap at the bottom the density is esti-
mated by means of empirically prepared tables.

A U-tube may be used in such a way that a certain column
of sea water in one limb is in equilibrium with a longer
column of distilled water in the other limb. From the differ-
ence in the heights of the columns the density is estimated,

(4) By a chlorine titration. It has been stated (see
p. 136) that the ratio of chlorine to total dissolved solids
is the same everywhere in the ocean. This is not absolutely
true, for water of very low density in the neighbourhood

194 AN INTRODUCTION TO OCEANOGRAPHY

of great rivers may differ in composition from that in the
open sea. Nevertheless, the uniformity of composition
is so very nearly perfect that no significant error is involved
in the assumption upon which the method depends. The
relation between chlorine and total dissolved solids has
been found by Knudsen over a considerable range of
values of the latter, and an interpolation formula has
been obtained. This is : —
Density at 0°C. = -0-069 + 1-4708 CI -000157 Cl^ + 0-00004 CI'

The results are tabulated, so that from the chlorine
value the total dissolved solids can be found. Thus
the salinity (see p. 137) can be obtained from the
results of the chlorine titrations. This is the method
of obtaining density that is now generally adopted.

The salinity has been defined as the weight of total
dissolved solids (given certain qualifications) measured in
grams and contained in 1,000 grams of the sea water in
question. The temperature will not matter. But the
density will vary with the temperature, for this function
is the ratio of a certain volume of sea water at t° to an
equal volume of distilled water at 4° and the same volume
of the former will contain more dissolved solids (and so
be heavier) when its temperature is lower than when it
is higher, because of the expansion or contraction of
volume with change of temperature.

Density varies with salinity in the way shown by the
" density " graph in Fig. 27 (p. 186). Here again the
actual ratios have been found experimentally ; an inter-
polation formula has been made and the results are tabu-
lated for a large range of chlorine titrations.

There are two main expressions for the density of sea
water : —

,,, The weij^ht of unit volume of sea water at 0°C.

the weight of unit volume of distilled water at 4°C.

,„v The weight of unit volume of sea water at ^°C. _

the weight of unit volume of distilled water at 4°C.

Given, then, the temperature of the water sample in situ,

PHYSICAL CHARACTERS OF SEA WATER 195

with the results of the chlorine titration and the density
at 0°C., that is, o-^ can be found from tables. In a
similar way o-^ can be found from the value of o-q .

There remains the correction for the compressibility
of sea water. The depth from which the sample was
taken is known and from this the hydrostatic pressure
can be found (given an approximate value for the saHnity
in situ for this does not affect the compressibility to a very
marked extent). At a depth of one metre, say, the volume
of a litre of sea water will be diminished to c litres, c being
the coefficient of compressibility for a water column of
1 metre in height. Call the density at the temperature
in situ and at atmospheric pressure o-< then the density
in situ will be nca , , n being the depth in metres. This
expression for the density is of importance in hydrodynamic
investigations. Finally the density is usually expressed
in an abbreviated form : unity is subtracted from the
number representing it and the result is multiplied by
1,000, Thus the numbers on the vertical scale in Fig. 27
are usually written 30, 25, 20, 15, 10 and 5 instead of
r030, 1-025, 1-020, etc.

Other physical functions of Sea Water. — The Osmotic
pressure. Suppose a small quantity of sea water to
be contained in a vessel, the walls of which are
semi-permeable, that is, they allow water molecules
to pass through but prevent the passage of the molecules
of the dissolved salts. Suppose the vessel to be closed ;
to be immersed in distilled water, and to be provided with
a. manometer so that the pressure of the liquid against
the walls may be observed. Water will pass into it from
outside, diluting the saline solution, and in a short time a
very considerable pressure will be established within the
closed vessel. For water of the same temperature this
osmotic pressure will increase as the salinity increases, and
its rate of variation is given by the graph in Fig. 27. These
values have not been observed experimentally but are
calculated from observations of the depression of the

196 AN INTRODUCTION TO OCEANOGRAPHY

freezing-point with increasing salinity. The importance
of this character is mainly theoretical.

The boiling 'point. This is higher than that of distilled
water. For the same atmospheric pressure it increases
as the salinity increases. The vapour pressure varies in
the same way. The viscosity or internal friction of sea
water is mainly a function of the temperature but depends
also on the salinity. It has little practical importance
in oceanographical investigation though it has been
correlated with the forms of marine Diatoms, Peridinians,
Radiolarians, etc. In equatorial regions, where the
temperature is relatively high, the viscosity is decreased
and many micro-organisms appear to exhibit adaptations
of form such as will tend to a slower rate of sinking through
the water. The electrical conductivity varies both with
the temperature and salinity. Distilled water is a very
poor conductor of electricity since it is dissociated to a very
small extent. Dissociation means the formation of ions,
which then carry the current. With increase in salinity
up to a certain limit (of course), there is increase in the
quantity of salt molecules dissociated, increase in the
numbers of ions present per unit volume of water and so
increase in the conductivity. Obviously the salinity
might be found from a determination of the conductivity,
and apparatus has been devised whereby the conductivity
of the water in situ can be estimated. The apparatus
is loM''ered down into the sea at the end of an insulated
cable and the electrical resistance of a known column of
the water is read on the instruments on board the ship.
The method is not always practicable, and it has not the
degree of accuracy given by a chlorine titration.

The optical characters of sea water. The refractive
index is greater than that of distilled water and
depends on the temperature and salinity : the function
has little significance in oceanography. The penetrating
power of light in sea water is very much more important
as upon the energy of solar radiation depends the

PHYSICAL CHARACTERS OF SEA WATER 197

ability of plant organisms to synthesise carbohydrate
from CO.^ and water. The penetrating power depends
mainly on the presence of fine particles in suspension in
the sea. This turbidity due to detritus is very small in
the water far away from land, but photosynthesis is far
less intense there than in shallow inshore seas, and so the
occurrence of finely divided suspended matter in the
water has much general importance. Where there are
rapid tidal streams near the land there is always much
matter in suspension, and sunlight does not penetrate to
much below 10 to 20 fathoms in such turbid seas.

The transparency of the sea has been estimated by
lowering a white enamelled disc of about two feet in diameter
to such a distance that it just ceases to be visible. This
depth may not exceed one or two fathoms in muddy
water off the mouth of an estuary but it may be as much
as twenty fathoms or more in the open sea far from the
land and in bright sunlight. When the tidal streams have
little velocity off a coast where there are no big rivers
and with a bottom of hard sand, the sea is always more
transparent than in lakes. This is because of the precipi-
tating effect of the ions in sea water on colloidal clay
entering the sea.

Even in water far from the land and containing little
or no mineral particles in suspension, there are always
plankton organisms in the surface layers, and these absorb
the light falling through the sea, altering the colour of
the water. As a liquid, however, and apart altogether
from the presence of solid particles, mineral or organic,
the sea water absorbs the solar radiation. This effect
has been studied by observation of photographic plates
exposed to the fight at various depths. The plates are
contained in a carrier with glass covers shielding them
from contact with the water and the carrier is covered
by a metal case. When it reaches the required depth,
a weight is allowed to sfide down the wire rope carrying
the instrument, and this releases a catch which allows the

198 AN INTRODUCTION TO OCEANOGRAPHY

plate carrier to slide out of the case. The plates are then
exposed for a definite time, when a second weight is lowered
and this closes the carrier again. The apparatus is then
hauled up to the surface and the plates are developed.
Colour filters can be used. Highly sensitive panchromatic
photographic plates are employed.

Such plates, then, are affected by the radiation at
a depth of 1,000 metres, but are certainly not affected at
a depth of 1,700 metres. Thus some light penetrates to
a depth of over 500 fathoms. Plates screened with blue
filters require 6 times, and those screened with green
filters 18 times the exposure of unscreened plates.

The composition of the radiation alters rapidly with
the depth. At even a few centimetres below the surface
the infra-red heat rays are absorbed ; then follows a
disappearance of the red waves and last of all the blue
and violet ones. At about 100 metres all the wave lengths
are still present, but at 500 metres the red has almost
disappeared while blue is still present.

The colour of sea water varies between green and blue.
Distilled water prepared very carefully has a pure, clear
blue colour when seen in some depth. Near the land
and off muddy estuaries the colour of the sea is often
that of the suspended mineral particles, but when the
latter are very few in number the colour is green or yellow-
green. Within a zone between 30° N. and S. latitudes
the colour is ultramarine changing to indigo in the Southern
Ocean and then to olive-green in the Antarctic. The
general blueness is due, of course, to the absorption of
most other wave lengths before the blue ones disappear,
and the green colour of the Antarctic is attributed to the
abundance of diatoms in those waters. Dissolved chemical
substances affect the colour : thus in the neighbourhood
of coral reefs there is much calcium carbonate in solution
and this gives the water a blue colour.

CHAPTER VIII
THE TIDES

It is quite impossible to do more in this Chapter than
deal with the outlines of our knowledge of the tides. The
dynamical theory of tidal movements is still very incomplete
and it is necessarily treated in such a way as to be quite
unintelligible to all those students who do not possess the
mathematical technique of the investigator. Bound up
with this is the problem of the prediction of the tides and
this, also, is exceedingly technical and probably quite
impossible, in its later phases at least, to anyone who does
not attempt actually to work out examples. Here, then,
we deal only with such parts of the subject as are capable
of exposition without mathematical symbolism. We take,
first, the tides as they are to be studied by ordinary
observation and measurements ; then we consider the
principal celestial phenomena associated with tidal changes
and lastly we say something about the methods of
prediction.

The tides from ordinary observation. — At most places on
the sea coast there is a periodic rise and fall in the level of
the sea. This can easily be studied by watching the rise
of the water on the foreshore or on the vertical face of a
cliff, dock wall, breakwater, etc. The extent of the rise
and fall varies greatly and it may even be non-apparent
in some exceptional localities.

The rise and fall of the tides. In the ocean, far from land
and over deep water, the tidal rise and fall can hardly be
observed, much less precisely measured. On the shores
of oceanic islands, however, it can be seen to be only a
very few feet. In inland seas, such as the Mediterranean
it is small — only two or three feet or less — and in great

199

200 AN INTRODUCTION TO OCEANOGRAPHY

lakes it does not exist in the typical form but is replaced
by peculiar water movements called Seiches. In shallow
seas, such as the North Sea, English Channel, or Irish Sea,
it is moderately large and it is maximal in large shallow
bays, estuaries and tidal rivers.

Associated with the vertical rise and fall of water level
is a horizontal streaming movement : this we consider
presently.

There are remarkable differences, even at stations no
great distance apart, in the extent of rise and fall of the
sea level and here we consider some typical place on the
coast of a shallow sea, but not in an estuary. Beginning
with the lowest level of the water on any chosen day — low
water — we find that for an appreciable time, say half an
hour or more, there is no apparent change in the sea level.
Then the sea begins to rise very slowly, then more quickly
and finally more and more slowly until it appears to cease.
For an appreciable time there is again no apparent change
in water-level and high water occurs. During the time of
rise the tide is said to be flowing.^ or it is flood-tide and
during the time of fall the tide is said to be ebbing, or it is
ebb-tide.

Tidal gauges. The rate, and extent of rise and fall
can be estimated very precisely by observations of a tide-
gauge. In its simplest form this consists of a plank set
up vertically on the outer side of a dock-wall or in some
other suitable place. The plank is marked in feet and
half-, and quarter-feet. At regular intervals of time the
height of the water is read off on the scale.

Automatic tide gauges are now generally set up by
harbour authorities. As a rule a deep well, lined by
brickwork, is sunk to such a depth that there is always
water in it no matter how low the tide may be. The well
is connected, by a pipe, with a part of the sea below the
lowest water observed. A float in the well rises and falls
with the water level and this is connected with a mechanism
which reduces the rise of fall, in some convenient ratio,

THE TIDES 201

say an inch to the foot. The mechanism records the rise
and fall by tracing a curve on a sheet of paper wound on a
drum. The paper is marked by a time-recording
mechanism and the heights of the curve are measured at
the time-intervals required.

Lately tide-gauges have been constructed on entirely
different principles. The water pressure at any spot on
the sea bottom depends on the height of the tide and the
varying pressure is made to affect a Bourdon pressure
gauge or to actuate a system of electric oscillators contained
in a water-tight vessel lying near the sea-bottom. In this
way the change in pressure from moment to moment can
be transmitted to a suitable recording station and registered
as such. The recording mechanism is calibrated so that
the head of water is read instead of hydrostatic pressure.

From the graphs obtained in these ways the times and
levels of low and high M^ater can be estimated with
considerable accuracy and the rates of rise and fall can be
deduced from the form of the graphs.

Chart data, ordnance data and mean sea level. Some fixed
level is adopted at each place where tidal observations are
made and, as a rule, this is an arbitrary mark on some
part of the foreshore which is generally accessible to
observation. The mark is chosen so that the tide will not
often fall below it, or it has been chosen for some practical
reason. The original tide datum at Liverpool was the
level of the old Dock Sill : this was a practical level mark
for the height of the tide above it was an indication of
value to vessels preparing to enter or leave the Dock.
The Dock does not now exist but the datum is preserved
in the form of a mark cut on the river face of the centre
pier of Canning Half-Tide Dock. All land surveys in
Great Britain are referred (as to level) to the Ordnance
Datum and this is taken to be a level of 4-67 feet above
that of the old Dock Sill at Liverpool. Elsewhere the
heights of the tides are referred to some local datum mark
which is specified on the local charts. The tidal heights

202 AN INTRODUCTION TO OCEANOGRAPHY

are given in feet and tenths above such marks unless in
the cases of exceptional tides, when they may fall below
the mark, then the distances beloiv the datum are
measured.

In nearly all places the tide, then, rises to variable
heights above some arbitrary level and exceptionally falls
below the latter. If the sea level is measured with respect
to this arbitrary mark and if the mean of a great number
of measurements, made at equal intervals of time, be taken
the result is mean sea level, stated with reference to the
datum mark.*

If a long series of measurements be made of the height
of the tide at low water and high water, and if the mean
of these be calculated the result is mean tide level. This is
not quite the same as mean sea level because the tide
seldom rises at the same rate as it falls. Mean tide level
is also referred to the local datum mark.

The range of the tide is the difference between high and
low water levels : it varies from tide to tide. .The rise of
the tide is the difference between the datum mark and
high water level : this also varies from tide to tide. The
height of the tide is the difference between the datum mark
and the water level : it varies from moment to moment.

Periodicity of the tides. Ordinary observation shows
that high and low waters occur alternately and about
twice a day : the interval between two successive high
waters being nearly 12i hours : thus the rise and fall of
the tide is approximately semi-diurnal. But it may also
be noticed that one of the two tides that occur each day
is bigger than the other one : thus there is a diurnal
periodicity in the change of water level as well as a semi-
diurnal variation, the interval between two higher high
waters being approximately 24| hours.

Spring and Neap tides. For several days during every
fortnight the high waters are higher than usual and the

* The question of " mean sea level " is much too complex to be
discussed here. Differences in level due to meteorological causes
have much importance in marine biology.

THE TIDES 203

low waters are lower than usual : these are the spring tides
and they occur a few days after the moon is new and full.
Also the highest springs occur, at any place, at nearly the
same time in the day. For a few days also, during each
fortnight, the high waters are lower than usual and the
low waters are higher than usual : these are the neap
tides and they occur about a few days after the moon is
in her first and last quarters. Like the spring tides the
lowest neaps also occur, at any place, at very nearly the
same time of day. Since the high and low waters are
about three-quarters of an hour later every day it follows
that, at any given place, they may occur at any hour
during the twenty-four, but for any one place the highest
spring tide occurs at very approximately the same time
in the day. There is thus a fortnightly periodicity in
the tides as well as half -daily and daily periodicities.

Twice a year, also, the spring tides are rather higher
than usual : this occurs about the times of the venial and
autumnal equinoxes. These extra high tides are the
equinoctial springs.

The range of the tides. Thus the rise of the tide has a
daily, fortnightly and six-monthly periodicity : these are
not the only periodicities but they are those that are most
easily apparent to ordinary observation. If now we
consider different places on the sea coast we shall find
quite notable differences in the range of the tides : that is,
between the levels, say, of ordinary springs. This is so
even with places that are not very far apart — say Liverpool,
where the range of ordinary spring tides is about 30 feet
and Greenock, where it is only about 10 feet. Some
examples of this geographical difference are given in
Fig. 29 which shows, for the same month, the rise and fall
of the tide for the ports of Avonmouth, Heligoland
and Gibraltar.

It will be noticed that though the extreme range varies
from about 49 feet (for Avonmouth) to about 3| feet (for
Gibraltar) we see the same sequence of springs and neaps

204 AN INTRODUCTION TO OCEANOGRAPHY

and also that the spring-tides occur in each case at nearly
the same time.

Types of tidal rise and fall. Our usual experience in
Great Britain is that of a semi-diurnal tide : that is a
succession of high waters occurring at intervals of half a
day. We see, however, that one of the two half-daily
tides is usually bigger than the other, so that we may
regard the whole tidal undulation as consisting of at least

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wVvVAvv^AAVvAA^AA/VW\AAAAAAAAA^v^AA'V'vvvvv\/v^^^

Fig. 29. Graphs of the rise and fall of the tides for the ports of Avon-
mouth, Heligoland and Gibraltar during the month of
March, 1922. The data are the predictions taken from the
Admiralty Tide Tables.

two parts : the predominant one (in the Atlantic Ocean)
being half-daily and the other (much less noticeable) one
being daily in their recurrence. In other parts of the world^
however, the opposite is the case : the daily part of the
tide is the more prominent and the half-daily part is much
less noticeable. This is illustrated by the graphs of Fig. 30.
Here we see tides consisting almost entirely of a half-
daily part in the case of Dover. In the other case the

THE TIDES

205

main part of the tide is daily in periodicity, but there is a
small half-daily part which adds a hump to the daily
curve. We may regard this hump as sometimes occurring
on the ascending part, and again on the descending part
of the daily curve.

Exceptional forms of tide occur in the British Seas.
Thus there are places on the South Coast of England,
between Portsmouth and Southampton where there are
double high waters, or double low waters, in every half-
daily period of 12^ hours. There are also places in the
North Sea and elsewhere where there is no tidal rise and

Fig. 30. (a) Fifteen days' tides at Dover and (b) fifteen days' tides at
Victoria, British Columbia. The data are the predicted
times and heights in each case.

fall. These are called " amphidromic points," and round
these exceptional conditions occur.
Summarising we find : —
Semi-diurnal tides characteristic of the North Atlantic

region. These recur at intervals of 12| hours.
Diurnal tides exceptional for the Atlantic but common
in the Pacific. These recur at intervals of 24| hours.
Quarter-diurnal tides exist but they are very exceptional.
Sjyring tides recur once a fortnight.
Neap tides recur once a fortnight.
Equinoctial spring tides recur at intervals of 6 months.

206 AN INTRODUCTION TO OCEANOGRAPHY

The tidal streams. — Associated with the rise and fall of
sea level due to tides there are generally horizontal move-
ments which often reverse themselves at about the same
times as the change of level. These horizontal movements
are the tidal streams. They are moderately small (that is,
from the seaman's point of view) in shallow sea areas like
the North Sea, English Channel and Irish Sea, they are
large in shallow bays, and they are maximal in estuaries
which shoal and contract hi width as we pass upwards
from the sea. Neither the tidal rise, nor the streaming,
have much practical significance in open shallow seas,
except that the rise and fall must be taken into account
in making use of soundings. In narrow waters, straits,
estuaries, shallow bays, tidal rivers and harbours the rise
and fall as well as the tidal streams have immense practical
importance.

The tidal undulation approaches the land from the ocean
as a wave of approximately harmonic form. As it enters
shallow water its energy tends to remain constant (apart
from dissipation of course), but its wave length shortens
and its amplitude (vertical height) increases. Also the
wave ceases to be harmonic (or approximately so) and
becomes steeper on the advancing than on the trailing side.
Wind waves or " seas " break in such circumstances but
the tidal oscillation seldom breaks. In such circumstances
the vertical displacement motion is far less striking to
ordinary observation than the horizontal one.

In general the stream will tend to flow in the same
direction as that in which the tidal wave advances. But,
coming into shallow water, the direction and velocity are
modified in a host of ways according to the conformation
of the land and the depth of the sea. Tidal streams tend
to flow parallel to the coast in the fairways of channels,
along gutters, or elongated depressions of the sea bottom,
through straits and along the courses of estuaries or tidal
rivers. They will strike across the mouths of small bays
but there is generally an indraught into wide bays. Their

THE TIDES 207

velocities are also exceedingly variable being, perhaps, half
a knot to one knot (a knot is a nautical mile per hour) in
an open shallow sea at some distance from the land, and as
much as 7 knots in narrow estuaries.

Very often the duration and direction of the tidal stream
are the same as those of the tidal wave front. Near the
time when high water and low water occur the directions
of the stream usually reverse and when this reversal occurs
within an hour or so before or after high or low water
it is customary to speak of the Flood-stream as that one
which runs while the tide is rising, and of the Ebb-stream as
the one which runs while the tide is falling. In tidal rivers
and estuaries, however, the rise of the tide is not generally
synchronous with the flood stream nor is its fall with the
ebb-stream. The water may continue to rise for nearly
the normal period of six hours but after three hours flood
the stream may reverse. Thus it is often said that the
tide may flow in a tidal river for a period of 3 to 5 hours ^
and ebb for 7 to 9 hours, but the water level may really
continue to rise although the tidal stream has reversed in
direction. So in reference to channels, fairways and
estuaries it is customary to speak of ingoing and outgoing
rather than flood and ebb streams.

When the change of direction of the stream occurs in
an open shallow sea at some distance from the land, it
usually does so in a certain way. Fig. 31 shows graphically
the results of a series of estimations of the direction and
velocity of the current at Ling Bank, in the North Sea,
on 7th and 8th August, 1906.

A small object floating on the surface of the sea is
affected far more by a moderately strong wind than by
a tide- way, but a large and heavy object, such as a big
ship at anchor rides to the tide and is aflected by the
latter more than by a moderate breeze. When the tide
is slack the ship w^ill be " wind-rode," but when the stream
begins again she will rotate on her anchor as a pivot in the
same direction as the hands of a watch turn. This is the

208 AN INTRODUCTION TO OCEANOGRAPHY

normal condition in the northern hemisphere : in the
southern one the rotation will occur in the opposite direction
to the movement of a watch hand.

In seas immediately off a coastline with prominent
headlands, bays, islands, etc., and with much variety in
depths the tidal streams become very complicated.
Islands will split them and, on reuniting, the streams may
reinforce or almost nullify each other, or form small
turbulent eddies, or very large ones with areas of slack

£1 cL^-t-.

^O metres 75 metres

Fig. 3]. Graphs of a series of readings of a current-meter at depths
of 2, 20 and 75 m. The lengths of the arrows are proportional
to the velocities of the stream and the directions are as shown
in the figure. North Sea.

water in the centre and comfortable anchorages. In large
bays and estuaries where there are numerous sandbanks
and channels and where a prominent river current enters
the sea the conditions may be extremely complicated.
The Admiralty Charts usually record the main directions
and velocities of the tidal streams, but much of this
information is very old and dates back from observations
made about 1848 and which do not appear to have been
repeated since then, (Lately, however, extensive observa-

THE TIDES 209

tions have been made in the North Sea). The conditions
in shallow, inshore waters are well known to fishermen,
pilots, coasting sailors and harbour engineers, but extensive
surveys of this kind do not appear often to have been
made — certainly there is little published information.

Obstructions to the tidal streams lead to change in
direction and velocity. Rocks, or bold sand banks on the
sea bottom, divert the stream upwards and in narrow
channels (such as in the Menai Straits between the Tubular
and Suspension Bridges), overfalls may be very prominent
during certain states of the tides : these are peculiar
boilings-up of the water which may be dangerous to small
vessels. Very bold headlands also divert the streams and
increase their velocities. Round such headlands there may
be a narrow zone close inshore, where the sea is calm, but
off this the tidal stream may run very strongly, and when
a strong wind is blowing in the opposite direction a very
violent sea may result. This is a tidal race and good
examples exist round the Mulls of Cantyre and Galloway,
on the coast of Scotland.

When a well-marked tide runs up an estuary which
shoals and narrows the wave becomes higher and departs
still further from the symmetrical harmonic form. Finally
it may become steep on the advancing front so that the
tide enters the narrower part of the estuary as a breast of
water several feet high. This may break, and be succeeded
by several smaller waves which may not break. Such a
tide is known as a Bore, and good examples exist in the
Severn and Solway in Great Britain and in some of the
Chinese rivers.

When the tide enters a very large and shallow bay
(Morecambe Bay, on the West Coast of England, for
instance) it may appear to rise very slowly at first. The
upper part of the foreshore may be nearly flat and may be
very extensive. A time comes (it may be at about half
tide, when the rate of rise is maximal) when the advancing
tide spills over the front of the bay and then rushes up the

210 AN INTRODUCTION TO OCEANOGRAPHY

shallow flat part as a stream deepening very quickly. In a
very short time vast areas of foreshore are covered by the sea.

So much for the more obvious characteristics of the
tides, we next consider

The tide-generating force. — In the theory of universal
gravitation, so far as it is applied to planetary movements,
the whole of the mass of a cosmic body is supposed to be
collected at its centre. The gravitational force between
two such bodies is directly proportional to the product of
the masses and inversely proportional to the square of the
distances between them. The bodies themselves are
regarded as being perfectly rigid.

r ^ S^Harth radii.

"^^^ ^ doSarth radii

J^oorh

■<-

6/ iZarfh radii

Fig. 32.

Let E be the earth and let the horizontal straight lines
lie in the direction of the moon's gravitative force. At E

the latter may be represented by ^^2^* where 0 is a

constant depending on the masses of the two bodies and

on the units adopted. At A it is given by ^r^JJ and at

B. by ^2^- Between the two places represented by the

extremities of a diameter lying in the direction of the
tide-generating force there is therefore a difference in

potential of \—-^ — ^-^„)C.
61- o9^

The earth-body itself is not quite rigid and it can be

shown that the tide-generating force does actually deform

THE TIDES

211

it because it must act with greater effect on a place near A
than on a place near B. The deformation, however, is
extremely small. But the envelopes of the earth are not
rigid and they are easily deformed. We need not consider
the atmospheric envelope, though tidal effects are said to
have been detected in it. The other envelope, the ocean,
is subject to a sensible deformation and this latter is the
tide.

Fig. 33.

Let the circle represent the earth and the inclined
line the direction of the attractional force : it can be
shown that the latter is inversely proportional to the cube
of the distance between the generating body and the
place on the earth considered. It is easy to see, by general
reasoning, that the tide-generating effect will be greatest
at the points x and y where the lines of force of the
gravitational field cut the ocean surfaces normal to the
latter. It will be zero at the ends of the broken line and
will diminish from x and y towards points on the earth's
surface 90° distant. All this follows very precisely from a
trigonometric discussion which is, however, not necessary
for our present purpose. Therefore the effect of the tide-

212 AN INTRODUCTION TO OCEANOGRAPHY

generating force is a heaping-up of the water of the ocean
round two places on the extremities of a diameter of the
earth which is in Hne with the direction of the tide-
generating force. The heaping up, in such an ideal earth,
uniformly covered by ocean as we are now considering
will amount only to a foot or two at the most.

Suppose now that the earth rotates (very slowly and
without friction) round its axis NS, which we take to be
inclined to the direction of the tide-generating force. The
tidal protuberances remain where they are, and the earth
slips round beneath them so that a wave will appear to
travel round it in a period of half a day (because there are
two protuberances). But a glance at Fig. 33 shows that
one of the latter la higher (it is h h') at the point a than it
is at the point b (where its height is h" h"). Therefore the
place a, when it has moved through 180° of longitude and
comes to position 6, will not only have experienced two
half-daily tides but one of the latter will be greater than
the other so that we have a semi-diurnal change of water
level as well as a diurnal one. The condition of this is
that the tide-generating body is not on the earth's equator
but is on a plane inclined to the latter.

Next we have to consider the

Variations in direction and intensity of the tide-generating
force. Every body in the solar system attracts the earth,
or contributes to the gravitational field in which the latter
is placed. The distances and masses of the other planets
are, however, not competent to produce significant effects
and so we need only consider the sun and moon. The
gravitational force due to the sun is a little less than half
of that due to the moon. The field, however, is a combined
one and its force varies continually with the relative
positions of the earth, moon and sun while the tidal elevation
at any place on the earth's surface undergoes corresponding
fluctuations. We now deal with the principal limiting
configurations of the system and this leads us to consider
the motions of sun and moon in relation to the earth.

THE TIDES 213

These are very complex and we can only deal with a few
of them. The reader must, therefore, miderstand that we
only mention such of the motions that will illustrate the
means of studying the variations in the tide-generating
force.

(1) The moon in syzygy. That is, the earth, moon and
sun are most nearly in line. Let the horizontal straight
line represent the plane of the ecUptic (that of the earth's
orbit round the sun). Then (neglecting decUnation) we
have the principal configurations of the moon in opposition
and in conjunction : —

-© @ e—

Afew moon FuTrfnoon

© .■■,..■,,..0 ©

Conjunction

Cjt^positi'on

^

Fig. 3.4.

These are the conditions for spring tides because the
directions of the tide-generating forces due to the moon
and sun are most nearly similar.

(2) The moon in quadrature. That is, the Hues joining
the centres of the three bodies are most nearly at right
angles to each other. We have the two cases of Fig. 34.

These are the conditions for neap tides. The effect of
the sun's gravitational field is to produce high water at
that place on the earth's surface where the effect of the
moon's gravitational field is to produce low water and
vice versa. But since the moon's effect is to the sun's
effect as 5 : 2 there will always be a residual high water
due to the moon.

The principal variations in the heights of the tides,
therefore, occur in correlation with the positions of the

214 AN INTRODUCTION TO OCEANOGRAPHY

moon in conjunction, opposition, first and last quarters.
Spring and neap tides follow the moon in her orbit romid
the earth.

The effect of varying declination. Next consider declina-
tion (when the conditions become much more complicated).
The plane of the earth's equator is inclined, at an angle of
23|°, to the plane of the ecliptic and the plane of the moon's
orbit round the earth is inclined at an angle of 5° to the
plane of the ecliptic. In the figure the angles and sizes of
the cosmic bodies are exaggerated.

Fia. 35.

Thus the attractional forces due to the moon and sun
are not normal to the surface of the sea at a place on the
earth's equator : these directions are indicated in the
Figure by the dotted lines with arrows.

Place the moon on the left of the earth leaving all the
angles as they are and we have the conditions while the
moon is in conjunction.

Place the sun on the left of the earth, leaving all the
rest of the figure as it is and we have the conditions
at the winter solstice. The moon may be on the right
and left giving the positions of conjunction and
opposition while the sun is at its position in the winter
solstice.

Thus when the moon is in syzyzy we have spring tides,
and when she is in quadrature we have neap tides. In the

THE TIDES

215

cases considered in Fig. 35 these conditions are accompanied
with the condition of maximum declination of the smi and
moon. This, by giving a less direct effect of the tide-
generating force, sets up the conditions of ordinary sjpring
tides.

Declination : the sun at the equinoxes. It is necessary to

\ Jlufumna-I
eo/uinox.

Kq/uinax

Fig. 36.

visualise the configuration at the vernal and autumnal
equinoxes. This is represented at A, Fig. 36.

Now regard the configuration as seen from the side of
the diagram. This is represented at B, Fig. 36.

For several days, about the times of the equinoxes, the
sun will be very nearly on the equator as the earth rotates
on its axis and so its declination will be zero, or nearly so.

216 AN INTRODUCTION TO OCEANOGRAPHY

For a day or two in every fortnight the moon is also nearly
on the equator. Therefore, about the time of each of the
equinoxes, there will be a short period when the declination
of the sun and moon will both be zero, or nearly so. At
such periods the combined gravitational field of both the
tide-generating bodies will most directly affect a place on
the earth's equator. When this configuration occurs while
the moon is in syzygy we have high or equinoctial spring
tides.

The moon in perigee and apogee ; the sun in perihelion
and aphelion.

When the moon is nearest to the earth (in perigee) her
tide-generating force is greater than the mean and vice
versa when she is furthest from the earth (in apogee).
These periods of perigee and apogee occur twice during the
lunar month.

When the sun is nearest to the earth (the latter in
perihelion) his tide-generating force is above the mean and
vice versa when he is furthest away from the earth (the
latter in aphelion). These periods of perihelion and
aphelion occur twice during the solar year.

Perigee or apogee may occur at any time relative to the
occurrence of new or full moon, or of the moon in
quadrature ; or with reference to the times when the
earth is in periheHon, or aphelion ; or relative to the times
of greatest or least declination of one or other of the tide-
generating bodies.

Thus the intensity of the tide-generating force must vary
from moment to moment at a place on the equator. But
if we take any other place on the earth's surface the same
conclusion follows. The various motions of sun, moon and
earth, upon which the above, and other configurations
depend are strictly periodic, are known and so the
configurations can be predicted. We may now summarise
the principal tidal cycles with reference to the various
astronomical phenomena with which they can be
associated.

THE TIDES

217

Tidal period Associated astronomical phenomena

Half-daily . The rotation of the earth.

Daily . . The rotation of the earth and the

declination of the sun and moon.
Fortnightly . The revolution of the moon in her orbit ^

syzygy and quadrature ; declination

of the moon.
Monthly . The revolution of the moon ; perigee and

apogee.
Half-yearly . The revolution of the earth in its orbit ;.

the sun's varying declination.
Yearly . The revolution of the earth ; the earth

in perihelion and aphelion.

Long periodic variations in the tide-generating force. In
addition to the periodic variations indicated above there

Fig. 37.

are, of course, others. Some are small and we do not
refer to them. Others occur after prolonged intervals of
time and have much general interest.

218 AN INTRODUCTION TO OCEANOGRAPHY

The plane of the earth's equator prolonged out into
space (the equinoctial) is inclined to the plane of the earth's
orbit round the sun (the ecliptic) at an angle of 23^°. The
inclination varies but we need not consider this here.

Fig. 37, A, represents the orbits of the earth and moon
as seen on the flat. Each of them is an ellipse (though the
eccentricity of the latter has jbeen greatly exaggerated in
the figure) and the straight lines forming their major axes
are the lines of apsides. The line of apsides of the earth's

earth's I af>sicses

Fig. 38. The line of the earth's apsides, the line of the moon's
apsides, and the Hne of nodes will coincide. This is the
configuration for an absolute maximum.

orbit rotates, but so slowly that we need not consider its
motion. The line of apsides in the moon's orbit also
rotates with a period of about 8-8 years. The planes of
the two orbits intersect each other in a straight line called
the line of modes and this rotates with a period of about
18-6 years.

The result of these various motions is that at intervals
of about 1,600 years a certain configuration of the three
bodies and the plane of their orbits arises :

It is shown in Fig. 38 where the projection of the planes
of the orbits at the side should make the configuration
clear. The motions that result in the latter are rather slow

THE TIDES 219

so that, for some time, it will remain. During this phase
there will be times when the earth is in perihelion, the moon
in perigee, the smi and moon in conjunction or in opposition
and both on or near the equator. Then we shall have
absolute maxima in the potential of the tide-generating
force.

The dates of such absolute maxima are calculable :
they occurred in 3500 B.C., 1900 B.C., 250 B.C., 1433 a.d.,
and the next maximum will occur in 3300 a.d. About
these times there must have been series of high spring
tides when there were very high tidal ranges (see further
at pp. 251-3). Subsidiary maxima to this one (that recurs
at intervals of about 1600 years) recur at intervals of 4J,
9, 19 and from 84 to 93 years.

Theories of the tides. Thus the moon and sun
establish a tide generating force which varies in intensity
from moment to moment passing through phases of
maxima and minima which depend on the relative motions
of the three bodies.

Suppose now that the earth does not rotate and that
sea-water is a perfect fluid without viscosity and moving
on the sea bottom without friction. We imagine the
ocean water " to have lost its inertia without losing its
gravitational properties." Suppose also the earth to be
covered by ocean to an uniform depth everywhere.

Then the surface of the ocean would assume a figure of
equihbrium in response to the tide-generating force. It
would become heaped up under the gravitation of the
generating body as two nearly spherical surfaces of
revolution with their axes passing through the centres of
the earth and the tide-generating body, as shown, in an
exaggerated degree, in Fig. 33. Directly in line with
the moon (say) the elevation would be maximal and
it would decrease to zero at distances of 90° along great
circles passing through the position of maximal elevation.

Suppose now the earth to rotate so that the tidal
elevations slipped on its solid surface without external and

220 AN INTRODUCTION TO OCEANOGRAPHY

internal friction. Then a tidal wave, with two crests at
a distance of 180°, would be formed and would appear to
travel round the earth with a period of half a day. This
is the principle of equilibrium. The modifications of this
simple, theoretical scheme made necessary by the motions
of the moon and sun relatively to the earth can easily be
traced. Neap and spring tides, with their variations
according to declination^ etc., can be deduced. Given the
hypothetical conditions indicated, then mathematical
investigation can find all the cases that arise. Such, in
short, is the ordinary " explanation " of the tides given in
the older text books.

Limitations of the equilibrium theory. The most obvious
limitation is that the ocean is not a perfect fluid but one
which moves on the earth's surface with considerable
external and internal friction. Next the depth of the
ocean is about 5 miles at the greatest and has an average
value of only two to three miles. This means that a wave
travelling freely would take much longer than the theoretical
time to run round the earth. However the tidal wave is
not free but is forced. It receives what may be regarded
as a periodically repeated impulse and so it travels, as a
forced oscillation, round the earth with the same period as,
but lagging behind the generating body.

Nextj the earth is not covered completely by the ocean.
The water and land surfaces exhibit a complicated figure
which, almost everywhere, obstructs the passage of a wave
round the earth. Also the depth of the ocean varies in a
highly irregular way.

The progressive wave theory. Taking account of these
qualifications a theory that makes an approach to
considering actuality has been elaborated. In the
Antarctic Ocean we have a zone of earth completely
covered by ocean. This zone, it is true, contracts in
width to some 600 miles between the Antarctic Continent
and Cape Horn, but let that be regarded as offering a
sufficient water way for a tidal wave. The latter, then.

THE TIDES 221

sweep? round the Southern Ocean as a forced oscillation,
kept going, so to speak, by the continual impetus of the
tide-gen«rating force. Its main constituents have periods
of about 12^ and 24| hours. It lags behind the impetus
so that, for instance, low water may occur at a place when
the equilib?ium principle would make high water. The
tidal waves have, however, the same periodicities as the
tide-generatijg forces.

The AtlantK, Pacific and Indian Oceans, we have seen,
may be regarded as gulfs opening out from the Southern
Ocean. Near tke equator the Atlantic coasts tend to east
and west so that there is an obstruction there which does
not exist in the other two oceans. The forced wave, then,
which sweeps romd the earth in the zone of Southern
Ocean initiates free waves as it passes across the mouths
of the oceanic gulfs. These free waves then travel up the
Pacific and Indian Oceans. One travels up the South
Atlantic, becomes obstructed near the equatorial latitudes
and initiates a new fret wave in the North Atlantic.

These " free " waves travel, in the main, from south to
north, undergoing dissipation of energy, but all the time
causing tidal phenomena. They are, however, not quite
free, for tidal waves are set up in the North and South
Atlantic basins, in the Pacific and in the Indian Ocean
and these are forced in a periodic way just as is the forced
generating wave in the Southern Ocean. Thus the periodic
impetus given by the latter to the subsidiary waves becomes
greatly modified. Any tidal wave in the Atlantic Ocean,
say, thus originates in the Antarctic and travels
progressively up to high nortiern latitudes. There must,
then, be a series of fronts as the wave advances : the
latter is in different latitudes at different times. These
fronts are represented by the elder figures of cotidal lines
which showed how the wave progressed as it passed from
south to north.

To go into some illustrative detail : the tidal wave
front was said to approach tht British Islands from the

222 AN INTRODUCTION TO OCEANOGRAPHY

south west, or thereabouts, and to spht off the sruth of
Ireland. A subsidiary wave thus enters the Enghsh
Channel (where the configuration of the coast I:ne plays
all sorts of tricks with it) and goes on into the Xorth Sea.
Another wave enters the Irish Sea through Sc. George's
Channel, and a third enters the same area through the
Channel between Ireland and Scotland. The parent wave
makes tides on the West coasts of Great Britain and Ireland
and then passes round the North of Scotland into the North
Sea. The older figures of cotidal linei? showed these
progressive waves very beautifully. The newer figures do
not. The " age " of the tide represented the time that
had elapsed since its origin in the Soutaern Ocean.

Finally, whatever difficulties in thij conception of the
tides are not explained can be accounted for very plausibly
by ascribing them to the influence o:' the land outlines or
to the great irregularities of depth of the oceans. Since
deviations in the progress of a free wave could hardly be
deduced a priori from a knowledge of these irregularities
such explanations cannot be questioned.

The stationary ivave theory, li a small shallow vessel
containing water, say an ordinary " developing " tray, be
gently tilted a wave of a special kind will be established.
The body of water will vibrate as a whole, the level rising
on one side and falling on the other while the level at the
middle part remains stationary. A single impulse will set
up a series of stationary wa\es of this kind and these will
rapidly disappear by dissipation of their energy. If a
regular timed impulse be given by rocking the dish with a
periodicity which can easily be found by watching the
oscillations of the water the latter can be maintained. If,
while the dish is rocking from end to end the tilting move-
ments be suddenly stopped and then the dish rocked from
side to side the wave will l>e turned through a right angle.
If the dish be rocked succtssively from side to side, corner
to corner and end to end waves having these directions
will be set up. The direction will change as the rocking

THE TIDES 223

motion changes. The central part of the water will remain
approximately level.

The recent work on the theory of the tides looks upon
the phenomena hi something like this manner and the
same kind of treatment applies to each ocean or sea region
considered as a unit. What we have is a system — the
solid earth and the water on it — that is made to oscillate.
The oscillations are established by the periodically repeated
tide-generating force set up by the gravitational field of
the sun and moon, and the intensity of this force changes
as the relative positions of the earth, sun and moon change.
These relative positions, and thus the variations in hitensity
of the tide-generating force, can be traced with considerable
accuracy.

The material that oscillates is the water of the oceans
and seas. The sohd matter of the earth itself can also
be shown to oscillate but the effect is extremely small and
need not be considered here.

The osqillations of the water are forced by the periodically
repeated impulses of the tide-generating force. But the
masses of water considered, say the North Atlantic Ocean,
or the North Sea, have certain shapes and depths and
there are tributary seas, bays, straits, &c., in open
connections with them. There are also capes, islands, &c.,
in the way of the forced waves, obstructing them. The
depths of the water varies from place to place. Further,
and very important, the whole mass of water considered is
in rotation with respect to the earth's axis, therefore it has
properties analogous to those of the rotating material of a
gyroscope.

In each ocean or sea basin, then, we have a system such
as is represented, in miniature, by the rocking developing
dish. In the centre of the ocean, or sea, there will be a
nodal point (an amphidromic point) where the water
streams but does not rise or fall. Usually there will be
several such nodal points according to the configuration
of the basin and its relations with other basins. Suppose,

224 AN INTRODUCTION TO OCEANOGRAPHY

■JO ^|o SO 4|o 30 2 0 /C| c /O flo

\^ \^ f Hk / .#

60

^— -r f ^

60

1 ""\ \ / MiJ&J

50

€^v y f/yj •

^0

40

^f — ^^3^^''^^''%^^^^ ^ ^

■/u>

30

1 /h/ l^fN#''

30

20

?0

10

"^^^^^ / ^^ij;!^^

/o

0

f ^^--C, l^\

ja.

10

%■■ ¥k / 5-

/o

2.0

5o

to

50

4o

I ■■■'C—-^ 0 ^^vH:

^-^
^

«o

f #^---______,.i--VV

50

f ^^— — — ^_,

...-^ 6\a/\

^

\

\\

7 0 60 J 0 A-p 3p 20 /O 0

/|o Bp 1

Fig. 39.

The co-tidal lines in the Atlantic Ocean. The small circles
are the amphidromic points. The radial lines mark the
successive positions of the forced wave that swings round the
nodal points. The numbers on the radii give the hours when
the crest of the wave has the positions marked by these
radii. The amplitude of the wave is zero at the nodal point
and maximal at the other extremity of a radius.

THE TIDES 225

for simplicity, that there is only one amphidromic point,
then, along radii from this the water will rise and fall.
The oscillations will swing round the nodal point in a
certain direction.

This short explanation will, it is hoped, give the reader
some idea of the properties of a tidal system, treated in
the modern way. Further investigation is, of course,
exceedingly technical.

Tidal predictions. — It is not difficult to see how the
times and heights of the tides, in the British seas at all
events, can be predicted with a degree of approximation
that is near enough for rough navigational requirements.
Such predictions would, of course, be entirely empirical
ones and must have been made, in the days before tide-
tables, by many old experienced sailors and fishermen who
were entirely illiterate. Doubtless they are stiU so
made.

To begin with, it must have been noticed that spring
tides were highest at a certain interval of time after the
moon was full and new. Then the rise and fall of the
water level must have been observed with considerable
accuracy because it was seen, not on a vertically standing
tide-gauge scale, but on a foreshore of very gentle slope,
when even an inch or two of rise makes a very noticeable
difference in the appearance of the beach. The tide-
marks made by shore debris also provide very exact
measures of the rise of the highest tides. Further the
correct estimation of the depth of the sea over sand banks,
bars, and in narrow channels is a matter of practice upon
which everything in fishing in inshore waters depends.
The time also can be estimated with considerable accuracy
by men who are able to note exactly the southing of the
sun at midday.

The highest of the spring tides must then have been
observed to occur at the same time during a certain day
after that on which the moon was full or new. Then it
would be noticed that the tides would " take off " for

Q

226 AN INTRODUCTION TO OCEANOGRAPHY

about a week by which time they would rise to a minimum
height, also at a definite time in the day. Then they
would " make " for a similar period and would again
reach the maximum height at the same time of day as
they did on the occasion of the last springs. It would be
noticed that there was a certain rough alternation of higher
and lower spring tides and that twice a year, about the
times when day and night were about equal, extra high
springs would happen. There would be an average rise
and fall of the height of the tides and variations from this
would be expected. It would be very apparent, of course,
that high water was a little later (by an uiterval of time
easily approximable) each day. It would be noticed then,
as it is now, that winds blowing in certain directions
tended to raise or " cut " the tides, or hasten or slacken
the ebbs. Such knowledge, all coming in the course of
the day's work would be Httle inferior in point of practical
value than that obtained from the use of a modern tide-
table.

First of all there is a basis of observation : the heights
and times of high and low water have been recorded by a
tide gauge and it is assumed that this is available
for a period of at least a year and a quarter. As a rule
longer periods than this are used. There are then two
principal methods by which the observations are
reduced.

(1) Empirical {non-harmonic) methods. These are the
methods by which the British Tide-Tables have been
calculated in the past and they can only be briefly indicated.
The Table on p. 228 may be taken to represent the
observations. They are really the predictions but the
difierence between them and the results, as given by the
tide-gauges, may be neglected so far as our present purpose
is concerned. Three months are chosen : January, March
and July, and in addition to the figures taken from the
Admiralty Tables some other astronomical data are
quoted. To save space only the first few lines of one

THE TIDES 227

Table are actually given. The reader can complete them
from the Admiralty Tide Tables and Nautical Almanac.

Col. (1) gives the day of the month ; (2) gives the times
of high and low waters and the heights above the datum
mark.* In general there are two high and two low waters
in each day but occasionally there is only one. The heights
are in feet and tenths and the times are " standard."
00 hrs. is midnight ; 12 hrs. is noon ; hrs. less than 12 are
a.m., more than 12, p.m. Col. (3) gives the times and
heights of the low waters. Col. (4) gives the times of the
moon's transit across the meridian (also in standard time).
The " upper " transit is that which occurs on the meridian
above the horizon ; the " lower " one occurs half a lunar
day later, below the horizon. Col. (5) we explain presently.
Cols. (6) and (7) give the declinations of the moon and sun
at apparent noon at Greenwich. These are the distances
in degrees and minutes N. or S. of the body from the equator.
Col. (8) gives the phases of the moon and the dates of her
positions furthest north or south of the equator, as well
as her positions in perigee and apogee. The moon's
transits are taken from the Admiralty Tide-Tables and the
declinations, phases, etc., are taken from the Nautical
Almanac. If the data for the transits and declinations
are to be used for any other place than Greenwich they
must be corrected for longitude. This, however, is most
conveniently done by a change in the scale when the dates
are graphed.

The times and heights of high and low waters must next
be related to the times when the moon is on the meridian
at the place of observation. These are given in Col. (4).
We take them one by one and look for the time of high
water of the tide immediately following a transit. Thus
on January 2, the moon crossed the meridian at 2 hrs.
57 mins. (her lower passage). The H.W. immediately
following this was that of January 2 at 13 hrs. 41 mins.

* When an asterisk is prefixed to a height it means that the latter
is really a distance below the datum.

228 AN INTRODUCTION TO OCEANOGRAPHY

We subtract 2 hrs. 57 mins. from 13 hrs. 41 mins. and get
10 hrs, 44 mins. — the entry in Col. (5) opposite the time
of high water on January 2 at 13 hrs. 41 mins. Again the
moon crossed the meridian above the horizon on January 2

Tides and associated celestial phenomena.
Liverpool, January, 1922.

1

2

3

4

5

6

7

1

High
water

Low
water

Moon's
transits

si

03 O

'i|
II

T3

o

n

h.

m.

feet

h.

m.

feet

upper
lower

H.W.

only

noon
0 1

noon
0 1

1

01
13

02
07

26.9
28.0

07
20

40
10

3-9
3-1

14 35
02 13

10 54

10 16

S.

23 02

S.

2

01
13

37
41

26-4
27-5

08
20

11
43

4-4
3-8

15 19
02 57

11 02
10 44

6 55

22 56

A]

3

02
14

12
16

25-7
26-7

08
21

44
15

5-2

4-8

16 01
03 40

10 53
10 36

3 17

22 52

4

02
14

48
54

24-9
25-7

09
21

19
49

6-1

5-8

16 44

04 22

10 47
10 32

0 28

22 46

Ec

at 15 hrs. 19 mins. and the high water immediately following
this was that occurring on January 3 at 02 hrs. 12 mins.
Take 15 hrs. 19 mms. from 24 hrs. and add 2 hrs. 12 mins.
to the result : we get 10 hrs. 53 nuns, as the interval that
elapsed between the moon's transit on 2 January at 15 hrs.

THE TIDES 229

19 mins. and the high water immediately following on
3 January at 2 hrs. 12 mins.

Thus the " Lunitidal intervals " in Col. (5) give the
periods that elapsed between the times of high water
(on the same Hne) and the times of the preceding
transits of the moon. It will be seen that high water
(at Liverpool) follows a transit of the moon after an
interval of from 10| to 12J hours. The same calculations
can be appHed to the times of low water but the appUca-
tions to times of high water will illustrate the method
well enough.

Imagine that the moon were to revolve round the earth
in the plane of the equinoctial, and with a period of 24
solar hours, and that there were no sun. There would be
a series of high waters, all of precisely the same height and
occurring at a certain constant time (the lunitidal interval)
after the moon had crossed the meridian of the place.
One observation of the time of transit, the time of high
water and the height of high water would enable us to
predict the times and heights of H.W. at any time in the
future.

Imagine now that the sun were to revolve round the
earth in the plane of the equinoctial and with the same
period as the moon, and further that it were to cross the
meridian at the same times. The tide caused by the sun
would now be added to that caused by the moon, but the
times of H.W. would remain the same. This would also
be the case if the sun crossed the meridian 12 hrs. exactly
after the moon.

If, however, the sun were to cross the meridian 6 hours
before or after the transit of the moon the tide caused by
him would have to be subtracted from the tide caused by
the moon : this follows from pp. 213-4. The times of H.W.
would still be the same.

Still suppose that the moon and sun to revolve roimd
the earth in the plane of the equinoctial but give these
two bodies each its real period. Let the sun cross the

230 AN INTRODUCTION TO OCEANOGRAPHY

meridian at intervals of 24 hours and the moon at intervals
of 24 hrs, 51 mins. There will now be a variable, angular
distance between the two tide-generating bodies. At new
moon and full moon they are both on the meridian at the
same time and the angle is zero : the two tides are now to
be added together. About 7 days later, the moon is in
quadrature, the angular distance between her and the sun
is about 90° : the solar tide must now be subtracted from
the lunar tide.

Therefore, in the course of a synodic month, the lunar
and solar tides reinforce each other at new and full moon
and we have springs. Halfway between these times the
solar tide diminishes the total one and we have
neaps. There is thus a continual variation in the
height of the tide and this has to be predicted. The
predictions must give the times of high and low water,
and the height of the compound tide generated by both
the bodies.

The establishment of the port. From the (completed)
Tables of p. 228 we might take all the moon's transits,
upper and lower, occurring within the twelve groups of
intervals, thus : — -

0 hour to 1 hour, 1 hour to 2 hours, etc... 11 hours to 12 hours,
and 11 „ „ 13 „ 13 „ „ 14 23 „ „ 24 „

Opposite each transit we place the heights of high water
and low water of the tide that immediately follows, and
also the lunitidal interval. Thus, from the tables we get
that on p. 231.

Thus the 7nean time of transit is 0 hr. 31 mins.
(If a sufficiently large number of data had been
taken this would have been 0 hr. 30 mins.) The wean
height of high waters following the transits of 0 hr. to 1 hr.
and 12 hr. to 13 hr. is 28-7 feet, and the mean lunitidal
interval between the transits and the following high waters
is 11 hr. 21 mins.

THE TIDES 231

Moon's transits occurring between 0 hour and 1 hour and 12 and 13 hours.

Times of transits.

Heights of the tides

following the
preceding transits.

Lunitidal intervals

between the transits

and the following tides.

H.

M.

FEET.

H.

M.

00

29

30-3

09

12

59

29-3

07

12

32

271

37

00

09

28-1

35

00

54

28-3

33

12

09

301

19

00

37

31-5

10

12

03

27-5

45

12

47

27-8

31

00

25

28-2

36

12

21

28-1

18

00

46

26-9

21

12

05

29-1

08

12

58

29-8

00

00

29

28-2

15

Means 0

31

28-7

11

21

Doing this for each group of transits we get the complete
table on p. 232.

Finally we plot the mean times of the transits, as
abscissae, against the lunitidal intervals as ordinates, and

232 AN INTKODUCTION TO OCEANOGRAPHY

draw a smooth curve through these points : this gives
the graph A of Fig, 40. Also the heights of the high waters
are plotted as ordinates against the mean time of transits

Times of
moon's
transits.

Mean times of
moon's
transits.

H. M.

Mean heights of

H.W. of the

tides following

the preceding

transits.

Mean
lunitidal
intervals.

H. M.

0 — 1 hour
12—13 „

0 30

28-7

11 21

1—2 hour
12—14 „

1 30

28-8

11 07

2 — 3 hour
14—15 „

2 30

28-6

10 57

3 — 4 hour
15—16 „

3 30

27-6

10 43

4 — 5 hour
16—17 „

4 30

26-2

10 35

5 — 6 hour
17—18 „

5 30

24-7

10 39

6 — 7 hour
18—19 „

6 30

23-3

10 50

7 — 8 hour
19—20 „

7 30

23- 1

11 19

9—10 hour
20—21 „

8 30

23-9

11 43

10 — 11 hour
21—22 „

9 30

25-3

11 54

11—12 hour
22—23 „

10 30

26-9

11 49

12— 0 hour
23—24 „

11 30

28-1

11 36

as abscissae and a smooth curve is drawn : this gives the
graph B of Fig. 40.

From these graphs we can interpolate any time of
transit and find the corresponding lunitidal interval and

THE TIDES

233

height of high water. The example given deals only with
three months' observations : in actual work some years
would be taken. Also the low waters would be treated in
a similar way.

These methods now enable us to find the times
and heights of high and low water of the average
luni-solar tide. Thus let it be required to predict the
time and height of the tide at high water on September
1st, 1922.

c

3

h-i"-

^<s^'\^ — ^^\

^'

ino

N\ \<^^ y^ ^

^SV^ /• \0 y/^

II oo

■ ^^a-^rda^V^

10-30

A ^-^^

0

loco

i?

>.

fi-

Jff^ '■ """.k,^^

ft

IS

^^^Nv y'^''^

<»■

2.1

>v^ )^sy^^

3-

rt-

lb

B X_^

3-

3.S-

3.M.

1j

X 3

-+

O 'l i. 3 J^ A- 6 ^ i ^ '" '''

iL /3 1^- IS- . '6 n le /f xo XI 3.1. i3

Ti mes or moon's fra-nsi -r.s.

Fig. 40.

We look up the times of the moon's transits on the day
before, and on that day. These are : —

August 31, upper transit at 19 hours 48 minutes.

lower „ „ 07 „ 22

September 1, upper ,, „ 20 „ 38 „

lower „ „ 08 „ 13

and next we find the lunitidal intervals for the tides
following the transits at 19 hrs. 48 mins. (that is, 7.48 p.m.)
on 31 Aug., and at 08 hrs. 13 mins. (that is, 7.22 a.m.) on 1st
Sept. The intervals read from Fig. 40, A are 11 hr. 26 mins.
and 11 hrs. 15 mins. respectively. The times of H.W. are

234 AN INTRODUCTION TO OCEANOGRAPHY

therefore 7 hrs. 48 mins. + 11 hrs. 26 mins. = 19 hrs. 14
mins. (or 7.14 a.m.), and 08 hrs. 13 mins, + 11 hrs, 15 mins.
= 19 hrs. 28 mins. (or 7.28 p.m.). The Admiralty predic-
tions give 7.16 a.m. and 7.50 p.m. respectively. Next we
find the heights from Fig. 40 B, by reading the ordinates
on the abscissae, 7.48 and 8.13 : these are 23.3 and 23.6
feet. The Admiralty predictions give 22.4 and 23.6
respectively.

A calculation of this kind, carried out on several years'
data (or those for one and a quarter years at least) give
the results called the establishment of the port. This
states that high water will occur at a certain average
interval after the moon has crossed the meridian (visibly
at full moon and invisibly at new moon). This interval,
as obtained from the three months data given on p. 232
is 11 hrs. and 28 mins. It also states that the average
height of high water will be 28-4 feet. As deduced from a
long series of observations recorded in the Hydrographic
Department of the Admiralty the average constants are : —

High water at full moon and change of moon occurs
at 11 hrs. 17 mins.

The height of spring tides at high water is 27-66 feet.

The former constant is abbreviated " H.W.F.C— 11,17."

The empirical predictions.. So far we have found what
will be the times of high and low water and the
corresponding heights of the tides provided that the
latter are generated by the moon and sun revolving round
the earth with their present speeds, in circular orbits and
in the plane of the equinoctial. The phase, that is, the
angular distance between the two bodies is always changing
but the method takes account of that and it is the combined
luni-solar semi-diurnal tide that we predict. It includes
what is called the phase inequality in the tide, that is, the
effect on the lunar semi-diurnal tide, of the solar semi-
diurnal tide.

But (1) the orbits are ellipses so that the moon and sun
are at variable distances from the earth and their tide-

THE TIDES 235

generating forces are proportional to the inverse cubes of
their distances and (2) the planes of the orbits are not in
the equinoctial. The moon's orbit is inclined 28^° and
the sun's 23^° to the equinoctial. These are the maximal
declinations of the two bodies. The moon may be as much
as 28|° North or South of the Equator and the sun may be
as much as 23|° North or South of the Equator.

It is necessary, then, to correct the values as obtained
by the above methods. Because of declination each of
the two principal tides — ^the lunar semi-diurnal and the
solar semi-diurnal — is accompanied by a diurnal tide.
Therefore one of the two tides of each day is usually
higher than the other. This means that two corrections
must be applied : —

(1) One for the lunar, tropical, diurnal inequality (or
diurnal effect on the phase inequality of changes in the
moon's declination). Period one synodic month.

(2) One for the solar, tropical, diurnal inequality (or
diurnal effect on the phase inequality of changes in the
sun's declination). Period, one solar year.

The changing declination of moon and sun also affect
the principal lunar and solar tides so other two corrections
must be applied : —

(3) One for the lunar, tropical, semi-diurnal inequality
(or semi-diurnal effect on the phase inequality of changes
in the moon's declination). Period, half synodic month.

(4) One for the solar, tropical, semi-diurnal inequality
(or semi-diurnal effect on the phase inequality of changes
in the sun's declination). Period, half solar year.

The changing distances of the moon and sun from the
earth also affect the principal lunar and solar tides and two
further corrections are required : —

(5) One for the lunar, anomalistic, semi-diurnal
inequality (or effect on the phase inequality of changes in
the moon's distance). Period, one anomalistic month.

236 AN INTRODUCTION TO OCEANOGRAPHY

(6) One for the solar, anomalistic, semi-diurnal
inequality (or effect on the phase inequahty of
changes in the sun's distance). Period, one solar year

Finally there is an annual variation in height due to
seasonal meteorological changes. Period, one solar year.

How these inequalities affect the heights and times of
high water may be seen from the graphs, Figs. 41 to 43.
In each of these the heights of the tides are represented by
a simple Linear run and fall. Above this the declination,
or distance in degrees above and below the equator, of the
moon and sun are shown. Above that again the phases
of the moon are recorded, with the positions of this body
in perigee and apogee.

13 I 4-\ £1 si 7 16 1-9 \/o\ //\/s\/3\/4-\/^\ /<>] I7\ IB\/9\so\Sl\SP\S3\S'h\P6

Fig. 41. Low spring tides.

P Fig. 41. — Low s'pring tides. This relates to the Liverpool
tides in July of 1922. The highest springs follow the moon
in conjunction after an interval of 1| days from the time
of transit. The sun is very near his highest northerly
declination. The moon is near her lowest southerly
declination and she is nearer apogee than perigee. The
sun is near his furthest distance from the earth. The

THE TIDES

237

maximal declinations lower the height of the tides and so
do the positions of moon and smi (both being near their
maximum distances from the earth). Therefore, we have
low spring tides, the range being only 26-3 ft.

Fig. 42. — High {equinoctial) spring tides. These are the
tides of Liverpool, March, 1922. The springs occur about
the middle of the month. On 13 March the moon is in
opposition, is on the equator (no declination) and is in

.5" I 6 I 7 ['■^T^ /'^l //|/g|/3|/»j /S\ /6| /7\/e\/S>\^<^ g/|gflga|^/|P^|g6|g7|gg

5" I 6 I 7 "g" S> lo\ II I I2\li\ m I5\ lb\ /7\/8\/o\po\? Asel^BP-^ ^£\S6\l:^7WS.

Fig. 42. High (equinoctial) spring tides.

perigee. The sun is nearly on the equator (the declination
is very small) and he is about at his mean distance from the
earth. The effect of no declination of the moon and very
Httle decUnation of the sun, with the moon nearest to the
earth, is to raise the height of the tide. The range is
great, 32-9 feet.

Fig. 43. — Very high {equinoctial) spring tides. These are
the tides of Liverpool, September, 1922. The sun and
moon are together almost on the equator on Sept. 20, on
which day the moon is in conjunction and in perigee.
The sun is about at his mean distance (on the 21st there
was a solar ecHpse). All these conditions make for high
tides and those graphed are the highest of the year.

238 AN INTRODUCTION TO OCEANOGRAPHY

Fig. 43. Very high (equinoctial) spring tides.

THE TIDES 239

Application of the corrections. The approximate prediction ,
obtained by the method of pp. 226-34 are now compared
with the actual records : there will be in each case, both a
difference in the times and heights of high and low waters.
This difference is the total correction which must be made
to each prediction and it is due to all the seven factors
mentioned on p. 235. Now by taking selected groups of
tides it is found that, in each group the effect of each of
the factors is different and, of course, in each of these cases
the group of factors can be equated to a total value for
the corrections. Thus we obtain a system of equations
and by solving these a value of each factor can be found.
The work is laborious and comphcated, but not difficult
and it involves a technique which is of interest only when
one attempts the computations. It has been sufficiently
indicated above.

(2) The harmonic method. Thus the periodic rise and
fall of water level that we caU the tide is considered to be
due to a wave established by the sun and moon. This is
an average wave and it is affected by a number of
" inequalities." A value for each inequality is found
empirically, and the average heights and times are corrected
accordingly. The method was developed during the 17th
and 18th centuries, and tide-tables for the ports of London
and Liverpool' were calculated by secret, or undivulged
rules and were published by private individuals. Early in
the 19th century the methods were studied by Whewell
and Lubbock, and new tables, accompanied by a description
of the modes of calculation, were published. It is incredible
to read, in the works of Whewell, that the pubhcation of
these new tables " was resented as an infringement of the
rights of property " though this is no more incredible than
the proprietary status, at the present time, of other things
which are public necessities. Towards the middle of the
19th century, however, an entirely new method of
calculating tide tables was evolved from the work of Lord
Kelvin, Sir George Darwin and others.

240 AN INTRODUCTION TO OCEANOGRAPHY

This is the method of harmonic constituents and it is
elegant in the extreme. To understand it we must
remember that the tides are generated by the variable
gravitational field established by the moon and sun, and
that the intensity of this field can only vary because of
the motions of the moon and sun relatively to the earth.
These motions are absolutely periodic, are known with
great accuracy and can be predicted. The configuration
of the land and sea, and the depth of the latter, affect the
progress of the tidal wave as well as its height, but these
topographical conditions are invariable throughout any
period of time that we need consider. By a study of the
motions of the earth, moon and sun we can predict the
variations in tide-generating force, and by an empirical
study of the local tides we can find how they vary as the
tide-generating force varies. There are certain aperiodic
causes of variation in the heights of the tides and these
we consider presently. Thus we find that- a real tidal
wave can be regarded as being made up of constituent
waves. It is easier, however, to think about the matter
in the following way.

Partial tides. The tidal wave is an undulation that is
periodic. If we make a graph of the rise and fall of the
water level at a certain place, and for a period of several
years we can get a graph that repeats itself in time. (Here
we neglect the very long periodic variations referred to
in p. 217). A property of such a graph is that it can be
decomposed into a number of partial waves, each of them
absolutely periodic and having a certain (harmonic) form.
The combination of these partial waves, that is, their
algebraic summation, gives us the observed tidal
undulation.

Thus the average tide referred to on p. 233 consists of two
partials, one due to the moon and one to the sun. The
period of the lunar partial tide is 12 hours 25 minutes and
14i seconds, and the crests of the waves follow each other
at exactly this interval.

THE TIDES 241

The period of the solar partial tide is exactly 12 hours
and its crests follow each other at such intervals. At new
and full moon the two bodies are together on the meridian
and the crests of their tidal waves are together then, or
at a certain fixed time afterwards. They reinforce each
other so that the total tide is represented by 5 + 2. At
first and last lunar quarters the crest of the lunar partial
wave coincides with the trough of the solar partial wave,
and the total tide is now represented by 5 — 2.

The tidal satellites. The following " fiction " also assists
in understanding the process of harmonic ajialysis. The
moon and sun move, not with an invariable orbit round
the earth,* but in eUiptical paths variously eccentric and
variously inclined to the plane of the equinoctial. The
variations are known and are precisely predictable. We
can now replace the actual moon by a series of imaginary
moons, each with a certain mass and orbit. The paths
of these orbits are all precisely circular and either in the
plane of the earth's equator, or in certain fixed planes
parallel to the latter. The distance between the earth
and each satelHte is constant. Each satellite has a certain
orbital speed and period and a certain phase. Acting
together they will generate the same gravitational field as the
actual moon does.

So also we replace the actual sun by a similar series of
imaginary suns so chosen as to their masses, distances,
orbits and phases of revolutionary paths as to set up
exactly the same gravitational field as does the real sun.
The advantage that all this additional complication gives
us is that instead of a real moon and sun moving round
the earth in paths that vary in disconcerting ways we now
have a number of sims and moons round the earth in paths
absolutely fixed and with constant periods, velocities and
phases. At once they become amenable to mathematical
treatment.

* We regard the motions of sun and moon as explainable for the present
jnirpose, in this way.

I

242 AN INTRODUCTION TO OCEANOGRAPHY

Tidal constituents. Each satellite, if it existed, would
set up a tidal wave on an earth covered completely with
ideal water to a certain ideal depth. This wave would be
harmonic in form. It does not matter that the earth is
as we know it, and that the water has friction, and that
the depth of the ocean varies in a troublesome way. At
any place these actual conditions retard and deform the
waves due to the satellites, but the retardations and
deformations are always the same. All the partial waves,
set up by the tidal satellites combine to give us the resultant
wave that we observe. We can now abandon our fiction of
" tidal satellites " and think about the actual tidal wave as
representing the effect of summation of a number of tidal
constituents. There are various classes of the latter, and
the most important ones are : —

(a) Semi-diurnal constituents.
Mg , Principal lunar ;
Sg, Principal solar ;
Ng , Larger lunar elliptical ;
Kg, Luni-solar declinational ;
Vg , Larger lunar evectional ;
Lg , Smaller limar elliptical ;
Tg, Solar elliptical ;
2N2 , Second order lunar eUiptical ;
yUgj Lunar variational ;
A-2, Smaller lunar evectional.

(6) Diurnal constituents.

Kj^, Limi-solar declinational ;

Oj^, Larger lunar declinational ;

PjL, Larger solar declinational ;

Qj^, Lunar elliptical ;

Jj^, Supplementary lunar elliptical ;

00 j^, Second order lunar declinational ;

p^, Lunar evectional ;

2Qj^, Second order lunar elliptical ;

0-, , Lunar variational.

THE TIDES 243

(c) Long period constituents.

Mf , Lunar fortnightly ;

MSf, Lunar fortnightly variational ;

Mm, Lunar monthly ;

Lunar monthly evectional ;

Three-monthly ;
Ssa, Solar semi-annual ;
Sa, Solar annual ;

Nineteen-yearly.

(d) Other constituents applicable to shallow water.

All the constituents have periods that are accurately
known and are deducible from astronomical constants.
The semi-diumals have periods of nearly half a day, and
so on. The symbols M and S denote the application to
the moon and sun respectively, and the suffixes 1,2, etc.,
indicate the periodicities.

Each constituent wave is of exactly harmonic form —
this is absolutely necessary for the success of the predictions.
Now an harmonic wave in deep water becomes non-
harmonic in shallow water, that is, the front of the wave
becomes steeper than the rear of the wave. But such a
non-harmonic wave can be represented by the summation
of two or more harmonic ones. If, then, the constituent
M2 becomes non-harmonic in approaching a shallow water
port it is replaced by the series of waves, M2, Mg, M,^ each
of which is harmonic. M3 has twice the speed of Mg, and
M^ three times the speed of M2. Added together, with
respect to phase, they make up a wave of non-harmonic
form.

An example. A concrete example will enable the reader
to visualise the process.

Here we have the complete wave that results from the
algebraic summation of the constituent waves, MgjSgjNg,
K^, 0^, K2, Pj^, M^ and MS^ (which is a compound of M^
and S2). The mean line of each graph represents a height

244 AN INTRODUCTION TO OCEANOGRAPHY

Fig. 44.

THE TIDES 245

above datum mark of 6-5 feet. To this are added, according
to their signs, the ordinates of the other graphs. Thus we
obtain the resultant, or predicted tidal undulation. The
horizontal scale shows time, in solar hours, and the vertical
one shows heights above and below the mean in feet.

The evaluation of the tidal constants : harmonic analysis.
Each constituent is represented by a wave, or oscillation,
which has an harmonic form. It can be put as a function
of the time, thus :

Height of tide = a sin {bt + c) •

Here there are three constants, a, b and c. The amplitude
of the wave, that is the distance it rises above, or sinks
below its mean level, is defined by a. The period of the
wave, that is, the time that elapses between every two
complete oscillations, is defined by b. The initial phase of
the wave, at zero time, is defined by c.

Now suppose there are only two tidal constituents in
the whole tidal wave : the principal solar {S^) and principal
lunar (M^) ones. At some one moment their high waters
are coincident. In 12 hrs. 00 mins. 00 sees, the solar
undulation again makes high water and this time (one
half solar day) is its period. But not until 12 hrs. 25 mins.
16|^ sees, will the lunar wave again make high water :
thus there is a difference of phase of 25 mins. 16^ sees,
between the two constituents. Also the lunar wave will
rise to a height of (say) 5h feet above sea level (h depending
on the locality) whereas the solar one will only rise to a
height of about 2h feet. These maximum heights of the
waves are their amplitudes.

The periods of the constituents are found from
astronomical theory. Thus the mean sun is in the same
position, with respect to some one place on the earth every
24 hours, but the mean moon comes back to the same
position after 24 hours 50 mins. 32^ sees. Thus we get
the periods of the tidal constituents by a priori reasoning,
but the ampUtudes and phase must be found a posteriori

246 AN INTRODUCTION TO OCEANOGRAPHY

by analyses of the recorded tidal data for the place in
question. The actual work is highly complex, but the
principles involved are fairly easy to understand.

Assume, then, that the whole tidal undulation is made
up of the two partial waves, M^ and S^. It may be
simpler if the reader visualises the matter and thinks of
the tide as being set up by M^, which is an imaginary
satellite revolving round the earth in the period of 24 hrs.
50 mins. 32| sees., and S.^ , which makes a similar revolution
in 24 hrs. 00 mins. 00 sees. There are, of course, two tides
for each satellite.

If only S^ existed the tide would recur every 12 hrs.
exactly, and if only M^ existed the tide would recur every
12 hrs. 25 mins. 16^ sees. As it is, however, the real tide
is due to both M^ and S^, and the records of the tidal
gauge, therefore, contain the effects due to the two
gravitational fields. We have to disentangle them. It is
assumed, of course, that there are absolutely no other
causes of variability, in the tides, than those due to M^
and jSg .

In the graph (Fig. 44a) the horizontal scale represents
hours of solar time, and the vertical scale represents the
heights of the two constituent tides, M^ and S^. Time and
tide, so far && S^ is concerned are one, for whatever makes
solar time also makes solar tides. At any given moment
on any number of days then, say 12 hrs. 00 mins. 00 sees
the height oi S^ will always be the same. Graph 44a (A)
is that of a whole series of tides occurring on the 1st, 2nd,
3rd — 15th days, say. Siuce there are two tides in each
day there must be 30 graphs in the figure, but they all
superpose exactly on each other to make one curve. High
water, low water, mean tide, &c., are always the same from
day to day — the same heights and the same times of
occurrence.

Fig. 44a (B) represents the corresponding series of tides
due to Mg. Now this constituent has the period of 12 hrs.
25 mins. 16 J sees., so that when it is graphed on the same

THE TIDES 247

horizontal scale {of solar hours) as that of A the individual
M^ tides will not superpose. Every high, water will be
about half an hour later in the solar day than the preceding
one. The vertical Une a — h in the two graphs represents
an arbitrary moment of solar time when we measure the
height of the resultant tide, as it is given by the gauge
records. Theory shows that the part of the tide due to
^2 will be exactly the same from day to day. So if we
knew the parts due to S^ on the 30 half days and summed
them and took the average, we should get a certain height.
Now notice that the line a — h cuts a series of about 14
curves, each representing a lunar tide occurring a.m. and
jp.m. on the first day, a.m. and jp.m. on the second day and
so on. If we could find and sum these 14 heights and
then take the average we should find that about 7 of them
were above the mean and 7 were below the mean, and
their average would be zero (approximately). Thus the
summation and averaging of the >S 2 -tides as given by their
heights at the same hour on each of a number of days will
give us a certain height, that of ^2 ^^ tha^t hour. But the
summation and averaging of the M^ tides, at the same
hour, on each of the same number of days will give us
mean sea level.

This will evidently be the case whatever hour during
the solar day we take. Draw a number of vertical lines on
the graphs so that they represent 12 noon, 1, 2, 3, 11 o'clock
and so on, and then measure the heights on the graphs.
In every case the average for M^ will be zero, while the
average for S^ will be the point on the A graph cut by the
vertical time-line.

We cannot, of course, isolate the ^Sg-tide from the
resultant graph given by the tidal gauge and treat it
separately from the Mg-tide. But in the summing and
averaging of the heights of the resultant tide, at the same
moment of solar time, on a number of days we really do
what has been imagined above. All the M^ parts average
out leaving mean sea level, while all the S^ parts remain

248 AN INTEODUCTION TO OCEANOGRAPHY

(all positive or all negative, or all zero, as the case may be)
thus giving us the height of the tide due to S^, above or
below mean sea level. We have actually eliminated M^
and left S^.

The principle of harmonic analysis should be clear from
this discussion. There are a number of constituents, M,^,

Fig. 44a.

(82, iVg, &c. All of them have different periods and all of
them contribute to the resultant tide. We take them
one by one. We take the solar day of 24 hours and then
measure the height of the tide at (say) 12 noon on a great
number of days, 1 p.m. on the same days, 2 p.m. on the
same days and so on. We sum and average and are left
with the height due to S^ at (say) 12 noon. M^^N^^&c.

THE TIDES 249

have been eliminated. We then take the lunar day of
24 hrs. 50 mins. 32^ sees, and divide this into 24 lunar
hours, each a little bit bigger than a solar hour, observe
the heights of the tide at 12 noon, 1 p.m., 2 p.m., &c.,
lunar hours, sum and average as before. This gives us
the height of the tide due to M^ and /Sg, iVg , &c., have been
eliminated. Then we find an N^ day, each hour of which
differs from both the solar and lunar days, and we go
through the same process, eliminating M^ and S^. We do
this for each of the 30 or so constituents of the whole
observed tide.

Also it is done for each hour in the particular kind of
day. Thus we find the rise and fall of the tide throughout
a complete period for each constituent.

Now note that the actual, practical methods of harmonic
analysis are rather different from that indicated above
(which would be impracticable because of the labour
involved). Very beautiful methods were evolved by Sir
George Darwin (and those who followed him) for shortening
the labour of computation. Still more fastidious methods
have been developed by the Liverpool investigators so
that the process of harmonic analysis is now made very
complete and accurate by ingenious artifices.

Harmonic synthesis. Thus we get a number of tidal
constituents (30 or thereabout in some cases). From 10
to 30 perhaps may be necessary for tidal prediction by
harmonic methods. Each constituent is a cosine function
of the form given on p. 245. Calculation enables us to find
the constants a, b, and c, and then solving the equations
for the various constituents, for each hour (say) of the day
we find the partial tides. (We can now dispense with the
fiction of the tidal satellites). The work of analysis is
done, once and for all for any particular port and is usually
based on several years observations of the gauge records.
The calculation of the times and heights of each partial
tide must be made anew for each year. Summing all the
partial tides we get their resultant — the predicted tide.

250 AN INTRODUCTION TO OCEANOGRAPHY

Combine this with the labour of the harmonic analysis
and the task of computation becomes prodigious.
Compared with it the auditing of the accounts of the South
African war — which, it is said, has not yet been completed
owing to the meagre staff of clerks at the War Office — is
as nothing ! However the harmonic analysis can be
greatly shortened as we have said, and the S3nithesis can
be carried out by very beautiful machines which auto-
matically sum the partial heights and record the results
on a clock face, or as a graph. The machines can take up
to about 30 constituents. They are " set " for the pairs
of constants applying to each constituent. The machine
is then actuated, when it writes a graph of the tides for
the whole year, showing against a scale of time the height
of the sea level from moment to moment. The graph is
then measured and the times of the heights of its
maxima and minima give the high and low waters of
each day.

The tides as marine agencies. — The effects of the tidal
streams, in co-operation with wave action in the erosion
of the coast line have been referred to in Chapter V. The
variations in height are of immense significance on those
coasts where the tidal range is significant, for it means
that a vertical distance of (say) some ten to twenty feet
is daily exposed to disintegrative marine agencies. Further
the flooding of extensive estuarine areas at high water
adds enormously to the extent of coast exposed to marine
agencies : the general tendency of tidal action in estuaries
must be to accelerate the gradual planing down of the
land, and the formation of submarine flats by attacking
the land from the inside, so to speak.

Obviously the distributing effect of the tidal streams on
eroded material is no less important. From the biological
point of view the effect is no less significant. The tidal
streams do not exactly reverse their directions when they
turn (as Fig. 31 indicates), and there must always be some
amount of residual drift in sea areas where the streams

THE TIDES 251

run strongly. The result of this is a gradual transference
of water in a definite direction, and this leads to the wide
distribution of the planktonic stages of demersal and
nektonic animals and plants. It has an effect also on
the distribution of the temperature and salinity of the sea
and these conditions are of the highest importance as
ecological agencies.

The effects of long-periodic variations in the tide-generating
force. These we have already referred to in pp. 217-9.
Take the maximum generating potential to be represented
by 100, then the variability can never be greater than
about 6% and is less than this in any constellation
considered above. Even such a variability in the
potential must have had large effects in altering the tidal
range, and so the velocities of the tidal streams. This has
led to effects of the greatest significance in regard to the
climate of Northern Europe : such effects have been
traced by Pettersson.

The general tendency of the movement of the water of
the North Atlantic is, we shall see, from South-West to
North-East. Coming, then, from regions of higher
temperature and salinity (because of increased solar
radiation and evaporation) the north-easterly drift must
transfer heat from South- West to North-East. This heat
is conveyed by ocean drifts on the surface, in the temperate
zones but by undercurrents in the more northerly zones,
since as the water cools it must increase in density and so
sink beneath the surface and continue to flow in the same
general direction as an intermediate or bottom current.
The original impetus which sets the water in motion we
shall consider in the next chapter, but it may be noted here
that it will be increased, or maintained, by the resultant
tidal drift and this will be increased again by an increase
in the tidal range.

Therefore, in those periods of increased tide-generating
force the quantity of heat transferred to high northern
latitudes must have been greater than when the generating

252 AN INTRODUCTION TO OCEANOGRAPHY

force had average values. The relatively warm and dense
surface currents must have gone further north before they
lost sufficient heat to cause them to sink and flow on as
bottom currents. Therefore polar ice must have melted
at a rather higher latitude then than it does now. There
must have been a greater disintegration of the pack ice
that fringes the polar sea area and a greater dispersal
south of bergs by the southerly flowing cold currents.
Floating ice must have been relatively abundant in northern
temperate latitudes compared with the present time.
Even now the limits of this distribution are variable
to a significant degree (from the point of view of
navigation).

Connected with the more vigorous water circulation
there were probably differences in the general distribution
of atmospheric pressure leading to more violent storms
than at the present, time. Occurring at times of high
water of the increased tidal ranges there must,
therefore, have been extensive tidal inundations on low
lying coasts. In such conditions (about 1287) the North
Sea transgressed on the Netherlands, breaking down the
dykes and forming the present Zuider Zee from a
previous lake.

In the Baltic exceptional conditions obtained during
periods of higher tides. At the present time the under-
current of relatively warm and dense Atlantic water that
flows in through the Skagerak is not very thick, though
even now tidal waves in this lower, denser and warmer
current are easily detected. In times of high tide-
generating force the depth of the undercurrent must have
been greater, that is, it was nearer to the surface and so
the upper, nearly fresh layers of the Baltic in the region
of the Belts and Sound became thinner and lost their heat
more readily during the winters. During these seasons
in the 13th, 14th and 15th (but not in the 16th) centuries,
the Baltic between Denmark, Germany, Sweden, Gothland
and Esthonia was frequently frozen over.

THE TIDES 253

Finally the increased tide-generating force had also its
effects on regions of earth-instability. There is evidence
that Iceland was nearly free from volcanic eruptions and
earthquakes during the period 800 to 1250, but that such
phenomena were relatively frequent during the period
1291 to 1348.

CHAPTER IX
THE OCEANIC CIRCULATION

Thus we have (1) a periodic change in sea level and (2)
a periodic streaming backwards and forwards of the water :
these are the tidal movements, and they represent the
responses of the hydrosphere to periodic change in the
gravitational field generated by the moon and smi. They
are complicated to a remarkable degree by the change
of depth of the ocean and by the irregular distribution of
land and sea.

In addition to them we have currents and drifts.
" Currents," in the older sense of the term, are movements
of the water of the ocean that are of importance in naviga-
tion and which, like the tidal streams, must always be
allowed for in fixing the position of a ship, " Drifts " are
slow movements of large bodies of water which are, as a
rule, imperceptible to seamen in the ordinary routine of
working a ship. The currents have velocities that are of
the same order of magnitude as those of the tidal streams,
or even of the flow of the great rivers. The
Gulf stream has velocities of from 1 to 5 knots ; the
Japanese stream flows at the rates of 2 to 3 knots, and the
Agulhas current has a velocity of about 2 knots. Drifts
are much slower : thus the North Atlantic Gulf stream
Drift moves at about the rate of 10 miles per day while
the creep of water along the sea bottom from the temperate
regions towards the equatorial zone is slower still. Currents
can be detected by direct measurements (see p. 298),
while the movements of drifts are usually deduced by
observations of surface or bottom floats set free and found
again after some considerable time, or by observations on

the salinity and temperature of the water.

254

THE OCEANIC CIRCULATION

255

The general scheme of oceanic circulation. — The primary
causes of oceanic circulation are everywhere the same, and
so we have a general scheme that applies to each of the
great oceans : the North and South Atlantics, the North
and South Pacifies and the Indian Ocean. The Arctic and
Antarctic Oceans are not included in this general scheme.

A- SouthPa-ci fie

e. North

Fig. 45. Diagrams of the main current systems in the North Atlantic,
the South Atlantic, the North Pacific, the South Pacific and
the Indian Ocean. The drawings have been made from
photographs of a marked globe.

Plainly there is a general scheme of circulation in each
case. The origin of the movement is the flow of water
along the equatorial zones from East to West — that is the
great equatorial streams. In the northern hemisphere the
current turns to the north, and in the southern to the south.

256 AN INTRODUCTION TO OCEANOGRAPHY

In each case there is a single or double closed circulation
or eddy, such that in the north the direction of flow changes
continually towards the right hand side, while in the south
the turning movement is towards the left. " Right " and
" left " are the directions apparent to an observer swimming
with the current. This deflection to the right or left is
due to the force exerted by the earth's rotation on bodies
free to move on its surface. Between the north and south
equatorial streams, there is a smaller counter- equatorial
stream flowing from West to East. From each main eddy
a drift takes origin, flowing to the North-East, in the
northern hemisphere, and to the South-East in the southern
one.

These latter drifts tend to enter the Arctic and Antarctic
Oceans from the North Atlantic and Pacific on the one
hand, and from the South Atlantic and Pacific on the
other. At the same time water flows South fr»m the
Arctic Ocean and North from the Antarctic, into the North
Atlantic, and South Pacific respectively. These latter
drifts are deflected towards the west in both cases,
that is, towards the right hand side in the northern
hemisphere, and towards the left hand side in the
South. In the North, and in each of the two oceans
(N. Atlantic and Pacific) the water turns round in the
direction of the hands of a clock. In the South, in the
S. Atlantic, S. Pacific and Indian Ocean the water turns
round in the opposite direction to that of the hands of
a clock.

Thus there is a general symmetry in all cases, but this
is modified locally and seasonally to a notable degree by
the configuration of land and sea, by the variations in
depth of the ocean and by the seasonal changes of the
winds. Further each of the great oceanic eddies shifts
North and South as a whole during the year in consequence
of the annual change in the sun's declination. This
general scheme we shall study in detail later, noting how
it is modified.

THE OCEANIC CIRCULATION 257

The general cause of oceanic circulation. — It will help the
reader if we consider first the circulation that would exist
if the earth were covered with water everywhere to a
uniform depth ; if it were devoid of rotational movement
and if the sun were to revolve round it, in the celestial
equator and with the apparent velocity that we observe.

In such conditions the water of the ocean would be
strongly heated in the equatorial region ; the quantity of
heat received per unit area would diminish towards the
North and South (being a fimction of the latitude) and
would be minimal at the pole. The strongly-heated
surface water near the equator would expand and become
less dense, and a difference in sea level would tend to
become established. Therefore there would be a surface
flow everywhere from the equator towards the temperate
latitudes in the north and south hemispheres.

At the same time the water would be evaporated to
some extent and so its salinity would increase. This
would tend to make it denser but the heating effect would
prevail, and so the northerly and southerly flowing water
would be less dense (because of its high temperature),
although it would really be more saline. But since it
flows into regions of lower air temperature it would lose
heat by radiation and so, at some intermediate latitude
(say 50° to 60°), it would fall to the temperature of the
water in situ. But being more saline than the latter it
would become more dense and would, therefore, sink
towards the sea bottom. Thus we have a surface flow
North and South from the equator and then a downward
movement towards the sea bottom as an undercurrent.

The evaporation at the equatorial zone would, however,
add large quantities of water vapour to the atmosphere
and this would be a continual process. It would be
compensated by an equal amount of precipitation of this
water vapour upon the surface of the sea as rain or snow.
Much of this precipitation wodd take place in the regions
round the poles. There would be a very low temperature

258 AN INTRODUCTION TO OCEANOGRAPHY

there, and so the precipitated rain and snow would
accumulate to form the polar ice-caps. This process, like
that of the excessive evaporation in the equatorial regions,
would be a continual one. The ice caps would tend
always to extend to south and north, breaking away as
icebergs which would, of necessity, drift outwards from
their places of formation into the sub-polar seas. There
they would slowly melt.

This melted ice would form water that would be less
saline than that of the sea round it. Because of its lower
salinity it would tend to be less dense, but because of its
low temperature it would tend to be more dense. As it
drifted towards temperate latitudes it would mix with the
more highly saline water in situ and so it would become
denser (because of its retained low temperature). It would,
therefore, sink towards the bottom at some intermediate
latitude and would continue to flow on in some direction
as an undercurrent.

The general features of these undercirculations must
also be noted. In the act of freezing, sea water becomes
denser, because a certain mass of sea ice contains less
salts than an equal mass of the sea water before freezing.
Therefore the " mother liquor " in which the sea ice is
formed becomes denser (by reason of its higher salinity)
and so sinks towards the bottom. At the poles, then,
there would be a downward movement of water from
surface to bottom. This would lead to a slow drift out-
wards in all directions from the poles along the sea bottom
radially towards the equator.

In equatorial zones the surface water becomes lighter
and flows along the surface towards temperate latitudes.
Cold water from the lower levels must, therefore, rise up
to take the place of that which has drifted away to north
and south, and thus the surface layers would be fed from
the bottom. Therefore there would be a slow drift, along
the sea bottom, of water from the temperate zones, towards
the equator.

THE OCEANIC CIRCULATION

259

Now so much can be deduced from first principles, but
the further study of these water movements by means of
hydrodynamical methods may be indicated, though the
mathematical reasoning cannot be reproduced. At any
depth in the sea the pressure on any small element of
surface can be found from the depth itself and the co-
efficient of compressibility of sea water. All points subject
to the same pressure lie on an isobaric surface and, in
general, the isobaric surfaces are parallel to the sea level.
The isobars increase, of course, in value from above down-
wards. As the pressure increases so the specific volume,
that is, the volume of unit mass of sea water, decreases

Surface

Fig. 46. (o) Part of a meridianal section of the ocean showing the
isobaric surfaces (thin Hnes) the isosteric surface (thick lines)
and the sections of the enclosed solenoids ; (6) A meridianal
section between 0° and 90° (shown on the flat) with the
deduced directions of water circulation.

(because of the compressibility). All points where the
specific volume has the same value lie on an isosteric
surface, and if the temperature of the sea were everywhere
the same these isosteric surfaces would also be parallel to
the sea level. The temperature, however, decreases from
the surface towards the bottom and from the equator
towards the poles (in the upper layers). Therefore the
isosteres will not be parallel to the isobars, but the two
systems of surface will cut each other as indicated in
Fig. 46 A, which represents a vertical section of the ocean
in a meridianal plane. Where the surfaces cut each other

260 AN INTRODUCTION TO OCEANOGRAPHY

they enclose prismatic blocks of water : these are called
solenoids and they are seen in section in Fig. 46 A. Now
the further study of the subject will show that, given this
distribution of isobaric surfaces, the movements of the
water particles can be deduced. Further than this it is
impossible to go here and we only note that a circulation
such as is represented in Fig. 46 B, can be deduced entirely
la priori, given only the distribution of pressure and
temperature.

Here we see the water rising vertically from the bottom
towards the surface, then streaming along the surface
towards the pole and sinking doAvn in latitude 40-50. At
the pole the water streams towards the equator and smks
down towards the bottom at latitude 60-50. From these
intermediate latitudes the water then streams along the
sea bottom towards the equator, on the one hand and
towards the pole on the other.

FerrelVs Law. — Next we consider the earth as still
covered everywhere to a uniform depth by the ocean, but
as rotating on its axis from West to East with a period
of one day. The effect of this is to establish a deflecting
force such that any particle free to move on the surface
tends always to be deflected towards the right in the
northern hemisphere, and towards the left in the South.
The deflecting force is a function of the latitude, is maximal
at the poles and is zero at the equator. It cannot set in
motion a particle at rest and can only deflect particles
that are already in motion.

I. — A particle moving eastwards on the earth^s surface.
The earth turns from W. to E. so that the centrifugal
force on the particle d is exerted in the plane of the circle
in which the particle rotates, that is, the parallel of latitude
ed. This force is represented in direction and magnitude
by db. At the same time there is a centripetal force due
to gravitation and acting towards the earth's centre.
This is represented by ad. Complete the parallelogram of
forces and we get dc as the component along the meridian

THE OCEANIC CIRCULATION

261

NE. When the particle is at rest this component balances
the poleward force (due to the tendency of the particle to
move to a position nearest to the earth's centre. When,
however, the particle moves to the East its motion is that
of the motion of a point on the earth's surface, the proper
motion of the particle. The centrifugal force da, therefore,
increases and with this the component dc. The poleward
component remains the same and so the particle d moves
towards the equator along the meridian NE, that is, to
the right.

Fig. 47.

//. — A particle moving to the West. With the construc-
tion of Fig. 47 it is easily seen that the resultant velocity
of the particle is the earth's velocity at the point in question
minus the proper motion of the particle. Since thfr
velocity of the particle is now less than it was relative to
the velocity of the earth beneath it, its centrifugal force
must decrease and, therefore, the poleward force is
unbalanced and the particle moves towards the North
along the meridian EN, that is, to the right.

(If the direction is between E. and N. there is an easterly
component. If between W. and N. there is a westerly
component. These components are treated in just the
same way.)

262 AN INTRODUCTION TO OCEANOGRAPHY

///. — A particle moving to the South. Fig. 48. Let the
particle be at a and have a proper motion to the South on
the earth's surface, that is, along the meridian NM. First
regard it as at rest at a, thus it has the velocity of a point
on the earth's surface in that latitude. Let it now be
given an impulse towards b ; obviously it will move South
still preserving the same easterly velocity conferred on it
at a by the earth. But at latitude o'b the velocity of a
point on the earth's surface is greater than at latitude oa
and so the particle will move more slowly, relatively to
the earth beneath it. Therefore it will lag behind the
earth and so move to the West, that is to the right.

■\Ai

IV. — A particle moving to the North. Simply reverse the
above construction. The particle will move to the East
as it passes from latitude o'b to latitude oa. That is, it
will move to the right. The above proofs apply to motion
in the northern hemisphere. Reversing them we get the
demonstration that deflection to the left occurs when a
particle, free to move, changes its position on the surface
of the earth in the south hemisphere. It is assumed that
there is no friction between the moving particle and the
earth but, of course, there is. Even when the friction is
considered the deflecting force is still great enough to be
a factor in the movements of water and air currents.

THE OCEANIC CIRCULATION 263

Effect of the earth's rotation. — We can now replace the
simplified scheme indicated above by one which still
assumes an earth covered everywhere by water to a
uniform depth but which rotates on its axis. The effect
will be that water no longer flows North and South from
the equator but to the North-East, in the northern
hemisphere (that is, to the right) and to the South-East in
the southern hemisphere (that is to the left). Also the
currents flowing southerly from the north pole will turn
to the right, that is, they will flow to the South- West.
Those flowing northerly from the south pole will turn to
tlie left, or will flow to the North- West.

The movements are, of course, continuous, that is, the
right or left-hand deflection is experienced at all parts of
the course of the currents. The latter will, therefore,
flow in great eddies turning back on themselves and
sinking beneath the surface as indicated on p. 258. We
should thus have two systems of eddies in the northern
hemisphere. (1) The water flowing from the equator :
this would gradually turn round to the East and then back
to the South as it sinks towards the sea bottom. (2) The
water flowing from the pole : this would gradually turn
round to the West and then back again to the North as it,
too, sank beneath the surface. A similar, but reversed
circulation would be established in the southern hemisphere.

The currents as they are. — The modifications of this
theoretical scheme must next be considered and we have
to see in what ways it is changed by the winds ; the
distribution of land and ocean ; the variations in depth
of water ; the temperature (as affected by the land) and
the salinity (as affected by the drainage of fresh water
into the sea. It is to be noted that, even when all these
modifying factors have been given due weight, the
theoretical scheme, as outlined above, can still be
recognised.

The winds. Wind blowing along the surface of the sea
exercises a very considerable force and this can be seen

2H AN INTRODUCTION TO OCEANOGRAPHY

at its maximum in the formation of spindrift, that is,
spray whipped off from the crests of waves and carried
l?y a strong gale. There are certain permanent features
of the atmospheric circulation that are of importance in
our study of ocean currents. To the North of the equator,
between latitudes about 30°N to 10°N we have the north-
easterly trade winds. These blow strongly throughout
the year. To the South of the equator and between
latitudes about 20°S. and 0°, there are the south-easterly
trade winds and these also blow strongly throughout the
year. Between the two belts of trade winds is the region
of equatorial calms and rains. North from the north-east
trades is the belt of south-westerly winds : this is between
about 40° and 60° N, Lat. in the Atlantic and Pacific.
South from the south-east trades is the belt of " brave
west winds " extending also from about 40° to 60° S. Lat,
aijd encircling the earth in the southern ocean.

The effects of these permanent winds is to modify the
theoretical directions of the ocean currents to a remarkable
degree. The trades blowing always from the North-Bast
and South-East, and thus converging on the equatorial
zone drive the water before them so that the direction of
the theoretical currents from the equatorial belt becomes
changed from North and South to due West. Thus we
have the two (North and South) equatorial currents in
each of the three great oceans and these are by far the most
prominent features of the oceanic circulation. Water
which rises from the depth is driven along the surface of
the sea from East to West by the trade winds. There is,
of course, the continual tendency for this westerly current
to be deflected to the right (in the North) and to the left
(in the South), but this is completely obscured by the
powerful effects of the winds. Thus warm, highly saline
water is driven up against the East coasts of Central and
Northern South America ; against the East side of the
Austral-Asian islands and against the East coast of
Africa.

THE OCEANIC CIRCULATION 265

The circulations in the North Atlantic and North Pacific
are also dominated by the great south-westerly wind
currents. In the Atlantic we thus have the very prominent
Atlantic European Stream, or Gulf Stream Drift, flowing
across from the North- West Atlantic towards northern
Europe with its main axis cutting the 45th parallel at a
small angle. In the Pacific we have the West Wind Pacific
Drift flowing over towards California from the Japanese
Islands and havuig its axis also near the 45th parallel.
Nothing of the kmd exists in the northern part of the
Indian Ocean, where the oceanic circulation depends
almost entirely on the seasonal monsoons, generated by
the extraordinary powerful heating and cooling effect on
the atmosphere of the great Asiatic land mass.

In the Southern Ocean the wind effect is even more
strikingly exhibited. The brave west winds blowing on
the average with considerable force propel the surface
water of the Southern Ocean always from West to East,
thus forming the Southern West Wind Drift round the
whole earth. But, impinging on the toe of South America,
this cold southern water creeps up the Pacific coast as
far as Peru ; deflected to the North (that is, to the left)
the West Wind Drift flows up along West Africa as the
Benguela current and, similarly deflected in the South
Indian Ocean, it runs up North as the West Australian
Drift. But the main part of the great Southern West
Wind Drift encircles the globe.

The effect of the land. — Land masses interposed in the
course of a current or drift thus divert the latter. So the
water driven along by the equatorial streams piles itself
up against the eastern continental shores and sets up
actual differences of sea level. Thus the level of the Gulf
of Mexico is raised significantly by the water of the Atlantic
Equatorial Stream entering it through the Islands of the
Antilles and the consequence is the rapid Gulf stream that
issues via the Straits of Florida. Much the same thing
happens in the Pacific, where the Japanese Current is the

266 AN INTRODUCTION TO OCEANOGRAPHY

analogue of the Atlantic Gulf Stream. But even then the
excess of water driven along by the trades is not wholly
distributed, and all along the axis of the North and South
equatorial currents in each of the three oceans there is
a counter-equatorial current. That is (as shown in Figs.
51-53), some of the water flowing West recurves inwards
towards the axis of the North and South streams and flows
back to the East. This occurs in the region of the
Equatorial Calms where there is heavy rainfall and so the
counter streams are warm and less saline than is the water
to the immediate North and South. These examples show
the effect of land barriers on the great scale, but everywhere
the same effect may be observed in localised conditions.
Some of these smaller deviations will be noted later in
this chapter.

The effects of variations in depth. Just the same
mechanical obstructions act when the sea bottom shallows.
All currents tend to flow along regions of sea where the
depth is maximal and a shoaling, up to, say, 100 fathoms
from the surface, offers a very considerable impediment
to the flow of the current. Thus the Gulf Stream Drift
flows freely across the Atlantic towards the coasts of
North East Europe and then further to the North East
along the West of the British Islands. But towards the
North of Scotland the sea bottom shallows and an actual
submarine barrier to the unimpeded drift is established
by the extension of the Continental Shelf between the
Shetland Islands and Iceland. At only one place is the
sea deep enough to allow of a notable current to the North-
East, that is in the Channel between the Shetland and the
Faeroe Islands. Here, however, the shoaling restricts the
north-easterly drift to the upper layers of the ocean and
leads to a very remarkable distribution of bottom tempera-
ture.

This well-known submarine barrier is represented in
Fig. 49. On the Atlantic side there are depths in the
neighbourhood of 2,000 metres, and on the Norwegian Sea

THE OCEANIC CIRCULATION

267

side the depth falls more rapidly to about 1,000 metres.
The water which is flowing, with the Atlantic Stream, over
the ridge has a temperature of 8°C. at a depth of about
600 metres, and the temperature at a depth of 1,000 to
1,200 metres in the Atlantic is about 7° to 8°C. But the
onflow of water is checked by the barrier so that, while
there are still temperatures of 8° to 9° on the surface in
the Norwegian Sea adjacent to the Ridge, the bottom
water is not affected by the North-Easterly Stream and
so we find temperatures of 0°C. to 3°C. at depths of 1,000
to 1,100 metres on the northern side of the Ridge.

/^j^res ^f/ant/'C acea

Norwe^'cin fi

Hex.

4<X>

boo

Boo.

/coo.

/eoo-

/6oo-

/eoo

Sooo

■35-. £5 °/oo

3 6 -as y^

Fig. 49. A diagrammatic section across the Wyville-Thompson
(Shetland-Faeroe) Ridge. The horizontal scale is much
smaller that the vertical one. The inset is a sketch chart of
the area. The soundings are in metres.

The effect of changes of salinity. — The enclosed, or
partially enclosed seas are of two main types : (1) Those
in which the temperature is high and where there is much
evaporation. Here the salinity tends to be higher than
the value which is normal for the zone of latitude in which
the sea is situated. Examples are the Roman
Mediterranean and the Gulf of Mexico, and a glance at
Fig. 22 will show that the salinity in these areas is markedly
greater than in the adjacent Atlantic, (2) Those in which

268 AN INTRODUCTION TO OCEANOGRAPHY

there is much precipitation (raiii or snow) or where large
volumes of fresh water enter from the land, via rivers.
Here the salinity tends to be lower than that normal to
the adjacent ocean. Examples are the Baltic and the
Arctic Ocean.

In such cases current systems are set up. Thus Atlantic
water flows into the Roman Mediterranean on the surface,
through the Straits of Gibraltar, because evaporation tends
to lower the level and there is an inward flow of water to
compensate this.

in
fathom

■*rAllanfic Mediterranean ->-

3 to 4 hours after L\fi. ^ /louis offer HVJ.

13'

387o«

17^

Fig. .50. A diagrammatic vertical section of the sea in the Straits of
Gibraltar. The vertical scale is very much greater than the
horizontal one. The line at the bottom represents, by its
variable thickness something of the order of the natural
scale — the horizontal and vertical ones being the same.

The western basin of the Mediterranean, that is, the sea
to the West of the Italian Peninsula has an average depth
of about 2,000 fathoms ; its temperature, on the surface,
is about 19° to 21°C., and the salinity is mostly between
37%o and 38%o. The water is thus relatively warm and
salt because of the high air temperature, much evaporation,
and a limited supply of fresh water from great rivers.
On the outside the Atlantic has a depth of 2,000 fathoms
at 200 miles from Gibraltar ; the temperature on the
surface is about 17°C. (the average for the year) and the
salinity is seldom higher than 36%o.

THE OCEANIC CIRCULATION 269

The Mediterranean communicates with the Atlantic
through the Straits of Gibraltar. Here the least width
between the European and African shores is about 8 miles.
The average depth in the straits is about 350 fathoms and
the least depth is about 250 fathoms. At a distance of
about 50 miles on either side from the shallowest part of
the straits the depth is about 500 fathoms. In the straits,
then, we have a " sill " separating the two basins, Atlantic
and Mediterranean. The former is about 2,000 to 3,000
fathoms, and the latter about 2,000 fathoms in
depth.

Because of the much greater evaporation in the
Mediterranean the level of the sea tends continually to
fall. At the same time the surface water must become
denser because of evaporation and so it will tend to sink
towards the bottom. The result of the latter condition is
seen in the Figure : at about 50 miles on either side of the
sill the depth is about the same — 500 fathoms — but on the
Atlantic side the temperature and salinity are about ]0°C.
and 36°/oo respectively while, on the Mediterranean side
the corresponding values are about 13°C. and 37-5.

Because the level of the Mediterranean tends to fall
there is an inflow on the surface of relatively cold and low
salinity water from the Atlantic and, at the same -time,
there is an outflow along the bottom of relatively warm
and dense water into the Atlantic. This is, of course, the
net efi'ect : what really happens is that the outgoing bottom
tidal stream runs stronger than the ingoing bottom one
and the ingoing, surface tidal stream runs stronger than
the outgoing, surface tidal stream. At about 3 to 4 hours
after low water the conditions are as shown in the figure
on the left hand side while at about 4 hours after high
water the conditions are as represented on the right hand
side. The positions of the arrows, as read ofE on the
vertical scale, give the depths of the currents, and their
lengths give the velocities : the longest arrow represents a
velocity of about 4 knots.

270 AN INTRODUCTION TO OCEANOGRAPHY

In the case of the Baltic the large influx of fresh water
from the rivers tends to raise the water level and so an
outward surface current is set up : this flows into the
Norwegian Sea through the Cattegat and Skagerak.
Accompanying these surface currents (as in the case of
the Mediterranean) there is a compensatory bottom
current.

The great oceanic currents. — We can now proceed with
a description of the main features of the superficial
water circulation in the various oceans, premising that
there is no end to the amount of detail with which this
subject might be treated. So we can only deal with the
broad, generalised, average current systems. Some of the
detail we shall refer to later on in order that the reader
may fully understand the complexity of the phenomena
dealt with in a description of oceanic current systems.

(1) The North Atlantic. The motive force of the North
Atlantic current system is the equatorial streaming of the
water. This occurs along a zone lying mainly to the
North of the equator, between parallels 0° to 10°N. Lat.
Driven to the West by the continual action of the North-
East and South-East Trade Winds the water flows along
the surface with a velocity of | to 2| knots as a warm
and relatively dense current. The axis of this lies between
N. Lats 0° and 10°, inclining to the North with the general
direction of the N. coast of N. America. Impinging on
the land barriers formed by the Greater and Lesser Antilles
the North Equatorial Stream is split. Part of it enters
into the Caribbean Sea and then into the Gulf of Mexico
where the temperature is relatively high and where the water
undergoes further evaporation. Part, however, flows to
the East of the West Indian Islands, as the Antilles Stream,
recurving continually to the North and East to form the
Sargasso Sea Eddy. This is probably by far the greater
part of the North Equatorial Stream. It is to be noted
that the deflection to the North is due, apparently, to the
land barrier formed by Central America and the West

THE OCEANIC CIRCULATION 271

Indian Islands, but it is certain that the deflecting force
due to the earth's rotation would lead to a turning of the
equatorial stream to the right hand side apart altogether
from the influence of a land barrier.

The Gulf Stream. The quantity of water thus driven
into the Gulf of Mexico is so great that an appreciable rise
of level is established. The salinity in the Gulf is over
36-5%o and the temperature rises to over 27°C., values
that are greater than those that are the averages for the
zone of latitudes within which the Gulf is situated. Issuing
from the Straits of Florida we have the Gulf Stream,
which at its narrowest part is about 5 miles wide, 60
fathoms deep, and has a temperature of about 26°C.
It flows to the North along the North American Coast as
far as N. Lat. 40° and W. Long. 50°, widening somewhat
as it tends to the North. At about 41°N, the Gulf Stream
impinges on the southerly flowing' Labrador Stream.

The Labrador Stream. This represents the theoretical
drift of water of low salinity and temperature from the
sub-polar to the temperate seas. It takes origin, for the
most part, in Baffin Bay and Davis Strait and it receives
relatively little contribution from the cold water that
flows down along the East Coast of Greenland. It tends
to be deflected to the right hand side, that is, to the West
and so it hugs the coast of North America, from Halifax
to Cape Cod, forming there the " Cold Wall " : about here
it tends to penetrate through and below the water of the
Gulf Stream so that a kind of variable stratification is
actually set up. It carries much ice, broken away as
bergs from the glaciers of West Greenland and these melt
in the sea to the West and South of Newfoundland,
depositing their stones and mud to form the Great Banks.
The area of distribution of this ice is variable with the
season and year and it depends to a very marked extent on
the volume, velocity and temperature of the Gulf Stream.

The Sargasso Sea and Atlantic Stream. Between N.
latitudes about 40° and 50° and W. longitudes 50° to

272 AN INTRODUCTION TO OCEANOGRAPHY

60° the Gulf Stream widens out, loses velocity and decreases
in temperature. It tends always to become deflected to
the right hand side, that is, to the South in this region and
then still to the right hand side and so to the West again.
Thus we have an anticyclonic circulation of the surface
water of the North Atlantic, that is, a continued turning
in the same direction as that of the hands of a clock.
This is shown in Fig. 51 and it should be noted that the
easterly part of the eddy is represented by broken lines,
that is, it is a drift of cold water there. " Cold " and
" warm " are, of course, relative terms, and since the water
of the Gulf Stream is continually losing heat as it flows
away from the Straits of Florida it is evident that by the
time that the drift has turned to the South and then to
the West again it has become colder than the water into
which it is flowing. It is also to be noted that there are
asterisks in Fig. 51 oft" tfie African Coast : these are placed
on the regions where water wells up from the bottom to
replace that which is being driven to the West by the
North-Easterly winds. This upwelling water is colder than
that on the surface and it joins the general Atlantic Anti-
cyclonic circulation, becoming heated as it is propelled
South- West to enter the North Equatorial Stream.

In the middle of the Atlantic Anticyclone to the West
and with its centre approximately in N. Lat. 40°, W.
Long. 50° is the Sargasso Sea. This is an extensive area
of ocean where there is no perceptible stream. It is
characterised by the presence of large quantities of drifting
" Gulf weed " (Sargassiim) carried out from the Gulf of
Mexico. This floats on the surface and carries a very
characteristic fauna. It is never so dense as significantly
to impede the way of a ship under sail or steam.

The Atlantic Stream (European Stream, Gulf Stream
Drift) takes origin from the Atlantic anticyclonic eddy
along its northerly margin. The main motive force of this
very prominent north-easterly drift of Atlantic water is,
of course, the prevalent south-westerly winds which blow

THE OCEANIC CIRCULATION 273

strongly in these latitudes throughout the greater part of
the year. The result is a slow drift of relatively warm
and dense water from the South- West towards the North-
East : this is the Atlantic Stream, It enters the coastal
water of North-West Europe from Spain to the EngUsh
Channel, penetrating through the Straits of Dover into
the North Sea and even entering into the Irish Sea through
St. George's Channel. The main core of the stream is
off the West Coasts of the British Islands, then to the
North of the latter, along the deeper sea bottom between
the Shetlands and Faeroes and then along the West Coast
of Norway and round North Cape.

The outlying branches of the Atlantic Stream. A branch
enters through the English Channel into the southern part
of the North Sea (the Flemish Bight) and another (though
much smaller branch) enters the Irish Sea. A prominent
branch flows round the North of the Shetlands and so
down to about the middle of the North Sea (see Fig. 51),
and then across towards the coasts of Holland and Denmark.
Part of the stream bends romid to the North- West, over
towards Iceland and Greenland and Atlantic water even
reaches the coasts of Spitzbergen. About the latitude of
North Cape, however, the water of the Atlantic stream has
cooled down to such an extent that it has become denser
(by reason of its higher salinity) than the water normal to
these latitudes and from- this region it flows on as an
undercurrent, close to the sea bottom. This also occurs
in the Skagerak with the Atlantic Water that has entered
into the North Sea. The result is that water which is of
Atlantic origin, and is warmer and denser than that
characteristic of the latitudes in question flows into the
deeper parts of the Baltic and also along the bottom of
the Barentz Sea. These warm and dense undercurrents
are seasonal to some extent.

(2) The South Atlantic— Fig. 51. The Southern
Equatorial Stream, flowing to the West just South of the
equator, impinges on the East Coast of South America,

274 AN INTRODUCTION TO OCEANOGRAPHY

Fig. 51.

The current systems of the Atlantic Ocean. Only the
surface currents and drifts are shown and the condition
represented is the average, generalised one : in many details
there is variation with the season of the year. Continuous
lines represent " warm " currents and broken lines
" cold " ones. The closeness to each other of the lines
gives some indication of the rapidity of the currents. The
stars along the continental coast indicate the principal places
where cold water wells up from the sea bottom towards the
surface.

THE OCEANIC CIRCULATION 275

splitting on Cape San Roque, From thence it flows to
the North- West, joining the North Equatorial Stream, and
to the South, along the East coast of South America, as
the Brazil Stream. Between latitudes about 20° to 40°S.
the Brazil Stream bends round to the East, being deflected
to the left hand side (just as the Gulf Stream is deflected
to the right) by the earth's rotation. This is the Western
half of the South Atlantic cyclonic water circulation.

Just as there is a drift to the North- West, in the North
Atlantic, due to the prevalent winds, so there also is a West
wind drift in the Southern Atlantic. This, however, is
much stronger than in the North, partly because the
winds are stronger and more persistent, but mainly because
of the uninterrupted sweep of the western wind drift
round the whole world in the Southern Ocean. The west
wind drift is continually deflected to the left and so it
turns round long before it approaches the West coast of
Africa and finally it enters the South Equatorial Stream.
Along the African coast the drift is known as the Benguela
Stream. It is cold (relatively to the water of the latitudes
into which it is flowing) because it has its origin in the
southern sub-polar seas and also because it contains water
which has welled up from the lower strata of the Atlantic,
The areas of this upward motion of the South Atlantic
water are marked by asterisks, placed near the coasts of
Patagonia and West Africa. As this cold, northerly
flowing, west wind drift water enters the South Equatorial
Stream circulation it becomes heated up.

Between the North and South Equatorial Streams
there is a counter- current, the Guinea Stream. Note that
there is a kind of convergence of the two great currents
so that the western part of the common stream must
move more rapidly than does the eastern part. This is
not all : there is an actual curving inwards towards the
axis of the joint equatorial stream, and then an eastward
flow of the surface water. This enters into the Gulf of
Guinea and there its further course is very complicated

276 AN INTRODUCTION TO OCEANOGRAPHY

and is also variable with the season. To this matter of
seasonal variation in the Atlantic Equatorial Stream
circulation we return in p. 288,

Thus there is an obvious symmetry in the water circula-
tion of North and South Atlantic. In each case there is
a great gyral occupj-ing the central parts of the two oceans.
That in the North is anti-cyclonic, while that in the South
is cyclonic in direction. In each case there is a westerly
drift — the Atlantic Stream (or North Atlantic West Wind
Drift) in the North, and the South Atlantic West Wind
Drift in the South. Here the water flows from West to
East. In each case there is a cold current flowing
eqiiatorially : the Labrador Stream, and the West Green-
land Stream in the North, and the Falkland Stream in
the South. The latter is the stronger of the two because
of the much greater volume of the Southern Ocean as
compared with the northern one.

(3) The Pacific Ocean circulation. — Fig. 52. We consider
the two circulations, those of the North and South Pacific
Ocean together. The conditions are rather more com-
plicated than they are in the two Atlantics, first
because of the lack of a geographical differentiation into
North and South Pacific basins, and second, because of the
great variety of depths in the western part of the Pacific.

We see, however, just the same strongly marked North
and South Equatorial Streams with the easterly flowing
counter stream in the axis : that is the westerly flowing
water continually turns inwards to South and North and
flows backwards towards the ocean off the coasts of West
Central America where it forms complicated cyclonic
eddies on a small scale. Then, as in the case of the Atlan-
tics, we have the two great gyrals, anticyc Ionic in the North
Pacific and cyclonic in the South. In the central parts
of these gyrals there are entensive regions where there are
no perceptible permanent currents. It is to be noted that
the North Pacific gyral extends over the greater part of
the ocean whereas the South Pacific one is markedly

THE OCEANIC CIRCULATION 277

restricted to the eastern region. This is because the
latter is the true South Pacific basin, the western region
being characterised by extensive areas of relatively shallow
water and numerous archipelagoes.

The circulation in the western part of the North Pacific
is analogous in every way with that in the western region
of the North Atlantic. As the North Equatorial Stream
impinges on the islands of the Australasian complex it is
deflected to the right hand side, that is, to the Nort.h and
it becomes concentrated as the Japanese Stream, which
flows strongly towards the North-East and is the Pacific
comiterpart of the Atlantic Gulf Stream. OfTshoots from
this enter into the Sea of Japan, the Yellow Sea and the
China Sea. Further a very evident north-easterly drift
takes origin on the northern margin of the North Pacific
gyral and here water, impelled strongly by the prevalent
West winds, flows over towards the coasts of North- West
North America. The communication between the North
Pacific and the Arctic Ocean (through Behring Strait) is,
however, very much more restricted than that between the
North Atlantic and the Arctic, and so the greater part of
the northern west wind drift in the Pacific is deflected
(by the earth's rotation) to the South and West again,
thus re-entering into the North Equatorial Stream. OfE
the coasts of California, this southerly flowing Califomian
current thus carries on the gyral circulation, but it also
contains water that has welled upwards from great depths
in the regions, off the coasts, marked in Fig. 52 by the
asterisks. It is also to be noted that, just because the
north-easterly drift into the Arctic Seas is much less
prominent in the North Pacific than in the North Atlantic,,
the gyral is more symmetrical and so the course of the
equatorial streams is almost truly parallel to the equator,
though (as in the Atlantic) the axis of the two streams
lies just to the North of the Equator.

Just as in the case of the North Atlantic circulation
there is a southerly flowing stream of cold water. Little

278 AN INTRODUCTION TO OCEANOCtRAPHY

of this (if any) comes through the Behring strait from the
Arctic Ocean and most (or all) of it takes origin in the
shallow North Pacific Ocean and in the Sea of Okhotsk.
Most of it flows southwards outside the Japanese Islands
impinging on, and cutting through the warm Japanese
current, just as the Labrador Stream impinges on the
North Atlantic Gulf Stream.

■3lo /8k> ItiB /M fS A^~

Fig. 52.

The same general scheme is to be seen in the South
Pacific oceanic circulation , though it appears, in the Figure,
to be much less regular. This is the result of the shoaling
which extends over the South Pacific in its western legions.
There is, however, a counterpart to the Brazil stream in
the strongly developed East Australian current which
carries a considerable volume of warm and dense water

THE OCEANIC CIRCULATION 279

from the Pacific into the Indian Ocean. Here, also, we
have a prominent southern west wind drift, as in the case
of the South Atlantic and this impinges on the western
coast of South America and is deflected to the North as
the Peruvian current : it is analogous, in every way, with
the Benguela Stream of the South Atlantic. Like the
latter it enters into the equatorial stream circulation.
The great southern west wind drift is more prominent in
the Pacific than in the South .Atlantic because of the much
greater area of ocean and the consequent greater effect
of the brave west winds in setting up a surface drift.

The Pacific Ocean, then, exhibits the two typical gyral
circulations, the northerly anticyclonic, and the southern
cyclonic ones. There is also the drift of cold and relatively
light water towards the equator, on both sides, from the
North and South.

(4) The Indian Ocean. — Fig. 53. The Indian Ocean
differs from the Atlantic and Pacific regions inasmuch as
it represents only one half of the theoretical scheme, the
northern basin being absent. We consider, then, the
region adjacent to the equator and South of this to the
great Southern Ocean. Here we have the theoretical
scheme in an evident form, although it is modified by the
absence of a great, elevated, continental land mass on the
eastern side, such as we have in both the Atlantic and
Pacific regions, particularly in the southern basins. In
the South Indian Ocean, then, we see the typical gyral
circulation, cyclonic in direction. The South Equatorial
Stream has its main axis (during the winter months)
between latitudes 10° and 15°S. and it impinges on the
eastern shore of Africa and Madagascar in the usual way.
Part (the major one) is deflected to the left, that is to the
South and East to form the cyclonic gyral and a smaller
part passes into the Mozambique Channel to form the
Agulhas Current, with a system of eddies between Africa
and Madagascar. The Agulhas Current flows South along
the African coast and a small part of it rounds the Cape

go 5p ■»|o SjO 0)0 -rp S\0 9|o /do i)p 7^

so

if^' MONGOLIA

^ ^ — — ■'^

SoE. 5IO ^|o 5|tj 6|o 7|o ^|g 5lo -o|o //|<> ~^o

■^-E "ik? 61^ 7ii3 g|i9 Pk? /Ofi /fljb /£|0

Fig. 63. The current systems of the Indian Ocean. As before
continuous lines represent warm currents and broken lines
cold ones. The upper figure represents the generahsed
conditions during the winter months and the lower figure
shows the summer conditions in that part of the Ocean
where the circulation reverses with the season.

THE OCEANIC CIRCULATION 281

and enters the Atlantic Ocean to join the Benguela Stream.
The greater part, however, becomes deflected to the left
(that is back to the East) by the earth's rotation and also
by the shoahng of the sea adjacent to the Cape. It then
disperses in the Southern Ocean, cutting through the
southern west wind drift and slowly sinking towards the
bottom.

South of the Cape and the Australian continent we see
the same prominent West wind drift that has been noted
in both the South Atlantic and Pacific Oceans. Here,
however, this has a more uninterrupted sweep to the East
and South of Australia, but it also suffers deflection to the
left, that is, to the North, so that it passes as a cold current
towards the West Australian coasts, then warms up and joins
the South Equatorial Stream circulation, completing the
Indian Ocean gyral. In the centre of the latter we have
the usual absence of any perceptible permanent streaming.

North of the equator the streaming of the Indian Ocean
is dominated by the monsoon wind systems. Fig. 53 (the
upper one) represents the winter conditions when the
North-East Monsoon has been established, while the lower
figure shows the streaming set up in the conditions of the
South- West Monsoon which blows during the summer
months. The monsoons are due to exactly the same
causes that lead to the land and sea breezes. During the
day the land becomes heated, by solar radiation, to a much
greater extent than the sea. It heats the atmosphere
in contact with it and so ascending air currents are
established. To compensate for these a breeze blows from
the sea towards the land, becoming deflected, of course,
by the earth's rotation. During the night the conditions
are completely reversed : the atmosphere over the land
cools doA\Ti more rapidly than that over the sea and so
descending air currents are set up and these blow out from
land to sea as the land breezes.

Such effects are produced on the great scale, but with a
period of a year instead of a day. As a rule the heating

282 AN INTRODUCTION TO OCEANOGRAPHY

and cooling effect of the continental land masses is
insufficient to do more than set up local modifications
of the prevailing wind currents, but the Indian Ocean, in
its relation to the great and high Asiatic continent, is a
striking exception. In the summer months the elevated
lands become so strongly heated that a wind system,
lasting for some months is established, this is the South-
West Monsoon. In the winter months the continental land
is strongly cooled and then a reversed condition is set up :
the North-East Monsoon is established and blows also for
some months.

During the period of the N.-E. Monsoon, then, the
streaming is represented in a generalised way, by the
upper figure of Fig. 53. A Northern Equatorial Stream
runs curving in towards the axis of both N. and S. streams
(in about 5° N. Lat.) as a distinct equatorial counter
current. There is no obvious gyral, for the drift of water
North of the equator is one to the South- West, that being
the point to ivhich the North-East monsoon blows. In
both North Atlantic and Pacific, it will be remembered,
we have West wind drifts : in the Indian Ocean in the
winter we have a North-East wind drift, that is a current
running to the South- West and joining the equatorial
COM w^er- current. In the Arabian Sea and the Bay of Bengal
the conditions are complicated and large eddy systems
are established.

During the summer months, however, when the S.-W.
Monsoon blows, the current system of the Indian Ocean
becomes very much the same as that in the North Atlantic
and North Pacific basins (except, of course, that there is
no current of cold and light water running South from
sub-polar seas). A North Equatorial Stream is established,
but there is no very evident counter current. There is a
West wind drift in latitude between 0*^ and 10°N., and this
is deflected to the right (that is, to the South) so that it
rejoins the North Equatorial Stream thus completing the
North Indian Ocean anticyclonic gyral. In the central

THE OCEANIC CIRCULATION 283

regions of this are places where the streaming is variable,
or quite unappreciable. Further a part of the North
Equatorial Stream is strongly deflected to the North by
the African coast, thus forming the very prominent Somali
current. While it runs this is the analogue of the Gulf
Stream of the Atlantic and of the Japanese Stream of the
Pacific.

Between Australia and the Asiatic continent the current
systems are very complex, and it would be unprofitable to
consider them except in detail and illustrated by large
scale charts.

(5) The Polar Seas. — Figs. 12, 51. Far less is knowTi
concerning the current system of the Polar Seas than is
the case with the g_eat oceanic regions dealt with above.
Exploration in these seas is much more laborious than
elsewhere, and the presence of ice introduces the most
formidable difficulties.

The Antarctic currents are still very imperfectly known.
Obviously there must be a resultant drift to the North
since icebergs, detached from the Antarctic ice-cap, are
found far North of the land where they have been formed.
The really important water movement (as apart from the
local seasonal streams that certainly exist in the neighbour-
hood of the continent) is that represented in Fig.
51. Round the whole earth in the great Southern Ocean,
there is a continuous drift of water from West to East,
set up by the very strong winds that blow mainly from
the West throughout the greater part of the year. This
is the southern west wind drift. It tends always to be
deflected to the left, that is, to the North, and it thus
enters the South Atlantic (as the Falkland and Benguela
Streams), the Pacific (as the Peruvian current) and the
Indian Ocean (as an ill defined current flowing to
the North along the coast of West AustraUa. Ultimately
the water entering the three southern oceans from
the Antarctic joins the South Equatorial Stream
circulations.

284 AN INTRODUCTION TO OCEANOGRAPHY

The Arctic currents. — (Figs. 12, 51.) These are better
known though few experimental investigations have been
made, and our knowledge is derived mainly from observa-
tions on floating wreckage and on the famous drift of
Nansen's ship, the " Fram " when she was frozen into the
ice. Fig. 12 shows that relatively warm and salt water
enters the Arctic Ocean with the Atlantic Stream. Off-
shoots of this penetrate as far as the West coast of
Spitzbergen and round North Cape into the Barentz Sea
(that is, the Arctic Ocean between the entrance to the
White Sea and the ice pack). Here, however, these drifts
of water sink to the bottom and flow on to the North and
West as under currents which are warmer than is the water
normal to those latitudes. This is the water that enters
the Arctic Basin ; it comes entirely from the Atlantic.
Far more water flows out from the North Polar seas than
enter it as oceanic streams whether superficial or deep.

The general drift across the Arctic Ocean is from East
to West and the main directions indicated in Fig. 12 are
based not only on the course taken by the Fram when she
was carried by the ice pack, but also on the general drift
of waterlogged wood, etc. The water thus moves from
the Siberian coast across the basin towards the North-East
coast of Greenland. Some appear to pass through Banks
Strait and Hudson Strait into Baffin Bay and there is also
a marked drift, through Kennedy Channel into Baffin Bay.
The main drift flows down the East coast of Greenland
through Denmark Strait, then round Cape Farewell into
Davis Straits. Reinforced there by water coming down
from Hudson's Bay and Baffin Bay the drift continues to
flow to the South as the Labrador Stream and the further
course of this belongs to the Atlantic system and is
indicated in Fig. 51. But there is also a drift of Arctic
water to the South along the East coast of Iceland and
this may extend well down in some years. Evidence that
water of Arctic origin may actually enter into the North
Sea is afforded by a study of the plankton, for organisms

THE OCEANIC CIRCULATION 285

having their normal habitat to the North may exceptionally
be taken in the Skagerak, or even in the North Sea itself.

Finally there is a restricted outflow from the North
Polar Basin into the Pacific Ocean through Behring Strait.
The volume of this is, however, much smaller than that
which enters the Atlantic.

(6) The Marginal Seas. It is to be noted particularly
that all that has been said so far constitutes a generalued
description of the main currents and drifts of the great
oceans. The charts from which Figs. 51 to 53 have been
drawn are all very small scale ones and what they represent
are average directions, the averages being really those
made from observations taken over considerable periods of
time. Thus the student will note a kind of symmetry
about them which certainly does not exist at any one
moment of time. The directions are shown by boldly
drawn sweeping " lines of least curvature " and this must
be the case whenever currents and drifts are shown on
small scale charts. If small parts of the North Atlantic,
for instance, were charted on a large scale the directions
of the Atlantic Stream would certainly be far more irregular
than they are represented in Fig. 51 and it would also be
noticed that the drift of relatively warm and salt water
towards northern Europe would advance rather irregularly.
On the whole, however, and taken over a long period of
time these irregularities would " aveiage out " and we
should have the generalised pictures that are shown by
Figs. 51 to 53.

And so, when the marginal seas are studied, the streaming
is seen to be much more complex than we have indicated.
There is an almost illimitable detail when small areas are
represented on a big scale. In these seas the tidal streams
are really the important and most striking movements
of the water and a current chart made for any particular
moment of time would, as a rule, show how the tides are
running, and nothing more. But the tidal streams do
not eocactly reverse themselves and so there is a general

286 AN INTRODUCTION TO OCEANOGRAPHY

resultant movement of the water which is indicated not
only by float experiments but also by observations of the
changes of salinity and temperature from place to place.
Further in any small sea area, and for any short period
of time, the winds are most important factors for they are
variable and so very considerable deviations from the
generalised directions may be seen from day to day in a
small sea area which is minutely investigated.

Fig. 54. The current system of the North Sea for the month of August.

The North Sea. It would be quite impossible to give
even a summary account of the water circulation in the
marginal seas without taking up very considerable space

THE OCEANIC CIRCULATION 287

and so the North Sea circulation only is referred to here.
It may be taken as illustrative of the complexity to which
we refer above.

Here we see a complicated system of vortices. The
main factors are (1) the entrance of Atlantic water that
comes from the Faeroe-Shetland Channel and is deflected
to the right and so into the northern part of the North Sea.
(2) The entrance of Atlantic water from the Enghsh
Channel, through the Straits of Dover : this entrance is
much more restricted than is the northern one and so the
volume of water coming from the South is much less than
that from the North. (3) The outflow of relatively fresh
water from the Baltic through the Cattegat and Skagerak :
this flows on to the North along the coast of Norway.
(4) The configuration of the North Sea itself, and the
varying depth of the bottom.

From a study of these factors, the observation of
directions of streaming taken over considerable periods of
time and the distribution of changes in the salinity of the
water the figure has been compiled. The resultant tidal
movement is considered and the effects of variable winds
are supposed to be eliminated by spreading the observations
over a period of time long enough to smooth out
irregularities traceable to these causes. Generalised in this
way we get the somewhat symmetrical arrangement showTi
in the figure, but at any one moment the picture would
certainly show considerable deviations from the average
condition.

The seasonal variations. — Almost everywhere there are
seasonal variations both in the directions and volumes of
the current systems of the ocean and the greater part of
the investigations now being carried on deal with these
seasonal variations and with their causes. The practical
value of this knowledge, to navigation and to the sea
fisheries is such that every well-known sea area has been
investigated to an extent that makes it impossible to deal
here with the results in detail.

288 AN INTRODUCTION TO OCEANOGRAPHY

Several instances may, however, be given: (1) the
Indian Ocean^ Fig. 53, shows the two conditions, (a) when
the North-East Monsoon blows during the winter months
and (b) when the South- West Monsoon blows during the
summer. In (a) the direction of the gyral characteristic
for a northern oceanic area is reversed. Thus we have a
South Equatorial Stream but no corresponding North
Equatorial one. There is an apparent counter-equatorial
current but this is a wind efTect being due to the cyclonic
circulation set up by the monsoon rather than to a backward
spilling of the equatorial streams. In (b) there is a North
Equatorial Stream, due obviously to the dragging of
water away from the East African coast by the S.-W,
Monsoon Drift.

Fig. 55. Upper : the Atlantic equatorial stream circulation in the
northern winter ; Lower : the same during July and
September.

(2) The Mid-Atlantic circulation. — The changes that
occur in the course of the year are represented in Fig. 55.

THE OCEANIC CIRCULATION 289

Here the current-systems are represented on the same
scale and with the reference lines of latitude and longitude
superposable. In the northern winter (the upper figure)
the axis of the South Equatorial Stream lies just South of
the equator while that of the northern stream may be
taken to be somewhere between latitudes 5° and 10°N.
There is a counter-current between the two streams : this
is the Guinea Stream, and its axis is two or three degrees
North of the Equator. It is not important, so far as its
volume and intensity are concerned. West of about 10°W.
long.

Now note how, during the months of July and September,
the whole current system is shifted bodily to the North.
The South Equatorial Stream lies mostly North of the
Equator while the northern stream lies well above 10°N.
lat. The counter stream is also very much stronger than
in the winter months : it has its axis in about 7°N. lat.
and it extends as far West as 4:0°W. long. The Guinea
Stream, as it is shown in the winter chart, still exists,
and it can now be seen to be due to the recurving backwards
of the South Equatorial current. There is, however, a
very evident counter stream in addition to this. The
increased volume of these latter streams is evidently due
to the greater quantity of water that is set in motion from
East to West.

(3) The Atlantic gyral. The matter may now be looked
at from another standpoint, that of the distribution of
salinity and temperature.

The two figures have been constructed from observations
which may, without much error, be regarded as
simultaneous and they indicate approximately the area
of ocean which is filled with highly sahne water of a
relatively high temperature. In March, then, the 36%^
water does not extend North of the 40th parallel of latitude
and nowhere does it come near the African or European
coasts. The isotherm on 15°C. has very much the same
general situation as the 36%^ isohaline. In November,

290 AN INTRODUCTION TO OCEANOGRAPHY

Fig. 66. Two charts of the distribution of sahnity and temperature
in the North Atlantic during the year and in the months of
March and November. The northern boundary of the area
of ocean filled with water of salinity of 36 or more is represented
by the heavy contour line. The dotted lines show the
isotherms of 15° and 20°C.

THE OCEANIC CIRCULATION 291

however, a very marked change has occurred : the 36%o
isohaHne now reaches up to the N.E. as far N. as the 50th
parallel and as far E. as about 10°W. long., while all the
coast of Europe and Africa, South of Cape Finisterre is
bathed with water of this, or some greater saUnity. The
15° isotherm has shifted North with the 36%o isohaline.
Evidently an extensive region of the North Atlantic,
towards the North-East, has been flooded with relatively
warm and salt water.

Now the primary cause of oceanic circulation, whether
this is set up directly by evaporation and rise of
temperature, or indirectly by the driving force of the winds,
is solar radiation. In the course of the year the sun moves
between the tropics ; he is directly overhead in 23|°S. lat.
at the winter solstice (Dec. 22nd) ; he is on the Equator
at the vernal equinox (March 21st) ; he is in 23|°N. lat.
at the summer solstice (June 22nd), and he is again on the
Equator at the autumnal equinox (Sept. 23rd). Where he
is overhead, there the ocean receives most heat from his
radiation and the intensity of the water circulation becomes
maximal for a short period.

Between the vernal equinox and the summer solstice,
then, the zone of ocean in the northern hemisphere most
greatly heated up shifts from South to North and so we
have the more northerly position of the axis of the North
Equatorial Stream in the summer, as shown in Fig. 56.
The result is, therefore, that the area covered by the
Atlantic Stream gyral extends northerly from about
March to November, then contracts to the South again.
The minimum extension North is in March, the maximum
one in November.

The gyral or " Gulf Stream Eddy," thus pulsates with a
period of about a year and accompanying this pulsation
there is an annual periodicity in the inflow of Atlantic
water into the northern marginal seas. Normally the
North Sea should contain water of about 34-5 to 35%^,
but water of over 35%^ enters it from the Faeroe-Shetland

292 AN INTRODUCTION TO OCEANOGRAPHY

Channel, in the North, and from the EngHsh Channel
in the South. In March the area of North Sea which is
covered by water of over 35%o is maximal ; in August it
is minimal. Thus the annual culmination in the flow of
the Atlantic Stream into the North Sea is in March. W9
have seen, from Fig. 56, that the greatest extension north-
easterly of Equatorial Stream water occurs in the Atlantic
in November : the period November to March represents,
then, the time taken for the water of the Atlantic Stream
gyral to drift from somewhere about 50°N. lat. and 20°W.
long, into the North Sea. In the Irish Sea the maximal
inflow of water occurs usually about May or June : thus
a little longer time is required because of the more restricted
entrance into this marginal sea.

Further North the same annual periodicity can be
observed. Thus the Baltic is penetrated — but by a deep
current of relatively warm and salt water — and this happens
later than in the North Sea, for the slowly moving under-
current has to pass through the Cattegat : in some years
it does not pass at all. It penetrates into some of the
deeper Norwegian fiords and here we have the interesting
condition of a distinct fortnightly periodicity. The dense
undercurrent is affected by the tides : that is, there is not
only a periodic rise and fall of the surface level of the sea
but there is also a rise and fall (a true tide) in the upper
level of the under stratum of Salter water. Daily changes
are difficult to notice but the fortnightly ones, brought
about by the alternation of springs and neaps have been
observed.

The Atlantic Stream, as we have seen, can be traced
into the Barentz Sea, and to the shore of Spitzbergen. In
the Barentz Sea it becomes an undercurrent, for by the
time the relatively salt water of the Atlantic has reached
these latitudes it has cooled considerably, has become
denser than that in situ and so sinks to the bottom and
flows on as an undercurrent. This culminates in November,
which is, therefore, the summer of the bottom water of

THE OCEANIC CIRCULATION 293

this remote region. The culmination of the Atlantic
inflow in the Barentz Sea, towards the end of the year is,
however, due to the flow to the North of water which was
present in the Atlantic Stream gyral one to two years
previously to its appearance in the extreme North, This
period of time, then gives us an approximate measure of
the rate of the drift.

Such studies of the translation of the Atlantic water to
the North are very numerous and their results are of much
scientific and practical importance. To some extent the
precise nature of the seasons, the air temperatures, the
times and abundance of harvests, the flowering of certain
plants, the quantity of ice in the sea, the times at which
certain Baltic and White Sea ports freeze up and re-open
again and even the abundance of certain marme fisheries
depend on this inflow of warm and salt water. The times
of these phenomena are not strictly periodic but vary a
little from year to year, and the variability may be of very
practical importance : a forecast, for instance, of the exact
times at which certain Baltic ports may be expected to
become icebound, or of the time of harvest in Northern
Germany, or of the frequency and southern limits of
extension of icebergs round Newfoundland are results
that have obvious practical significance. There is little
doubt that persistent investigation of the periodic and
aperiodic changes in the North Atlantic gyral will ultimately
give us this information.

It is mainly in the North Atlantic, and its marginal seas,
that investigations of this kind have been made. Lately
however, similar research has been prosecuted by the
American oceanographers in the Pacific Ocean ofT the
Califomian coasts.

The vertical circulation. — So far we have only considered
the movements of water on the surface of the ocean.
Besides these horizontal surface currents there are also
deeper ones : this is evident from a consideration of the
theoretical scheme whereby a circulation is deduced from

294 AN INTRODUCTION TO OCEANOGRAPHY

the effect of solar radiation in heating and evaporating the
superficial water. This raises the sea level so that a
current is established and the theoretical direction of this
is from the Equator towards the poles. Somewhere
between the tropics and the sub-polar seas this highly
saline water cools, becomes denser and sinks towards the
bottom.

In the polar seas the sea water freezes. Some of the
dissolved salts are then incorporated in the ice so that
immediately around and beneath the latter the water in
situ contains more dissolved salts and so becomes denser
and, therefore, sinks towards the bottom. When the ice
melts in the following summer the water which is formed
is less saline than that in situ and being at about the same
temperature it floats on the surface and flows away from
the polar seas towards the temperate zones. Somewhere
there it mixes with the water into which it flows and so
becomes denser. Since it is still colder than the seas into
which it enters it therefore becomes denser and tends to
sink below the surface.

At the same time there are ascending currents. Water
rises up from beneath in the Equatorial zones. Along the
coasts of Western Africa and America (see Figs. 51, 52)
there are regions where water " wells " up from beneath
to take the place of that which is driven towards the West
in the Equatorial Streams.

Besides these descending and ascending currents there
appear to be horizontal ones, both at the bottom and in
intermediate levels. It appears to be the case that there
8 almost everywhere in the ocean a slow creep of cold
water from the polar seas towards the equator. Also
there appears to be a greater volume of water driven from
North to South, across the equator on the surface in the
Atlantic and Pacific and Indian Oceans, than is propelled
in the opposite direction : therefore, a lower current,
crossing the equator at an intermediate stratum has been
postulated. This mid-level current returns to the North,

THE OCEANIC CIRCULATION

295

the excess of water which enters the Southern Ocean on
the surface.

Fig. 57 represents in a very schematic way, the probable
(and generalised) system of lower currents : it is based on
the results obtained during the German " Valdivia "
expedition. The detection of the directions and velocities
of bottom and intermediate currents and drifts is far more
difficult than in the case of those flowing on the surface,
which are much more accessible to direct observation. In
shallow seas they can be measured by means of the readings
made with current meters (see p. 298), but the use of these
instruments is hardly practicable in great depths. Bottom
currents can be traced by the use of bottles or other floats
so weighted that they drag or roll along the sea floor.

9o''s.

So"

3o'

looo
Sooo
3ooa

Sooo

'•^^'^ir^<rTri

O'

^r^.

>tr

r^

ji?"

6o°

go-'Ai

^^^^^

^^^^

IMI-

Sea holla m.

Tig. 57. An imaginary section of the ocean in a north and south
direction. The superficial, intermediate and bottom currents
flow in all directions, and generally obliquely to a meridian
but only the meridional components are represented in the
section.

It is necessary that they should come ashore, or become
taken by trawls, in order that their average paths may be
deduced. Thus direct observation of intermediate or
bottom currents is practicable only in the marginal seas,
and it is mainly there that such investigations have been
made.

The figure represents, then, the main features only of
the vertical circulation, and it must be admitted that
there is considerable doubt even about these. It is
probable, however, that the principal interchange of water
between the equatorial and temperate zones goes on in a
rather limited stratum which does not extend to a much

296 AN INTRODUCTION TO OCEANOGRAPHY

greater depth than about 1,000 metres. Below this we
probably have an intermediate drift of water from the
Southern Ocean into the North Atlantic and Pacific Oceans,
and below that again there is the very slow creep of water
along the sea bottom at the greater depths from both
North and South poles towards the equatorial regions.

The methods of investigating ocean currents. — ^Dead
reckoning. To the seaman a current, or tidal stream, is
an effect that can often be deduced from the difference
between a position that he expects, and that which is
actually observed. The position after so many days
sailing may be expected from the " dead reckoning," that
is the account which is kept of the course steered and the
distance sailed (as shown by observations of the log towed
astern). An astronomical observation, or a landfall, may
show a very different position and if there is no reason to
suspect unusual error in observation, or leeway that is
not accounted for, the conclusion will generally be that
the course of the ship was affected by a current. In some
cases these differences between expected and experienced
positions of ships at sea are the main reasons for giving
values to the directions and velocities of ocean drifts.
Currents having the velocities of tidal streams can usually
be detected in other ways.

Drifter observations. Large slow movements of the sea
water can generally be traced by observations of floating
objects, and much of the investigation of ocean currents
and drifts have been made in this way. Sealed bottles,
weighted so as to float with their corks just submerged,
and containing papers or cards with instructions for
reporting their occurrence at sea or on shore, are set free
at certain positions. They are found at other positions
and the average drift and time occupied by this are then
ascertained. A small light object floating at the surface
of the sea is generally affected to a greater extent by the
winds than by any movement of the water independent
of the winds. Therefore the bottle is usually a rather large

THE OCEANIC CIRCULATION 297

one and is so weighted that it is just submerged. When
it is desired to observe the drifts at the sea bottom the bottle
is so weighted that its specific gravity is just a Httle more
than that which is expected of the sea water in situ. It
therefore sinks to the bottom and is slowly rolled along
there.

Sometimes a long, stiff wire is used as the weight and
this drags on the bottom with the bottle bouyed upwards.
Thus friction is minimised and the bottle is exposed to the
drift to as great an extent as is practicable.

It is particulariy to be noted that these methods can
usually give only average results. In many parts of the
ocean the winds are variable, and so objects floating at the
surface of the sea may drift about very irregularly as the
wind-direction and velocity change. In shallow water
near the land the tidal streams act powerfully on the
drifters and since they change their directions every six
hours or so a bottle under their influence may simply
oscillate backwards and forwards over a path of several
miles, at least, in length. The tidal streams also vary
during the fortnightly period of neaps and springs and
they and the winds may, together, give the drifters all
sorts of paths. If, then, the time that elapses between
the liberation and recovery of the drift-bottles is short,
say only a few days, the paths taken by the individual
drifters may vary in the most perplexing way — this is,
indeed the usual experience. The point noted here is
important because its full consideration shows that ocean
currents are far more comphcated in detail than the figures
representing them usually indicate.

Nevertheless the method is valuable. When a sufficiently
long period elapses between liberation and recovery of the
drifter, say several weeks or months, it may usually be
assumed that the wind has been blowing in many directions
and with very variable velocities. Also the tidal streams
may have been moving about as much in one direction as
the other and any combination of the two factors may

298 AN INTRODUCTION TO OCEANOGRAPHY

have occurred. Therefore the observations — especially if
they are numerous — may give us the average or resultant
effects of the winds and tides in causing movements of
the surface water and this is very often the information
that is wanted. Very much of what we do know about
ocean currents and. drifts has been obtained by such
observations as those of drift bottles and other instruments ;
floating wreckage ; derelict ships ; exotic seeds, plants,
timbers, etc.

Direct current measurements. Various instruments have
been designed to measure and record 4:he movement of
water at the surface of the sea or at lower levels. A log
streamed out from a ship at anchor will record the velocity,
and the direction is easily observed. There are, of course,
numerous errors to be expected and compensated in such
observations but we cannot deal with them here. Current
meters can be used at any depth. In the best known of
these instruments (the Ekman meter) a large vane sets the
apparatus in line with the direction of flow of the water.
A delicately pivoted and balanced propeller revolves as
the water flows past it and this actuates a mechanism that
counts and records the revolutions of the propeller. The
direction is indicated by a compass attached to the
instrument. This has a heavy magnetic needle bent
downwards on either side of its point of suspension on the
pivot and there is a slot on one of the arms of the needle.
At certain intervals the recording mechanism allows a
small metal ball to fall on to the needle and it then rolls
along the slotted upper part of the latter and drops on to
the compass card. The latter is a metal box divided into
32 compartments. The box is rigidly fixed to the frame
of the apparatus, but the needle sets itself in the magnetic
meridian and so its direction relative to the box is recorded.
The velocity is deduced from the number of turns of the
propeller made in unit time.

In other forms of current meter there is a pendulum
which is lowered to a certain depth. From the direction

THE OCEANIC CIRCULATION 299

and amplitude of swing of the pendulum the direction
and rate of current is deduced. In others the pendulum
swings above a compass card which is covered with wax.
The centre of suspension of the pendulum is vertically-
over the centre of the card. When the apparatus has set
itself the frame carrying the pendulum is allowed to fall a
short distance, and then the pointed bob makes a mark
on the compass card which indicates direction in which
the current has swung it and the extent of the swing. In
other instruments again a vane is suspended by two wires
and the direction and velocity of the current are deduced
from the torsion of the wires. And so on. All these
forms of current meter must be calibrated by observations
made in a flow of water of knowTi velocity, or in a ship-
model tank.

Salinity and temperature observations. At any place in
the ocean the salinity and temperature have values which
are to be regarded as " normal " to that place and for a
certain season. Thus the average temperature of the
Atlantic in N. lat. 40° and W. long. 30° is about 16°C., and
the average salinity at the same place is about 36%^,,
Assume that the temperature varies throughout the year
with the sun's declination. The salinity, however, does
not vary in this way yet in some years its value may be
36%o and in other it may be 36. 5%^. These variations
can only be explained by assuming variability in the
quantity of highly saline water drifting up from the S.-W.
Atlantic. Or take the isotherms in the same region : we
must suppose that these would be parallel to the Hnes of
latitude if the temperature at any place depended only
on the declination of the sun yet they are bent up towards
the N.-E. in a very remarkable way. This can only be
explained by postulating a flow of warm water from the
S.-W. Atlantic towards the N.-E.

Plankton observations. The pelagic animal and vegetable
microscopic life of the sea varies from place to place, and
its nature and abundance depend on the local conditions

300 AN INTEODUCTION TO OCEANOGKAPHY

and on the time of year. Thus there are plankton animals
(such as the Salpidce) which are oceanic in habitat yet in
some years they may be found in great abundance in the
North Sea. This can only be explained by assuming an
invasion of water from the open Atlantic carrying the Salps
with it.

Thus large bodies of water are characterised by their
salinities, temperatures, faunas and floras. In these
respects the water masses change very slowly. If these
characteristics are found exceptionally in some sea area
the conclusion is that they are present because of water
movements. The latter can be traced by observation of
the physical and biological characters. Animals of the
planktonic category behave as minute drifters.

CHAPTER X
SECULAR CHANGES IN THE OCEAN

We are now in a position to consider, in rather more
detail, some of the matters discussed in Chapter II. The
prominent features of the surface-morphology of the earth
are : — (1) the shield-lands with their surrounding
continental regions ; (2) the oceanic depressions and (3)
the continental margins. Both the shield-lands and the
oceanic depressions have a certain character of endurance
in time when compared with the transitional regions
between them. These latter regions comprise that zone
of sea bottom round the continents which is less in depth
than about 1,000 fathoms and the lands adjacent to the
coasts but, on the whole, outside the areas of the shields.
Relative instability, when compared with either the shield-
lands, or the ocean bottoms, characterises this transitional
region.

Its general character varies in different parts of the
world. Consider first the continental shelf : we have seen
that this has a relatively great width in the North Atlantic
and on the South-West of the Pacific, and that it is mostly
narrow and contracted along much of the South Atlantic,
on the East and North Pacific and on the East side of
Africa. Now take the continental land margins and we
find analogous differences. In the North Atlantic the
coastal land has, in general, moderate elevation and it is
old land which is much eroded and is relatively stable.
On the other hand the coastal lands bounding the eastern
and northern sides of the Pacific have high elevation, and
the presence of numerous volcanoes indicates their relative
instability. But, again, there are other coastal lands —
those on the western side of the Pacific, in the West Indies

301

302 AN INTRODUCTION TO OCEANOGRAPHY

and in the Cape Horn Antarctic region, where the elevation
is moderately low, but where the presence of chains of
islands indicate an immediate past, or a present instability.

How did these prominent earth-features originate, and
to what extent have they endured throughout the great
geological periods ? Several hypotheses of earth-shaping
were discussed in Chapter II. These were : (1) the older
hypothesis based on a gaseous-molten stage in the evolution
of the earth. The materials of the latter solidified, first at
the centre and then progressively towards the surface.
The central parts retained heat for very loDg periods but
continued to contract, and do still contract. With the
contraction the surface layers became distorted in various
ways. One of these ways is very frequently supposed to
have given the earth a tetrahedral figure and the coigns,
or sohd angles of the tetrahedron (twinned in the way
some crystals are) are now said to be represented by the
shield-lands ; (2) Still assuming a gaseous-molten stage it
is argued that a difference in position between the centre
of figure of the earth and its centre of rotation would arise
as the materials of the earth stiffened into the rigid state.
Then a certain distribution of elevated surfaces above the
mean level, and depressed surfaces below the mean level
would come into existence ; (3) Assuming that the earth-
body grew by the accretion of planetesimals there were
probably differences in the rates of rotation. The latter
would gradually steady down to its present value as the
materials of the earth became rigid under the strains set
up by self-gravitation. But while the earth-body grew
and was assuming this condition of high rigidity it must
have yielded whenever the rate of rotation changed. It
would tend to yield along certain zones.

Now on these hypotheses there would be regions of
elevation and depression (1) ; regions of deformation set
up by internal strains (2) ; and zones of segmentation (3) :
in any case the earth-body would tend towards a condition
of stability (which is already attained, or is approaching)

SECULAR CHANGES IN THE OCEAN 303

such that the surface features — that is, the continental
elevations, the oceanic depressions and the marginal zones
— would be as they are now. The continents and oceans,
as we know them, would be permanent features of the
earth's surface.

It now remains for us to examine the nature of the
marginal, continental-oceanic zones, and what other
evidence we have as to the past distribution of land and
sea, in order to test this conclusion of oceanic and
continental permanence. We have already considered the
morphology of the oceanic bottoms and we may, very
summarily, say something about the continental elevations
and their marginal regions.

The shield lands. — These are — in the Eastern Hemisphere
— (1) Angara (Northern Asia and Siberia) ; (2) Baltica (the
Scandinavian-Russian region) ; (3) Ethiopia (the greater
part of Africa) ; (4) and (5) Lemuria (Madagascar across
to India) and (6) Australia (the major part of that continent).
In the West we have (1) Laurentia (the region round the
Gulf of St. Lawrence) ; (2) Columbia (in Mexico) ; (3)
Antillia (the West Indian region) ; (4) Amazonia (Brazil
and N.-W. North America) ; (5) Archiplata (Patagonia).
There is also the Antarctic shield-land (Antarctica). The
terminology, and descriptions of the shield-lands differ
somewhat in the books and Suess' original conception has
been greatly extended.

The shields are represented by the roughly oval areas
on the series of maps, Figs. 64, A — F. They consist
mainly of archean rocks which are highly metamorphosed
by long-continued stresses. In these masses of
metamorphic rocks are igneous intrusions of more recent
date. On the shields there may be strata of Paleozoic or
later origin. The general level of the shield-lands is rather
low, and the fact that they may be overlaid with
sedimentary rock indicates that they have, at times, been
at the bottoms of shallow seas. What distinguishes
them from other continental regions is the absence of

304 AN INTRODUCTION TO OCEANOGRAPHY

folded strata and this means that they are highly resistant
land masses which have been able to withstand horizontal
pressures, or thrusts, exerted from the regions around them.
This character of stability they must owe to some conditions
in the earth-mass beneath them and so the existence and
situations of the shield-lands is, in some way, to be traced
back to the processes that have made the earth body as
a whole.

Whatever we may conclude as regards the permanence
of the oceanic depressions we may be fairly certain that the
shield-lands have always been regions of elevation above
mean spheroidal level. They are the nuclei of the
continents : fixed points in a shifting, superficial earth-
structure.

The oceanic depressions. — What we do know with regard
to the beds of the great oceans has been summarised in
the earlier chapters of this book. The floor that is indirectly
accessible to our observation is, of course, a layer of ooze
or mud and we know it only to a depth of about a foot or
so. Its nature depends on the processes — chemical,
physical and biological — that are in operation in the
waters lying on the sea bed. What lies beneath the
stratum of ooze that we can sample we do not know and
there is no immediate prospect of such knowledge being
attained. For our ideas, such as they are, about the
nature of the lithosphere beneath the floors of the ocean
we have to depend on the examination of the rocks that
compose the substance of the oceanic islands. These are
volcanic and that is all we can say about them with safety.
The oceanic regions, then, represent depressions of the
earth's surface beneath the mean spheroidal level. The
nature of the lithosphere beneath these depressions is
unknown, and there are no sedimentary rocks accessible
to our observation that indicate, in any way, that they
have been deposited on the floor of a deep ocean.

The oceanic-continental margins. — Round the shield-
lands, but still on the general continental region we have a

SECULAR CHANGES IN THE OCEAN 305

great variety of conditions. On all this area, and at some
time or other in the past, the sea has transgressed, so that
much of the continental surfaces have formed the bottoms
of epi-continental seas, or great lakes, or lagoon formations.
These lands have been eroded, or have undergone
peneplanation, or even subsidence. Although all this may
have occurred over the continental regions we need not
assume that there has been a depression of level there to
a depth greater than over the average continental shelf
region. We are not concerned here with the vicissitudes
of the continental regions between the shield lands and
the oceanic margin. That is a subject of pure geology.

The marginal zones, that is, the continental shelves and
the borders of the continents inland from the coasts, are
regions of instabiUty and topographical change. The
causes of this instability may be put into two categories : —

(1) Those due to the evolution of the earth-body itself.
From the gaseous-molten hypothesis we deduce that the
earth is still cooling and, therefore, contracting. The
contraction is least of all at the surface and so the thin
film of sedimentary rock there must be thrown into folds
or wrinkles as it falls inwards upon an earth-body that is
shrinking away from it. The folding or crumping will,
obviously, be greatest on the borders of the continents,
since here we have, not only a slope from elevated to
depressed regions, but also strata which, on the one hand
are being lightened by erosion, and on the other, are being
weighted by deposition. The marginal zones are, therefore,
instable.

Also on such a hypothesis as that formulated by
Chamberlin from the planetesimal conception we have
regions of weakness in the earth-body itself. These will
obviously tend to be the transitional zones between regions
of elevation and depression.

(2) Apart from any hypothesis of the shaping of the
present superficial features of the earth we can easily see
how the oceanic-continental margins must be zones of

306 AN INTRODUCTION TO OCEANOGRAPHY

instability — just because they are the transitions between
elevated and depressed regions. There is, everywhere on
the surfaces of the continents, erosion by the gases of the
atmosphere, by rivers and by ice. On the coast itself
there is disintegration of rock by the action of the seas.
Rivers and glaciers and winds transport material in the
form of boulders, stones, mud and dust, while gravels,
sands and muds are transported by tidal streams, heavy
seas and currents. The result is that enormous masses of
material are removed from some regions and re-deposited
on others — in general eroded rock materials are taken from
regions of higher elevation and laid down in other regions
of lower elevation. This may occur in the interiors of
continental regions where sediments are being deposited
in lakes and lagoons, but the deposition must occur, on the
great scale, on the sea bottom adjacent to the continental
land. Eroded earth material is, therefore, taken from the
elevated continental land and is laid down on the adjacent
sea bed.

But it is very important to note that the deposition
must take place almost entirely on a very restricted zone
of the sea bed close to the land. Stones and gravel can
only be transported for great distances out to sea by
icebergs, and this accounts for relatively little material.
Gravels and sands are not carried in suspension by currents
which have the velocities which we have studied, but are
rather moved along the sea floor itself by breaking seas
and wave action. At very moderate depths wave action
must cease to affect the sea bottom in so far as the carriage
of gravels and sands is concerned. Muds may be carried
in suspension by water which is moving with the velocities
of spring tidal streams (in British seas), but even at a very
few miles from the land, muds are usually deposited largely
because of the precipitating effect of the ions in sea water.
Therefore, nearly all the materials that are eroded away
from the land and are carried down to the sea by rivers
are deposited on the continental shelf usually within a

SECULAK CHANGES IN THE OCEAN 307

distance of, say, ten to a hundred miles from the land.
The quantity of material taken from the land and laid down
on the ocean bed beyond the 1,000 fathom line is so small
that it may be neglected altogether.

That means that nearly all the material resulting from
the erosion of very extensive land regions is laid down
along a very narrow zone of sea tottom immediately
adjacent to the land. A glance at the charts of depths
given in Chapter II. will show that extensive masses of
eroded materials can only accumulate indefinitely on the
continental shelves in the vertical direction. It is indeed
possible that the continental shelves may grow out seawards
as they become loaded with sediments, and this may be
the case with the shores of the North-East and North- West
North Atlantic. But it seems probable that long before
this outward growth of the shelves has occurred the loading
will have proceeded so far as to bring about isostatic
readjustment.

That is to say, there must be an always increasing load
on the sea bottom adjacent to the land, in such regions
where there is extensive land erosion. Masses of sediments
must, therefore, accumulate and press downwards on the
stratum of lithosphere where the earth materials are self-
crushed or are potentially " flowable " or plastic. The
processes mentioned on pp. 22-23, occur in the yielding
stratum and the material of the latter is squeezed out, so
to speak, laterally from the zone of excessive loading. It
may " flow " into the regions beneath the ocean bed or into
those beneath the land and, in the meantime we assume
that the latter direction is taken. Therefore an horizontal,
lateral pressure will be exerted from the loaded continental
shelf region upon the strata beneath the adjacent land.
This is a thrust.

Its effect will be to fold those strata — should that be
possible without rupturing them. How the fold occurs can
easily be represented. Place the tips of the spread-out
fingers on the right hand page of this book laid open on

308 AN INTRODUCTION TO OCEANOGRAPHY

the flat, and then push gently towards the left. The upper
page or pages (the strata) will rise up in a fold and the
crest of this will move from right to left with its advancing
side steeper than the traihng one. At the centre of the
book the fold v/ill turn over so that the advancing side
turns in underneath the crest. This represents an over-
fold and its effects. Now place the spread-out fingers
of the left hand lightly on the right hand page along a
line about an inch or so from the centre and push as
before. A fold will form and will move towards the left,
but its advancing side will be less steep than the trailing
side and the latter will ultimately turn in under the crest.
This represents an undsr fold and its effects.

Such thrusts on a gigantic scale must be produced, on
the theory of isostasy, when a large part of the continental
shelf becomes so weighted down by accumulated sediments
as to lead to readjustment. The load just off shore is
largely increased while that over the eroded area is reduced.
Thrusting from the continental shelf must, therefore, have
notable effects on the uneroded horizontal strata of the
adjacent land regions, folding them over or under. What
actually will happen must, of course, be highly complex
depending on a host of conditions. Upper strata under-
going the thrusting pressure may be broken and faulted
with the production of earthquakes. Enormous pressures
may transform into heat with the result that accumulations
of fluid rock (magmas) may form in the cores of the folded
strata and give rise to volcanic effects. The underlying
strata may be folded and metamorphosed while the
superficial ones may be broken into series of faults. Such
foldings will form the flanks, at all events, of mountain
ranges but, it would appear, some other factors leading to
actual uplift must be at work to account for the elevations
that are to be seen on the margins of the oceans.

The regions where such earth-folds occur are those
between the coasts and the margins of the shield-lands.
The latter show no indications of mountain ranges produced

SECULAR CHANGES IN THE OCEAN 309

by the foldings of strata : they resist the lateral thrusts set
up by loading of the adjacent sinking areas and upon the
directions in which this resistance to the thrusting pressures
is exhibited, as well as upon the ways in which the loads
are distributed in the area of sedimentation will depend
the directions of the earth-folds.

Thus the marginal oceanic regions (the shelves) are
characteristically areas of sedimentation and vertical
loading, resulting in periodic crises of thrusting stresses.
The continental margins will exhibit the results of these
stresses in under- and over-thrusting pressures sustained
by the strata and leading to folds that mark the directions
of coastal (or even submarine) mountain ranges. There
will be the subsidiary effects of yielding of the strata by
fault-formation — and thus earthquake phenomena. There
will be great heat development that finds its expression
in volcanic effects. In these ways the instability of the
oceanic- continental margins is manifested.

We may now examine the conformation of the oceanic
margins with the above observations in mind.

(1) The Pacific Ocean. — The following sketch chart gives
a general representation of the morphology of the Pacific
Ocean. The main features are (1) the long, slightly curved
folds that mark out the situations of the American, coastal
mountain ranges ; (2) the shorter, curved folds on the
Asiatic coast and round Australia, and (3) the curved
chains of islands on the western coast. These are the
most elevated parts of earth folds that are mostly
submarine.

The long arcuate folds. These lie along the coasts of
South and North America and mark out the situation of
the great Pacific geo-syncline. They give us the existing
mountain ranges — the South American Andes, the Andean
Cordilleras, the Sierras in Chile and Mexico, the Cascade
Mountains in North America and the Alaskan ranges.
The folds run in long, gentle curves with the convexities
turned towards the ocean. On the Pacific side the slopes

310 AN INTRODUCTION TO OCEANOGRAPHY

are less steep than they are inland. Behind these coastal
arcs are others, or the indications of others, which mostly
face the other way : thus in North America we have the

/g|0 /4)0 /6|0

IB}p 11^ ^o /QP /(>P Mo /g|Q ^

Fig. 58. The Pacific Ocean showing (by double contours) the lines
of foldings ; the shield-lands and the ocean deeps. The
shields are cross hatched and so are the deeps (over 3,000
fathoms). The continuous contour line on the surface of
the ocean follows roughly the 2,000-fathom depth contour
but it is also drawn so as to include between it and the land
most of the large and small Pacific Islands and to exclude
the deeps. The short arrows indicate the directions of the
thrusts (when known) and the long curved arrows suggest
the general directions along which the Pacific floor has been
tilted downwards, or depressed, with the Australian Shield
as a kind of pivot. The reader should examine the sketch
chart, along with the one on p. 67, and a large scale general
chart of the entire region.

Rocky Mountains running roughly parallel to the coastal
ranges and between the two is the situation of the Great

SECULAR CHANGES IN THE OCEAN 311

Basin, the locus of a North American inland sea of
Cretaceous to Tertiary date. Several series of thrusts
have contributed to the formation of these long arcuate
foldings — from the ocean to the land, and from the
Cretaceous sea outwards to the coasts of this old basin in
both directions.

The coastal ranges mark out a region of instability
characterised by the events that have lifted up the mountain
folds on the continental margin and depressed the sea
bottom on the oceanic one. There are both earthquake
and volcanic phenomena along this geo-synclinal zone.
It has been regarded as the typical oceanic margin — a
narrow continental shelf, outside which the ocean bed goes
down to great depths and within which the land rises to
as great heights. So far from being typical the Eastern
Pacific coast is really exceptional.

Round Australia and on the Asiatic Continental Coast,
in China and Siberia we have similar foldings laid out in
long arcs and turned with their convex sides to the Pacific.
Far more characteristic, however, of this Western Pacific
region are : —

The Insular arcuate foldings. These are represented in
Figs. 59 and 60. What we see here are earth-folds that
are mostly buried beneath the ocean, showing only their
most elevated parts as chains of islands. The latter are
laid out in short curved lines and also turn their convex
sides towards the ocean. Here also we find evidences of
instability in the presence of numerous volcanic vents
along these island arcs and in the occurence of earthquakes.
On the oceanic side of the island arcs the ocean bottom
goes down to greater depths than on the outside of
the great American geo-syncline. Within the arcs, that
is, between the islands and the continental coasts we have
epi-continental seas. These are not merely situated on
the continental shelf but may go down to depths of 1,000
to 2,000 fathoms.

The great Pacific Island arcs are : (Figs. 58-9) — (a) The

312 AN INTRODUCTION TO OCEANOGRAPHY

Aleutian arc which begins with the Alaskan mountains
and runs out into the Pacific as the Alaskan Peninsula.
It is continued West as the line of Aleutian Islands ending
on the East on the Kamtchaktan Peninsula, Within it is
the Behring Sea with depths of 1,000 to 2,000 fathoms.

(6) The Kurile arc. The Kamtchaktan Peninsula is
continued to the South as the line of Kurile islands. These
turn gently towards the East and end in the Japanese
island of Tezo. Within them is the sea of Okhotsk with a
depth of rather over 1,000 fathoms.

^urmia

Fig. 59. The East Indian Islands showing the hnes of foldings. The
soundings (to be multiplied by 100) are in fathoms.

(c) The Japanese arc. The Japanese Islands run mainly
, meridionally and then turn towards the West. The fold
represented by them is continued on to the Asiatic Continent
through the Korean Peninsula. Within this arc is the Sea
of Japan with depths of over 2,000 fathoms. Outside the
Kurile and Japanese arcs is the greatest of the Pacific
deeps, where soundings of over 5,000 fathoms have been
recorded.

SECULAR CHANGES IN THE OCEAN 313

(d) The Lu-chu arc. This starts in the North with the
Japanese Island, Kinsiu, and is continued by the line of
Lu-chu Islands. It turns to the South-West and ends in
Formosa. Within it is the Yellow Sea. This is situated
on the continental shelf though we may regard it as an
epi-continental sea rapidly filling up with sediments.

The Festooned East Indian Arcs.

(e) The Philippine — Bornean — Malayan arc is formed by
the Islands of Formosa, the inner Phihppines, the earth-
folds on the North- West border of Borneo, a series of small
islands and the Malayan Peninsula, through which it is
continued into Cambodia. Within it is the epi-continental
South China Sea with occasional depths of over 1,000
fathoms. Without it are the island festoons.

(/) The Philippine — Sulu arc which is formed by the
Western Phihppines and the Sulu Islands connecting the
latter with the North-East comer of Borneo. It, and the
inner Philippine arc enclose between them the Sulu Sea.
This is an epeiric sea situated on the continental shelf.

(g) The Philippine — Celebes arc. Outside the last island
chain is another beginning with a strung-out series of
Phihppines and carried on into the northerly process of
Celebes. Between it and the Phihppine — Sulu arc is the
Celebes Sea, an epi-continental with occasional depths of
over 1,000 fathoms.

(h) The Javanese arc. Obviously the elongated islands
of Sumatra and Java form an earth-fold continued to the
East through Flores, Timor and the Moluccas and ending
in the eastern process of Celebes.

All the arcs mentioned so far are of late Mesozoic date.
But the two southerly processes of Celebes appear to
represent arcs of Pre-Cambrian age. Within the Javanese
arc, and bounded by Celebes and Borneo, are the Javanese
and Banda Seas which are epi-continentals with occasional
depths of over 1,000 fathoms.

The exterior West Pacific arcs. Well outside the East
Indian festoons there are indications of yet other submarine

314 AN INTRODUCTION TO OCEANOGRAPHY

earth-folds. The island chains of the Ladrone and
Caroline groups suggest such a line of folding carried out
away from the Philippines and then in towards Celebes.
From Celebes again another fold seems to run through New
Guinea and then into the New Zealand group. Doubtless
a more detailed survey of the Pacific sea bottom on this
central part would indicate the existence of other lines of
folding. In these cases we could have, of course, no
indications of the actual geological structure, and the
interpretation of the series of elevations as actual earth-
folds is based on the analogy of the lines of elevation with
those other ones the structure of which is known.

The Pacific borders, then, are formed by great earth-
folds produced by thrusts from the oceanic, towards the
continental regions. These thrusts have raised up the
strata into long, gently curved folds (the American coastal
regions), or into short, more sharply curved folds (the
Western Island series). The convex sides of all the folds
turn towards the ocean and nearly everywhere the folded
regions are characterised by volcanic and earthquake
phenomena. To these characters of the Pacific we return
at the end of this chapter.

(2) The Atlantic Ocean. — At first sight the Atlantic
differs strikingly from the Pacific Ocean. Nowhere do we
have such a coast in the Western Ocean as we see in the
West margins of the American Continents. The long,
sweeping folds with their convexities turned to the ocean ;
the narrow continental shelves sloping down quickly to
nearly 3,000 fathoms and the lines of volcanoes on the
back slopes of the mountains are features which are absent
from the Atlantic oceanic-continental margins. In the
northern part of the Western Ocean we have a remarkable
extension of the Continental Shelf that we do not see in
the Pacific region. The chains of islands arranged in
arcs of quick curvature are indeed present in the Atlantic
but not to nearly the same extent as they are exhibited
in the West Pacific region. Finally the great stable land

SECULAR CHANGES IN THE OCEAN 315

mass of Africa dominates the South Atlantic (as it does the
Indian Ocean). There the long, arcuate, coastal foldings
are almost entirely absent and instead of them we have a
great, fractured table-land : a " mosaic of alternating
plateau and plain " covering and surrounding the shield-
areas. In spite, however, of these apparent differences we
shall find that the morphology of the Atlantic region is
fundamentally the same as that of the Pacific and that
it is the phase in the process of evolution that distinguishes
between the two oceans.

The characters represented in Fig. 60 are really the
same as those shown in the two charts of the Pacific on
pp. 310, 312. The long arcuate folds, as well as the short,
sharply convex ones, also occur but the fundamental
resemblance is obscured by the condition that the Atlantic
is an older oceanic basin. Thus emphasis is laid, in Fig. 60,
on the distinction between longitudinal and transverse
coasts, the former being represented by mountain ranges
running roughly parallel with the sea margins and the
latter by ranges running down into the sea perpendicularly,
or at high angles to the general directions of the coasts.
In the Atlantic such transverse coasts are the loci of Rias
and Fiords.

Ria and Fiord Coasts. The typical Ria coasts are those of
Oalicia, where the Calabrian mountains tilt downwards
into the ocean. Here are ancient earth-folds, the oceanic
extremities of which have subsided, so that we have
valleys in which the seaward parts are drowned, so to
speak. They are the depressions between the crests of
the folds. Passing down them towards the ocean both
the widths and depths of the valleys increase. Similar
Ria coasts are to be seen on the shores of Nova Scotia,
Cape Breton Island and Newfoundland. Here the ancient
Appalachian mountains range dips down into the Atlantic.

Probably fiords have much the same origin but their
subsequent history has been very different. They are
depressions of elongated form, with rather greater depths

316 AN INTRODUCTION TO OCEANOGRAPHY

Transyersf: coasts
Longifuainai do.

Fig. 60. The Atlantic Ocean showing the shield lands and the
characters of the oceanic-continental margins.

SECULAR CHANGES IN THE OCEAN 317

than the Rias, with very much the same width and depth
throughout except at the seaward extremities where there
is usually a shoaling of the bottom in very much the same
way as a dock sill separates the basin from the sea outside.
These peculiarities are ascribed to the scouring action of
ice on the bottom of the fiords. As the glaciers which
formerly filled them projected outwards into the sea they
broke off as icebergs, or melted. Therefore material was
laid down at the mouths of the fiords to form the sills
while the bottoms, landwards from the sills, were ground
away by the ice.

The longitudinal and transverse Atlantic coasts
nevertheless reveal to us just the same series of long
arcuate and insular folds that we have studied in the
Pacific. But the Atlantic oceanic-continental margins
have undergone long-sustained changes in comparison with
the Pacific, so that our longitudinal coasts are a kind of
marginal debris and the transverse ones are the ends of
other folds, the greater parts of which have been eroded
away. So far we are rather emphasising the similarity
between the two basins, but we shall see later (p. 334) that
there are some characters in which the Atlantic appears to
differ in a striking way from the Pacific.

The long arcuate folds. These are represented in Fig. 60,
by the broken lines running along the East coast of N.
America ; the coast of Scandinavia and the East coast of
S. America. They are old folds, of early Mesozoic origin
and are much eroded. Without doubt, the East N.
American fold, represented now by the Appalachian
mountains, was originally continuous with the Scandinavian
fold, which extended across the North Sea and Scotland
as the ancient Caledonian mountains. A rather similar,
ancient, long arcuate fold is seen as the debris of mountain
ranges running along the East coast of S. America. We
refer further to these structures on p. 332.

The short arcuate folds. The only typical instance, in
the Atlantic, of a short sharply curved fold is that which

318 AN INTRODUCTION TO OCEANOGRAPHY

encloses the Mediterranean. The Sierras in Granada
appear to curve round to the South across the Straits of
Gibraltar and become continuous with the coastal
mountains in Northern Morocco,

The Insular Arcuate folds. Just the same conditions
are to be seen in the Atlantic as those which we have
studied in the case of the West Pacific coasts.

Fig. 61. The West Indian Archipelago showing the arcuate arrange-
ments.

The West Indian arcs are represented by rows of islands
arranged in sharply curving Unes,

The great foldings that run along the East Pacific
coasts, turn inwards in Central America and Southern
Mexico, and run transversely across the Continent to
emerge into the Atlantic as the Islands that bound the
Gulf of Mexico and the Caribbean Sea. The Venezuelan

SECULAR CHANGES IN THE OCEAN 319

fold runs out into the Atlantic at Trinidad and is continued
as the line of the Lesser Antilles and terminates at San
Domingo. The Mexican fold runs out through Florida
and is continued as the Bahamas, also terminating in San
Domingo. There are two interior arcs also pivoting on
the latter island : one turns in towards Honduras, the
other runs through Cuba to Yucatan. Within these
island arcs are true epi- continental seas of over 1,000
fathoms in depth. They are the loci of numerous volcanoes
and are thus regions of earth instability.

The American- Antarctic arc is not so well known. The
great Andean mountain system of folds turns East in
Tierra del Fuego, is continued along a sharply curved line
marked by the Falklands, South Georgia, the Sandwich
group, the South Orkneys and Shetlands and is terminated
in Graham Land in the Antarctic Continent. This island
arc is peculiar in that there is behind it no continental
hinterland, as is the case with both the West Pacific and
the West Indian arcs.

The Indian Ocean. — Something must be said about the
larger features of the Indian oceanic margins although less
that is interesting is known about this region than in the
case of the Pacific and Atlantic.

The only features that we notice here are : —

(1) The apparent dominance of the African continental
shield-land. Here we have the greatest area of land on
the earth that is unmarked (except in the extreme North
and at the Cape region) by mountain folds. It has been
elevated and depressed and great land blocks are marked
out by lines of faults but the thrusts from the ocean have
not been powerful enough to form the folds that are
characteristic of the American and Asiatic coastal margins.

(2) The relative shallowness of the western margin. The
continental shelf, and the zone of sea-bottom between
1,000 and 2,000 fathoms in depth is greater in area
on the western than on the eastern side of the Indian
Ocean.

320 AN INTRODUCTION TO OCEANOGRAPHY

Madagascar, the Seychelles, the Chagos, Maldives and
Laccadives Archipelagoes may very probably have been
the highest parts of a land region connecting Africa with
India and lying to the East and South of an Arabian
basin — a former epi-continental sea.

Fig. 62. The Indian Ocean region, showing the shield-lands (cross
hatched) ; the greater earth-folds (double lines) ; the ocean
deeps (cross hatched) and the depth-contours. The latter
follow approximately the 2,000-fathom line and are drawn so
as to include the shoal regions and to exclude the deeps (over
3,000 fathoms).

(3) The absence of deeps on the western side. This is in
strong contrast with the frequency of oceanic depressions
on the eastern side of the Indian Ocean.

SECULAR CHANGES IN THE OCEAN 321

THE HISTORY OF THE OCEANS.

We are now in a position to examine the question of the
permanence, throughout geological time, of the great oceanic
depressions. There have always been two opinions — (1) that
founded on the various hypotheses of earth-origin and
holding that the oceans have always been regions of earth
depression, and have occupied their present places, and
(2) the other holding that oceans and continents have
changed places more than once during the period of time
represented by the series of sedimentary rocks. Nowadays
the latter hypothesis has been modified to some extent.
It is generally believed that the shield-lands have always
been regions of elevation, and that they are traces of an
original earth-structure on the great scale. It must also be
made clear to the reader that all parts of the continental
regions may, at one time or another, have been at the
bottoms of shallow seas, even though thay may have yet
been placed above the mean spheroidal level. Our question
then, is this : have bottoms of the present oceans been at
any past time, dry land ?

The evidence available. — In considering this question
various lines of evidence are available. These are — (1) the
larger geological structures, in particular those of the
oceanic-continental margins with regard to the earth-folds,
and the lines of weakness that are indicated by volcanic
and earthquake phenomena ; (2) Stratigraphical facts,
that is, the occurrence and distribution of marine sedimen-
tary rocks on the continental regions ; (3) The evidence
afforded by fossils as to the lines along which both terrestrial
and marine animals have migrated ; (4) The absence, in
the sedimentary rocks, of any strata similar to the oozes
that are now being deposited at the bottoms of deep
oceans. These various lines of evidence may now be
briefly summarised.

(1) We have dealt with in the earlier part of this chapter.
Presently we shall look into its significance.

Fig. 63 A to C. Central Europe at various times during the Tertiary
Period. Horizontal lines = sea ; broken lines = lagoon lands.

SECULAR CHANGES IN THE OCEAN 323

(2) Is a matter for pure geology. It is sufficient to say
here that in very many places on the continental regions
there are sedimentary rocks that must have been laid
down at the bottoms of shallow seas. Large parts of
North America and Central Europe, for instance, have been
under water (the sea, fresh lake water, or lagoon water).
To illustrate this we give here three charts showing Europe
in tertiary times.

(3) We must deal with in rather more detail.

The paleontological evidence. — It is now generally agreed
that all species of plants and animals on the earth have
evolved from simpler forms and that the process of
evolution has been a single one. It is possible that large
regions of the earth may have heated (by radio-active
changes in the interior) so strongly as to have destroyed
all hfe and that evolution may then have begun anew.
It is simpler, however, to assume that this has not been
the case and that there has only been one evolutionary
process — all the existing form of animal life being thus
blood-related. Further we assume that each great
group of animals (the mammalia for instance) have
originated in some one area and have spread over all
accessible and suitable regions from this original focus.
As they so spread they became modified or evolved :
with the spreading there was adaptation to new conditions.
The process is one of adaptive radiation.

In spreading, the animals encountered barriers. Broad
seas would be a barrier to the movements of terrestrial
mammals and so would extensive desert areas, very high
and long mountain ranges, or great forests populated with
enemies — say parasitic insects. There might be other less
unfavourable conditions such as relatively high or low air
temperatures, etc., but to these the migrating animals
might be able to adapt themselves.

So also with other terrestrial animals and plants.
Barriers or obstructions, depending on the habits of the
species would exist in many cases.

324 AN INTRODUCTION TO OCEANOGRAPHY

Similarly there would be barriers to the spreading of
marine animals. Many such can only live and reproduce
on the sea-bottom in relatively shallow water : many
species of molluscs, Crustacea, polyzoa and foraminifera,
for instance. These animals, although living in shallow
water on the inner edges of the continental shelves may
produce pelagic larvae, that is, young stages which drift
about in the sea near the surface. But these pelagic (or
planktonic) stages are of short duration. At the end of
its development the animal must find its habitat in a sea
bottom below shallow- water. If this metamorphosis from
the larval, to the adult stage occurs in the open ocean,
over deep water, the animal must perish. Thus broad and
deep oceans are barriers to the spreading of these bottom-
living, shallow water (or neritic) marine animals. There
might (as in the case of the terrestrial mammals, be
unfavourable conditions. Thus the Cod is a northerly,
cold-water fish, and the Hake is a southerly, warm- water
one, each inhabiting water of a certain range of temperature
and avoiding sea regions where the conditions are outside
these ranges. In course of time, however, adaptation
would lead to new varieties and so to the range of such
animals.

Further the question of convergent evolution must always
be considered. Species of animals which appear to be
very like each other need not have evolved from the same
common stock but may have evolved quite independently.
We can usually trace this convergence in character, although
accompanied by divergent origin, in the embryological
history of the species in question. In paleontology,
however, we generally have only the hard parts, or skeletons,
of animals. We can conclude, however, that if the hard
parts are alike so will be the other parts, but this is not
necessarily the case.

The paleontological evidence is, then, not absolutely
rehable still it is safe to accept it so long as it is consistent
with the other lines of evidence available. Now consider.

SECULAR CHANGES IN THE OCEAN 325

as instances, the present distribution of (1) mammals and
(2) certain neritic foraminifera, and the distribution of
the same animals in past time, as shown by their fossil
remains.

Marsupial mammals are found only in Australia (and
some adjacent islands) and in South America. We are
justified, at present, in concluding that both the Australian
and South American pouched mammals had a common
origin. Therefore there must have been land connections
between Australia and South America in the past. It is
very probable that Australia was connected with Asia
and that America and Asia were connected across Behring
straits. It is also probable that Australia and S. America
were both connected with Antarctica. So we have the
land connections — via the North of Asia and North
America, on the one hand, and via Antarctica on the other
— necessary for the spread of mammals from South
America into AustraUa, or vice versa.

But, again, there is evidence of the existence of very
similar fossil neritic marine species on both sides of the
Atlantic Ocean. These animals (Foraminifera, Polyzoa
and Mollusca) are such that they could, apparently, only
have migrated along a zone of shallow sea bottom not far
from a coast hne. It is true that the northerly and
southerly land connections just mentioned afford this
coastal zone of shallow water but, on the other hand,
there was probably a considerable difference of temperature
between the sub-tropical regions in which these fossil forms
were living and the northerly and southerly seas. This
temperature difference would, very probably be a barrier
to the spreading North and South of our sub-tropical
neritic species, whereas no such marked tempera-
ture-differences would exist in the East and West
dimension.

The ancient land and sea. — Considering the lines of
evidence indicated above we seem obliged to assume such
a distribution of land and sea, on the face of the earth, in

326 AN INTRODUCTION TO OCEANOGRAPHY

former geological periods, as would permit of the distribu-
tion of the various forms of terrestrial and marine life.
This leads us to the conclusion that the Pacific, Atlantic
and Indian Ocean depressions cannot always have been
such that they now are. Reasoning, then, from the
results of paleontology many reconstructions of past
geography have been made and from these we have
selected a series illustrating what is, at least, a possible
evolution of the surface features of the earth.
Looking at these reconstructions we see that : —

(1) There has always been continental land round the
situations of the Shields ;

(2) From Devonian until about Tertiary times there was
a Pacific Continent ;

(3) There were North and South Atlantic Continents
until the end of the Mesozoic period and between these
land masses there was an inland sea — the Tethys ;

(4) There were land connections between Australia and
the African and South American Continents ;

(5) There was, apparently, much less water on the earth
from Devonian to Tertiary times than there now is.

In considering the probability of these conclusions we
must see, first of all, whether the structure of the oceanic-
continental margins — as we have dealt with it in the
earlier part of this chapter is consistent with the conception
of past continental lands on the regions now occupied by
deep oceans and next what consequences flow from the
conclusions. It is to be noted that we reckon with stable
and permanent continental bosses, or shield-lands. There
is nothing to indicate that these have ever been at the
bottoms of the deep sea basins. And we have not to
account for the elevation of a former oceanic floor — all
that the charts of Fig. 64 assume is the subsidence of
continental lands to the bottoms of the present Atlantic,
Pacific and Indian Oceans. So we are not troubled by
the fact that no sedimentary rocks known to us are such
as they would be had they been deposited at the bottom

SECULAR CHANGES IN THE OCEAN 327

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328 AN INTEODUCTION TO OCEANOGRAPHY

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SECULAR CHANGES IN THE OCEAN 329

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330 AN INTRODUCTION TO OCEANOGRAPHY

of a deep ocean-because the charts do not assume the
elevation of a past abyssal sea bottom.

What, then, appears to be the meaning of the present
oceanic-continental margins ?

The Pacific region. The character of the whole region
suggests instability. On the East and North-East the
instability is tending towards stability. The present West
American coast is an almost continuous geo-syncline
formed by foldings superposed on foldings in roughly
parallel rows. The folds are generally gently convex
towards the ocean and behind them the continental land
is high. There has been actual uplift brought about by
the intrusion of magmatic rock into the cores of the folds.
There are volcanoes and earthquake phenomena so that
the region is still unstable. In front of the foldings there
is a slope down to sea level, next a narrow continental shelf
and then a continuous depression of about 2,500 fathoms
in depth, that is, about the average for the Pacific. There
are few deeps. The whole American Pacific coast looks
like one that has resulted from a process that is nearly
completed.

Turning to the North and West margins the appearances
are rather different. It is not the actual continental
margin that is folded but rather the adjacent oceanic-
bottom margin. This means that sea bottom itself
consists of rock capable of being thrown up into folds —
that is, sedimentary rock. The northerly Pacific folds are
long ones and their highest parts are represented by the
line of Aleutian Islands. In front of this island arc there
are deeps, but these are not profound though their depths
are greater than the average depth of the Pacific.

The next system of foldings are those arcs indicated
by the Japanese and Philippine Islands. They are not so,
long as the Aleutian arc and they are more sharply curved.
In front of them are deeps and these go down to much
more than the average Pacific depth — to over 5,000 fathoms
in some places.

SECULAR CHANGES IN THE OCEAN 3a

Then we have, South of the Japanese and Philippim
foldings, the much shorter and still more sharply curvei
ones that form the East Indian festooned system. Soutl
of this again the whole series of West Asiatic arcs seem tc
pivot on the Australian shield.

Undoubtedly we find a progressive process at work here.
Such other evidence as there is shows that the Pacific
originated in late Mesozoic or early Tertiary times. Its
site we may take to have been a great land surface broken
up by seas of moderate depth which were filling with
sediments : this condition is far more probable than that
the Pacific area was purely an elevated land region. On
the eastern side these postulated epi-continental seas were
more extensive than elsewhere and sedimentations went on
more rapidly. There was loading of the regions . of
deposition, and isostatic readjustment, which set up
thrusts against the West American continental margin,
thus establishing the great systems of foldings that we
have already studied. Subsidence occurred over the
entire East Pacific region, and extended towards the
North and West — how it was that so great an area became
involved we do not know and it is certain that factors
other than isostatic loading of even an extensive marginal
area must have been in operation : just as some other
factors than isostatic readjustment only seem to be required
in order to account for the uplift of high continental regions.
But a general process of subsidence starting on the eastern
side of the postulated Pacific land area, and spreading
towards the west, does seem a plausible hypothesis — a
general tilt downwards on the great scale from the
Austrahan shield-region to North-East and East and
South-East (see Fig. 58).

Beginning then, at the eastern side the whole Pacific
area subsided so that the process is now being completed
off the Siberian and Chinese coasts. As the epi-continental
seas within the island arcs there fill up and exert thrusts
in both directions we shall have such conditions as are

3 2 AN INTRODUCTION TO OCEANOGRAPHY

row seen on the North American Pacific margin — a line of
coastal folds and behind these, and separated from them by
flled-up basins, other parallel foldings. The eastern
?acific is thus older and more nearly stable than the
yestem part of the ocean. In the South- West the stable
Australian land mass dominates the general area just as
\frica does the South Atlantic and Indian Oceanic regions.
I There is, of course, nothing " inconceivable " in the idea
w a subsidence on such a scale — indeed it is consistent with
(vhatever other evidence we have. Paleontological results
[)oint to it, as we have seen in the series of charts reproduced
from Haug (Fig. 64). Darwin's hypothesis of coral reef
formation assumed a general depression of the Pacific
tegion in order to account for the numerous atolls and
fringing reefs. The later hypotheses of Semper and Murray
pointed rather in the other direction in that they indicated
reef building in regions of actual elevation. Later still,
liowever, the American geologists, Daly, Vaughan and
others have shown that coral reefs are generally laid down
on platforms eroded from a subsiding land surface.

Very notable also are the indications of exterior island
arcs (see Fig. 58). With a more exhaustive hydrographic
Survey of the Pacific Ocean than we now possess insular
tires East of those that pass through the Ladrones and
Carolines may probably be found. Now does an earth
fold always indicate a crumpling of stratified rock ? If so
the conclusion seems inevitable — a large area of the Central
[Pacific Ocean consists of sedimentary rocks laid down on
Ithe bottoms of shallow seas.

I The Atlantic Ocean. In the infrequency of occurrence of
volcanoes and earthquake phenomena on the Atlantic
margins, when compared with those of the Pacific, we see
a more stable condition. Here the supposition we take
(on the paleontological evidence mainly) is that subsidence
has occurred in both the North and South Atlantics. This
occurred at the end of the Mesozoic period, prior to which
lending the great folds now represented by the Appalachian

SECULAR CHANGES IN THE OCEAN 33

and Caledonian systems in the North and th^
contemporaneous Brazihan coastal foldings in the Souti
occurred. It seems probable that the northern foldingi
formed a great geo-syncline round the West, North anq
East of the North Atlantic, similar in all respects to th«
geo-syncline that bounds the Pacific on the East and North-
East. In the process of subsidence the northern part oj
this system of folds became destroyed and its materia
is now represented by the very extensive continental sheli
that forms the real margin of the North Atlantic. Thuj
there was a Paleozoic, or early Mesozoic North Atlantc
Continent, subsiding in late Mesozoic times, with destructim
of its northern land margins. Into the South of the oceati
so formed the Sea of Tethys opened out. On the Soutl-
East the process of subsidence still continues and its resilt
is the formation of the series of West Indian Island ar<s.
The " transverse " coasts of the North Atlantic, on the
Newfoundland side as well as on the Scandinavian-Scottsh
side represent the broken-off ends of the originaly
contiimous North Atlantic geo-syncline.

The South Atlantic seems to be similar except in so ar
that the African shields have given a character to tUs
region which is not represented in the North. The ancieit
Brazilian coastal folds, on this view, formed the western
part of the South Atlantic geo-syncline which extendtd
along the East coast of South America and then, fron
about Cape San Roque, across towards Africa, abuttiig
on the latter continent somewhere near Cape Verde. Hffe
we have traces of an African " transverse " coast, foHs
which are perpendicular to the oceanic margin. Thus Te
have a South Atlantic land mass subsiding, with the resilt
that a northerly and westerly geo-syncline was establishd.
behind which was a folded region crossing the preseit
Atlantic region from about Cape San Roque to Caje
Verde. North of the land connecting Africa and Brail
was the Sea of Tethys. Subsidence occurred and tfe
South Trans- Atlantic land connection was broken throuj

334 AN INTRODUCTION TO OCEANOGRAPHY

As in the case of the North Atlantic the process of subsidence
was latest in the South- West, where insular arcs, essentially
similar to those in the South- West Atlantic and the South-
West Pacific are to be seen in the Cape Horn — Graham
Land chain of islands.

The new feature in the Atlantic. In the Atlantic, however,
we have a new feature, one which — so far as we know — is
not exhibited by any other oceanic depression. This is
the remarkable Central Atlantic Rise, or Swell — the
alongated elevation of the sea bottom that runs from
Sforth to South on the average, and nearly equidistant
Tom the Old and New World Coasts. It is very tempting
t) try and explain this feature on the theory of isostasy —
aid the explanation " strikes one in the eye," so to speak,
m looking at a chart of the depths of the Atlantic Ocean.
The depression is an old one — as old as the end of the
IV^esozoic, or the beginning of the Tertiary period. There
his been enormous erosion on all the Atlantic Coasts
aid here and there the continental shelves are exceptionally
wde. Therefore there was loading of the isostatic,
yelding stratum, a " flowage " of material out from
bneath the loaded zone and thrusts. Are the latter
eierted only towards the continental margins ? We
appear to have no good reason for assuming that they are
oily exerted towards the nearest land and we may suppose
tiat they are also directed towards the axial line of the
03ean, that is both East and West. Readjustment due to
isostasy may, therefore, have led to the lifting up of the
(Antral Atlantic Rise, which is thus to be regarded as a
g-owing sub-oceanic anticlinal region.

The other oceans. So little is known about the Indian,
i.rctic and Antarctic oceanic regions (in comparison to our
hiowledge of the Pacific and Atlantic) that not much can
le said as to their past history. The great stable table-
Imd of Africa dominates the Indian Ocean on the West :
b the East the latter is to be regarded as a part of the
lacific region. On the western side there is apparent

SECULAR CHANGES IN THE OCEAN 335

evidence of general subsidence. But the absence of the
arcuate earth folds on the East African margin is
remarkable. Here we have had instead a process of
faulting on the great scale along the broad zones marked
out by the Red Sea, and the great African lakes.

The Arctic Ocean has all the appearance of an old
epi-continental sea on the big scale, filling by sedimenta-
tion : thus we have the very extensive continental shelves
indicated in Fig. 12, The Southern Ocean is simply to be
regarded as the extreme South parts of the three great
Oceanic basins with Antarctica placed nearly symmetrically.
The structure of the South Polar land, and the depths of
the adjacent oceans, are far too little known to justify any
speculations as to the morphology of these regions.

Some consequent results. — (a) It seems to be probable :
(1) that the shield-lands have always been regions of
elevation and thus the nuclei of the continents ; (2) that
the present oceanic regions have either been areas of
depression or of very low elevation with respect to mean
spheroidal level and (3) that the zones between these two
series of regions have always been areas of marked
instability. Therefore there is a definite surface
morphology of the earth the origin of which must be traced
back to the processes by which the earth-body assumed
its present magnitude and figure.

(6) There has been an evolution of the surface features
of the earth. It does not appear to have been simply the
case that regions of elevation and depression originated
with the assumption of the present figures of the earth
and have persisted throughout geological time without any
changes other than that " embroidery " of outhne and
elevation which has always been, and still is in operation.
There has been a kind of " setthng down " of the lithosphere
as the earth-body lost heat and became more rigid. In
this process of setthng down the present distribution of
land and sea has been attained.

(c) Apparently there has been an increase in the quantity

336 AN INTRODUCTION TO OCEANOGRAPHY

of water on the earth. The charts of Fig. 64 seem to show
that the ocean was far less extensive in the Paleozoic and
Mesozoic periods than it is now. The variation may have
been apparent only : it may have been the case that
water was accumulated in the polar areas — either as
liquid water or as solid ice-caps. There may have been
changes in the rotational speed of the earth causing a
shifting of water from polar to equatorial regions or vice
versa ; the Paleozoic and Mesozoic oceans may have been
much deeper than they now are or there may have been
an actual increase in the quantity of water on the earth,
the origin of this being either from volcanic magmas
reaching the surface from the deeper layers, or from outer
space. Such consequences of the distributions that we
have assumed will be investigated with results that must
be in some ways the tests of the probability of those
distributions.

APPENDIX

PRINCIPAL AUTHORITIES CONSULTED

The following notes refer to some of the more important
books and papers consulted in the preparation of the
present work. The general books are : —

Chamberlin, T. C. " The origin of the earth " ; University of Chicago

Press, Chicago, 1916.
Chamberlin, T. C. and Salisbury, R. D. " Geology " ; Vols. I and

II, London, J. Murray, 1909.
Clarke, F. W. " Data of Geochemistry " ; Bulletin No. 695, U.S.

Geological Survey Govt. Printing Office, Washingtdh, 1920.
Darwin, G. " The tides and kindred phenomena in the Solar System " ;

3rd Ed., London, J. Murray, 1911.
Fowler, G. (Editor). " Science of the Sea " ; London, J. Murray,

1912.
Grew, E. S. " The growth of a planet " ; London, Methuen, 1911.
Hobbs, W. H. " Earth evolution and its facial expression " ; New

York, Macmillan Co., 1921.
Kri^mmell, O. " Handbuch der Oceanographie " ; Vols. I and II,

Stuttgart, Engelhorn, 1907 and 1911.
Murray, J. and Hjort, J. " The Depths of the Ocean " ; London,

Macmillan, 1912.
Murray, J. and Renard, A. J. " Deep Sea Deposits " ; Challenger

Report, 1891.
ScHOTT, G. " Geographic des Atlantischen Ozeans " ; Hamburg,

Boysen, 1912.

There are, of course, many other well known works on
oceanography and allied subjects and an exhaustive list
cannot be given here. The following notes refer to the
above works and to special papers.

Chapter II. Earth origin, &c. Chamberlin's books deal

with the planetesimal hypothesis. See also Grew, and

Chamberlin and Salisbury. For the nebular hypothesis

and its development and criticism see Darwin, Chamberlin

and Hobbs, Grew gives a short account of Love's
T 337

338 AN INTRODUCTION TO OCEANOGRAPHY

hypothesis of earth features by harmonic deformation.
J. W. Gregory, in " The making of the earth " (Williams
and Norgate, Home University Library), gives a good,
short account of the tetrahedral earth hypothesis. For an
account of the shield-lands (and for a good account of the
ocean as a geological agent) see L. V. Pirsson and
C. Schuchert, " Text book of geology," Wiley & Sons, New
York, 1920. Suess' " Face of the earth," of course, must
be consulted.

Chapter III. Oceanic depths. The chart of the Atlantic
is simplified from Schott. Those for the Pacific and
Indian Ocean are taken from Murray and Hjort. The
North polar chari is from Schott. The South polar chart
has been prepared from the latest Admiralty chart and the
German " Deutschland," observations are included (see
Brennecke, Archiv. Deutschen Seewarte, 39, 1921). Other
Admiralty charts have been used in this chapter in the
preparation of sections.

Chapter IV. Sea-bottom deposits. Schott's chart of the
Atlantic is simplified as Fig. 17. The charts of the Indian
and Pacific Oceans are from " Science of the Sea." Some
of the illustrations of deposits are from the Challenger
Report and others are original. A fine, general accoimt of
sea-bottom deposits, in general, is given by J. Murray in
the " Depths of the Ocean." The later views of the origin
of coral reefs are taken from a paper by Wayland Vaughan
in the Annual Report of the Smithsonian Institution for
1917 (1919), where there are other references. See also
Crossland, "Desert and water gardens of the Red Sea,"
Cambridge University Press,1913. (This book is otherwise
of much general interest.)

Chapter V. The ocean margins. The charts and sections
are based on the British Admiralty charts. A good,
general account of "the theory of isostasy is contained in
the Smithsonian Institution Reports (Willis, 1910 ;
Chamberhn, &c., 1916 (1917), Weichert, 1908 (1909). .
There is a symposium on the subject, published in Bull.

APPENDIX 339

American Geol. Soc, Vol. 33, 1922. These papers give
lists of references. For the variations of latitude, tidal
stresses, &c., in the earth-body see the appendices in G.
Darwin's book. Suess, of course. See also Hobbs. A
translation of Wegener's book on the " Displacement
of continents' hypothesis " is being published by
Methuen.

Chapters VI. and VII. Chemistry and Physics of sea
water. The literature is most abimdant and no summary
account can be given. The analyses of sea water are
collected by F. W. Clarke and there is a wealth of data in
Kriimmell's book. There is an enormous quantity of
descriptive detail, as regards variations in salinity and
temperature of the sea in all sorts of publications. (See
the " Bulletins " and " PubUcations de Circonstance " of
the International Council for the exploration of the sea ;
the English fishery blue books ; the Danish Meddelelser
Komm. f. Havimders)2(gelser ; the " Annalen die Hydro-
graphie " for instance).

The subject of ionic dissociation is well done by W. C.
McC. Lewis in his " System of Physical Chemistry,"
London, Longmans, 1918. For gases in sea water see Fox,
" Pubhcations de Circonstance," Nos. 21 (1905), 41 (1907),
44 (1909). Some later work is described in Archiv.
Deutschen Seewarte, XL, 1922 by B. Schulz. See also
Murray and Hjort. For CO „ -assimilation, H-ion investiga-
tions see B. Moore ; Report Lancashire Sea Fisheries
Laboratory for 1914 ; Liverpool, C. Tinling, 1915. See
also W. R. G. Atkins, a series of important papers (with
further references) in Journal Marine Biological Association,
Vol. 12, No. 4, Plymouth, 1922.

The salinity and temperature distributional charts of
the Atlantic are from Schott. Those for the Pacific and
Indian Ocean are from " Science of the sea." The graphs
in the chapter are mostly taken from data given by
Krbmmell. For the interesting superficial micro-measure-
ments of the temperature gradient, see A. Merz, Veroff.

340 AN INTRODUCTION TO OCEANOGRAPHY

Inst. Meereskunde Berlin, N.F., Geog-Naturwiss. Reihe
5,1920.

The classical work on nitrogen, phosphorus and silicon
compounds in sea water has been done by Raben. See
the Reports of the Kiel and Helgoland Biological
stations.

Chapter VIII. The tides. The only popular book is
G. Darwin's " Tides." See also Warburg, " Tides and
tidal streams," Cambridge University Press, 1922.

Methods of prediction of the tides, in the old way, are
explained (with difficulty) in the " Tide Tables " published
by the British Admiralty. These methods, and also the
newer ones, are dealt* with by Warburg.

Harris' " Manual of the Tides," (U.S. coast and Geodetic
Survey) made a new start after G. Darwin. Lately the
study, both of the theory of the tides and of the methods
of prediction, has been greatly developed by the Tidal
Institute at the University of Liverpool. (See " Reports
of the Tidal Institute," Liverpool University Press, 1920,
1921, 1922).

The Liverpool workers have shown that there is
considerable inaccuracy in the existing tidal predictions
and have developed marked improvements on the
Darwinian methods. See J. Proudman, " Harmonic
analysis of tidal observations in the British Empire " ;
British Association Report for 1920 (1921) ; A. T. Doodson,
" Intensive analysis of tidal observations " and " Harmonic
analysis of tidal observations " in Brit. Assoc. Reports for
1920 and 1921 (1921 and 1922). In this series of papers
the question of the accuracy of the predictions is
investigated and new analytical methods of power are
developed. The effect of meteorological changes on the
tides is also examined ; see Brit. Assoc. Rept. for 1923.
Bibliographies are given in these papers.

There is a description of the various tidal machines in
Publication No, 32 of the LT.S. Coast and Geodetic Survey ;
Govt. Printing Office, Washington, 1915.

APPENDIX 341

A short historical account of tidal investigations is given
by H. A. Marmer in " Science Monthly," March, 1922.

The figure of co-tidal lines on p. 224 is from Sterneck,
(Sitz. Akad. Wiss. Wien. ; Math.-Nat. kl. ; Abt. Ila,
Bd. 130, Vienna, 1921). Figs. 30 and 44 in Chapter VI are
modified from Warburg's book. Figs. 29, 41-43 are
plotted from the " Tide tables " and the " Nautical
Almanac."

The interesting subject of long-periodic tidal effects is
the work of Otto Pettersson. See Svenska hydro.-biol.
Komm. skrifter V. 1914.

Chapter IX. Ocean currents. There is a huge literature
on the subject. See Kr'iimmell for detailed accounts of
the currents in the various oceans and seas (there are,
however, no satisfactory illustrations in this work). The
various government hydrographic departments (Great
Britain, Germany and the United States, in particular)
publish periodic charts, pilot charts, atlases of currents,
sailing directions, &c. There is also very much in the
publications of the fishery departments of Great Britain,
Germany, Norway, &c. See also the publications of the
International Council for the exploration of the sea : the
Bulletins Hydrographiques, the Publications de Circon-
stance, and the Proces-Verbaux (particularly Vol. III. of
the latter series). There are also the reports of the voyages
of exploration, which are very numerous.

The figures showing the current-systems of the Atlantic,.
Pacific and Indian Oceans are reduced and simplified from
the Weltkarte prepared by G. Schott and published by the
Deutsche Seewarte in 1917. Fig. 12, the North Polar
circulation is from Schott's book on the Atlantic Ocean.
Fig. 54 representing the currents of the North Sea is from
G. Bohnecke, Saltgehalt u. Stromungen der Nordsee.
Veroff. Inst. f. Meereskunde, Berlin, N.F. geog.-naturwiss.
Reihe 10, 1922. The figure on p. 295 showing the vertical
ocean circulation is based on the work of the German
Valdivia-Expedition.

342 AN INTRODUCTION TO OCEANOGRAPHY

For the theory of solenoids (Fig. 46, p. 259) see Sandstrom
and Helland- Hansen, " Mathematical investigation of
ocean currents," Rept. Fishery and Hydrographic investiga-
tions in the North Sea and adjacent waters. Blue Book,
Cd. 2612, 1905.

Fig. 50 ; streaming in Straits of Gibraltar, is based on
the data given in "Depths in the Ocean " — ^so also Fig. 49,
the streaming in the Faeroe Channel. Fig. 56 showing
the salinity and temperature in the Gulf Stream Drift,
is from P. Cleve. Svenska Hyd.-biol. Komm. Skrifter,

Chapter X. History of the oceans. The appendix for
Chapter V, oceanic margins, contaias most of the authorities
used in writing Chapter X. The charts showing the
island arcs in the Atlantic and Pacific (Figs. 58 and 59) are
drawn from Admiralty charts and are based mainly on
Hobbs. The figure on p. 316, showing the structure of
the Atlantic shores is reduced and simpUfied from Schott.
The series of paleogeographic Mercator charts (Figs.
64, A-F) are from Haug.

The three paleogeographic maps of Europe in tertiary
times are from Osborn, " Age of Mammals " Macmillan
Co., New York, 1910. Here the reader will find the
hypotheses of geographical distribution of animals well
described.

INDEX.

Abyssal life, 89.

Nutrition of, 90.
Achil Head, 40.
Adaptive radiation, 323.
African yield tracts, 26.
Age of tide, 222.
Air in seawater, 146.
Andrews (Corals), 84.
Agulhas current, 254, 279.
Alaskan folds, 309.
Aleutian arc, 312, 330.
Alkalinity, 148.
American yield tracts, 25.
Amphidromic points, 205, 223,

224.
Andes, 124, 129.
Andes, foldings, 309.
Andes, soundings off, 43.
Animals, marine, 157, 324.
Animal colonies, 76.
Animal migrations, 324.
Antarctic continent, 11, 53, 58.

Yield tracts, 23.
Antarctic ocean, 1, 47.

Currents of, 283.

Sectors of, 2.

Nitrogen in, 159.
Antilles stream, 270.
Antillean yield tracts, 26.
Anticyclonic currents, 272, 276,

282.
Appalachian mountains, 219,
325.

Foldings, 124.
Aral sea, 110.
Arabian sea, currents, 282.
Arctic ocean, 48, 49, 110.

Morphology of, 335.

Currents of, 284.

Islands of, 48.
Arcs (earth folding), 311.

Insular, 319, 331.
Arcuate foldings, 309.
Ascension Island, 53.
Atmospheric circulation, 30.
Atolls, 81.
Atlantic ocean, 5.

Ancient, 333.

Arcs on coast, 315, 317.

Central rise, 52, 334.

Chart of, 51.

Coasts of, 124, 314.

Atlantic Ocean — cont.

Coastal folds, 317.

Continental shelf, 51.

Currents of, 273, 274.

Depths, 55.

Depth contours, 50.

Evolution of, 333.

Equatorial stream in, 270.

Equatorial stream variations,
288.

Morphology, 316, 332.

Nitrogen in, 159.

Oceanic islands of, 53.

Salinity of, 142, 289.

Sea bottom deposits, 93.

Section of, 42.

Temperature of, 175, 191.

Yield tracts of, 27
Atlantic stream, 273, 274.

Variations of, 293.
Australia and Antarctica, 325

and South America, 325.

Barrier reef, 80.

Currents at, 278, 283.
Avonmouth, tides at, 203.
Azores, 53, 56.

Bacteria, nitrogen, 146.

and corals, 87.
Baffin Bay, 49, 108, 271.

Currents in, 284.
Baltic Sea, 108, 110.

and Atlantic inflow, 292.

Currents of, 252, 287.

Freezing of, 252.

Temperature of, 177.
Bank, 45.

Banks straits, currents, 284.
Barentz sea, currents, 284, 292.
Barriers to migrations, 323.
Barrier reefs, 80.
Basin, 45.

Bathymetric contours, 46.
Behring sea, 108.
Behring straits, 48.

Currents in, 285.

Salinity in, 145.
Bengal Bay currents, 282.
Benguela stream, 275, 283.
Benthos, 88.
Bermuda, 79.
Black sea, 110.
343

344 AN INTRODUCTION TO OCEANOGRAPHY

Bottom samplers, 77.
Brazil stream, 275.
Brazil coasts, 124.

Calcareous algae, 76.

Canary Islands, 53.

Cape Breton, 124.

Cape Horn, 48.

Cape Verde islands, 53.

Carbonic acid in sea, 146, 148,

158.
Carbonic acid and heat

absorption, 172.
Carboniferous land and sea,

327.
Caribbean sea, temperature,

177.
Cascade mountains, 309.
Celebes Sea, 109.

Arcs off, 313.
Chagos islands, 320.
Challenger expedition, 39, 135.
Chamberlain, planetesimal

hypothesis, 15, 23, 26, 305.
Champlain, lake, 110.
Charts, 45.
China Sea, 108.

Currents of, 277.
Clay, Bed, 96.
Coasts, age of, 301.

Atlantic, 124, 126.

Biscayan, 125.

Brazil, 124.

Canadian, 125.

European, 125.

Greenland, 125.

Growing, 126.

Longitudinal, 124, 317.

Pacific, 124.

Ria, 315.

Scottish. 125.

Transverse, 124.
Collecting apparatus, 77.
Colloids in seawater, 72.
Compressibility of seawater,

166.
Continents.

Antarctic, 2, 47.

Antipodal to oceans, 5.

Characters of, 126.

Elevation of, 122.

Erosion of, 122.

Forms of, 6.

Movements of, 12.

Origin of, 10, 12.

Outlines of, 102.
Continental regions, 29.

Continental shelf, 40, 45, 54.

Atlantic, 54.

British, 40.

Deposition on, 123, 306.

Gradients of, 44, 107.

Irish, 40.

Loading of, 127, 307.

Movements of, 124.

Width of, 40.
Continental margins, 113.

Instability of, 114.
Continental displacement

theory, 127.
Continental slope, 41.

Gradient of, 41, 43.
Contour lines, 40, 45.

Making of, 46.
Convection currents, 171.
Copepods, 91.
Corals, 79.

and depth, 86.

Distribution of, 79.

Disintegration of, 81.

and hydrography, 86.

and lime metabolism, 87.

and sea temperature, 184.
Coral sediments, 87.
Coral mud, 81.
Coral precipices, 80.
Coral polyps, 85.
Coral reefs, 80, 332.

Origin of, 82.

Channels, 81.

Elevation theory, 85.

Ecology of, 85.

Foundations of, 80.

Fringing, 80.

Glacial control theory, 84..

Growth of, 81.

and isostasy, 83.

Lagoons, 86.

Platforms, 82, 84.

Subsidence theory, 84.

Talus, 80.
Corallina, 76.
Coral-like organisms, 79.
Cordilleras, 309.
Cosmic spherules, 64, 97.
Cotidal lines, 222, 224.
Cretaceous land and sea, 329.
Croll (cause of ice age), 173.
Cup dredge, 62.
Currents, 254.

Ascending, 294.

Causes of, 257.
Convection, 257, 259.
Descending, 294.

INDEX

345

Currents — cont.

Equatorial, 264.

Ice, 258.

Investigation of, 296.

Southern, 265.

Seasonal, 287.

Vertical, 259.
Currents and

Depth, 266.

Earth's rotation, 256.

Evaporation, 257.

Declination of sun, 256, 291.

Land barriers, 180, 189, 265.

Plankton, 299.

Precipitation, 257, 266.

Salinity, 267, 299.

Shoals, 180, 266.

Temperature of sea, 178, 299.

Winds, 263.
Current meters, 298.
Cyclonic storms, 172.
Cyclonic currents, 275.

Daly (coral reefs), 84, 332.
Darwin, C. (coral reefs), 82,

332.
Darwin, G. (origin of earth),
14.

Tides, 239, 249.
Davis straits, 271.

Currents of, 284.
Diatoms, 91.
Diatom ooze, 96.
Deeps, ocean, 45, 320, 330.
Deformation of earth, 32.
Demersal organisms, 89.
Denitrifying bacteria, 87.
Denmark strait, currents, 284.
Density (seawater), 193.
Depressions, 45.
Deposits, see " Sea bottom

deposits."
Depths, ocean, 39.

near coral islands, 42.

off Japan, 42.

off Peru, 42.
Devonian land and sea, 327.
Disruptive approach. 16.
Dover, tides of, 205.
Dredging apparatus, 62.
Drifts, ocean, 254.
Drift bottles, 296.

Earth,

Age of, 161.

Earth — cont.

Contraction of, 10, 14.

Density of, 115.

Depressions of, 30, 302.

Elevations of, 30, 302.

Growth of, 22.

Interior of, 114, 117.

Internal heat of, 9.

Tides in, 14.

Eigidity of, 115.

Rotational stresses in, 28.

Segmentation of, 24, 28.

Telescopic views of, 1.

Temperature of, 114.

Yielding of, 22, 333.
Earth, figure of, 12.

Changes of figure, 21.

Diameters of, 22.

Rotation of, 260, 263.

Rotational changes, 13, 21, 24.
Earth folds, 125, 305, 309, 311,

319, 330.
Earth, orgin of, 8, 10, 302.
Earth, superficial features, 8,

11, 335.
Earthquakes, 116, 308.
Earbones (in bottom deposits),

97.
Ebb tide, 200.
Ecology (and bottom deposits),

76.
English Channel (currents),

273, 287.
Eocene land and sea, 329.
Epicontinental seas, 108.

Temperature of, 177.
Epeiric seas, 108.
Eruptive suns, 16.
Establishment of a port (tides),

230.
Euler (change of latitude), 13.
Europe in tertiary period, 322.
European stream, 272.
Evolution, convergent, 324.
Equatorial streams, 264, 270,
275, 277, 279, 281, 288.

Counter streams, 282.

Seas, nitrogen in, 159.

Faeroe islands, 48.
Currents at, 287.
Falkland islands, 53, 319.
Falkland stream, 276, 283.
Ferrell's law, 260.
Fiji, corals at, 83.
Fiords, 125, 315.

346 AN INTRODUCTION TO OCEANOGRAPHY

Fishes (aud sea temperature),

324.
Floodtide, 200
Florida current, 271.
Foraminifera, 91, 113.
Formby shore, section, 105.
Foreshore, 103.

Charts of, 101.

Deposits on. 111.

Gradients of, 104.

Sections, 105.

Tides of, 100.
Fram (polar currents), 284.
Fringing reefs, 80.
Funafuti atoll, 42.

Galicia (rias), 315.
Gases in sea, 154.
Geosynclines, 43, 127, 309.
Geoisotherms, 9.
Gibraltar, 9, 203.

Tides at, 268.
Glaciation, 84.

and coral reefs, 84.
Glacial periods, 174.
Glauconite, 75, 122, 163.
Globerina ooze, 92.
Gondwanna, 129.
Gradients (sea bottom), 39.
Graham land, 48, 319.
Gravels, organic, 76.
Gravity, variations, 118.
Greenland, 48.
Greenland currents, 273, 276,

284.
Greenock, tides at, 203.
Guinea stream, 275.
Gulf stream, 176, 271.
Drift, 272.

Drift and temperature, 176.
Eddy, 291.
Velocity of, 254.
Gulf of Mexico, 109.
Salinity of, 139, 267.
Temperature of, 177.
Gulf of St. Lawrence, 108.

Yield-tracts of, 27.
Gulf weed, 272.
Gyrals, atmospheric, 30.

Harmonic analysis (tides), 245.

Synthesis (tides), 249.
Haug (paleogeography), 332.
Hecker (variations of latitude),

116.
Heligoland (tides), 203.
Heteropods, 90.

Himalayas, 129.
Holophytic organisms, 90.
Horsts, 126.

Hudson (sounding cup), 37.
Hudsou's Bay, 108, 178.
Humphreys (corals), 84.
Hydrogen-ions, 149, 153.
Hydrometer, 193.
Ice (sea), 184.
Barriers, 48.
Caps, 48.

Ages, causes, 174.
Forms of, 188.
Formation of, 187.
Iceland, 48.

Currents at, 273, 284.
Indian Ocean, 58.
Currents of, 279.
Corals in, 79.
Islands of, 60.
Morphology of, 319, 334.
Nitrogen in, 159.
Salinity of, 144.
Temperature of, 178.
Insular arcs, 311, 332.
Irish Sea, 101.

Atlantic inflow, 292.
Isohalines, 146.
Isotherms in ocean, 174.

and currents, 176.
Isostasy, 117.

and coral reefs, 83.
Isostatic layer, 119.
Isostatic equilibrium, 32, 120.
Isosteres (currents), 259.

Japanese deep, 43.

Arc, 312, 330.

Current, 277.

Seas, 108.

Stream, 254.
Javanese arc, 313.
Joly (age of Earth), 161.
Jurassic land and sea, 328.

Kant (nebular hypothesis), 8,

16.
Kennedy Channel, 284.
Knudsen (seawater analysis),

137, 194.
Kurile arc, 312.

Labrador stream, 179, 271, 276,

284.
Laccadive islands, 59, 320.
Lagoons, Coral, 81, 87.
Lands, ancient, 325.

INDEX

UT

Lands, ancient connections, 326.
Lands End, 40.
Land and sea breezes, 281.
Laminarian seaweeds, 89.
Laplace (nebular hypothesis),

8, 16.
Latitude, variations of, 13.
Leads, deep sea, 37.
Lime metabolism, 87, 162.
Lithothamnion, 76.
Littoral deposits, 68.
Liverpool, tides at, 203.
Lough Neagh, 110.
Love (earth features), 31.
Lubbock (tides), 229.
Lucas sounding machine^ 36.
Lucas bottom sampler, 37.
Lu-chu arc, 313.
Lunitidal intervals, 229.

Madagascar, 59, 320.
Magmas, volcanic, 308.
Maldive islands, 59, 320.
Manx foreshore, 105.
Mammals, migrations of, 323.
Manganese nodules, 97.
Marginal seas, deposits of. 111.

Currents in, 285.
Marg;ins, oceanic, 7.
Marine life, abyssal, 89.

Categories of, 88.

Density of, 89.

and light, 89.
Mars, surface of, 9.
Marsupials, migrations of, 325.
Mediterranean seas, 109.
Mediterranean, currents of, 268.

Nitrogen in, 159.

Salinity of, 139, 267.

Temperature of, 177.
Metamorphic rocks, 22.
Michelson ('rigidity of earth).

Migrations of animals, 324.

and barriers, 325.
MoUuscan shells, 75.
Monsoons and currents, 281.
Moon, surface of, 9.

Origin of, 9.
Morecambe Bay, 100.
Moulton (planetesimal theory).

Mountain folds, 308.
Muds, agglutination of, 72.

Blue, 73.

Distribution of, 73.

Green, 73.

Muds — cont.

Lines, 72.

Red, 73.

Volcanic, 73.
Murray, J. (coral reefs), 82,
332.

Sea bottom deposits, 66.

Nansen (polar currents), 284.
Nansen-Pettersseu water

bottle, 37.
Neap tides, 202.
Nebular hypothesis, 8.

Criticism of, 14.

Consequences of, 14.
Nebulae, spiral, 16, 19.
Nekton, 88.
Neritic animals, 324.

Migrations of, 325.
Neritic sea bottom deposits, 66,
74, 113.

Composition of, 75.

Distribution of, 76.

Foreshore, 112.

Shallow water, 112.
New Brunswick (rias), 124.
Newfoundland, currents, 271.

Rias, 315.
New Zealand, plateau, 58.

Yield tracts, 27.
Nitrogen compounds in sea,

158.
Destruction of, 163.
Nodal points (tides), 223.
North Sea, 108.

Currents, 273, 285, 292.

Depths, 40.
North American islands, 28.
Norwegian Sea, 48, 109.
Nullipore gravels, 76.
Ocean.

Antarctic, 1, 47.

Arctic, 3, 48.

Atlantic, 5.

Indian, 4, 47.

Pacific, 3.

Polar, 5.
Ocean.

Abysses, 107, 122.

Accumulating, 336.

Antipodal to continents, 5.

Bottom, 304.

Bottom deposition, 306.

Circulation, 34, 255

Depressions, 30, 32, 304.

Floor of, 44, 61.

Gradient of floor, 44.

348 AN INTRODUCTION TO OCEANOGRAPHY

Ocean — cont.

Gulfs, 3.

Heat received by, 172.

History of, 321, 326.

Permanence of, 7, 31.

Salts in, 135, 160.

Soundings (methods), 34, 40,
56.

Volume of, 131.
Oceanic islands, 78.
Oceanic regions, 29.
Oceans, pairs of, 29.
Ocean margins, 7, 100, 124, 304.

Instability of, 7, 309.
Ocean, origin of, 7, 10, 12, 131.
Okhotsk, Sea of, 108.

Currents in, 278.
Ontario, Lake, 110.
Ooze, deep sea, 66.

Diatom, 96.

Globerigina, 92.

Pteropod, 92.

liadiolarian, 94.
Ordnance datum, 201.
Orloff (variation of latitude),

116.
Organic skeletons in bottom

deposits, 74.
Oscillating tidal systems, 223.

Pacific Ocean, 56.

Ancient, 330.

Arcs in, 311, 313.

Chart of, 57.

Coasts of, 124, 311.

Coral islands, 79.

Currents of, 276.

Evolution of, 331.

Geosynclines, 43, 309, 330.

History of, 57, 332.

Islands of, 58.

Morphology of, 309.

Salinity of, 144.

Temperature, 178, 191.

Yield-tracts of, 27.
Pacific slope, 42.
Patagonian seas, currents, 275.
Patagonian coast, 53.
Paleontology and ocean history,

323.
Partial tides, 240.
Pelaeic deposits, 66, 88, 97.
Peridinians, 91.
Persian Gulf, 145.
Peruvian current, 279, 283.
Petterssen (tides), 251.
Philippine islands, 313, 330.

Philijisite, 97.
Photosynthesis, 151.

Rate of, 152.
Physics of sea, 165.
Planetary systems, origin of,

16.
Planetesimal hypothesis, 15, 18,
20, 23, 28.

and ocean, 132.
Plankton, 77, 88.

Categories of, 90.

and sea-bottom deposits, 98.

Density of, 91.

and currents, 284.
Plant life in the sea, 157.
Plateau, 45.

Poles, shifting of, 13, 116.
Polar oceans, 6.

Currents in, 283.

Temperature of, 191.
Polyzoan gravels, 76.
Potassium in sea, 163.
Prediction, tidal, 225.
Pressure in sea, 165.
Production in the sea, 152.
Primitive ocean, 31.
Pteropods, 90.
Pteropod ooze, 92.
Pumice, 64.
Pycnometer, 1 93.

Eadiolariau ooze, 94.

Eed clay, 96.

Eed Sea' (salinity), 139, 145.

Reefs, 45.

Reef-building corals, 79.

Relict seas, 109.

Renard (bottom deposits), 66.

Ridge, 45.

Rias, 45, 125, 315.

Rise (ocean floor), 45.

Rock, cleavage of, 22.

Contraction of, 23.

Flowage of, 22, 116, 307.

Strength of, 22, 119.
Rocky Mountains, 125, 310.
Ross Sea, 48.

Salinity of sea, 137.
Atlantic, 142.
Bay of Biscay, 139.
Charts of, 140.
(and) density, 194.
English Channel, 139.
Epicontinental seas, 143.
Gulf of Mexico, 139.
Indian Ocean, 144.

INDEX

34d

Salinity of sea — cont.

(and) latitude, 139.

Mediterranean, 139, 143.

Pacific, 144.

Red Sea, 139.

("and) rivers, 140, 144.

(and) trade winds, 144.

Variations of, 138, 289.
Salts of sea, 161.

Composition of, 134.

Dissociation of, 150.

in sea ice, 187.

Origin of, 133, 162.
Salt in rivers, 162.
Salt marshes, 102.
Sandhills, 106.
Sand, wind blown, 106.
Sandwich islands, 54; 319.
Sargasso Sea, 271.

Eddy, 276.
Sargassum, 272.
Scandinavian coast, 125.
Schistosity of rocks, 22.
Sea Areas.

Aral, 110.

Baltic, 108.

Barentz,

Behring, 108.

Black, 110.

Caribbean, 109.

Celebes, 109.

China, 108.

Japan, 108.

North, 108.

Norwegian,

Okhotsk, 108.

Yellow, 108.
Seas, ancient, 325.

Epicontinental, 108.

Epeiric, 108.

Landlocked, 111.

Marginal, 107.

Relict,
Sea level, 202.

Sea and land, interchange, 122.
Sea-bottom deposits.

Analysis of, 63, 70.

Calcareous, 64.

Categories of, 66.

Chemically altered, 64.

Classification of, 65.

Clays in, 71.

Collection of, 62.

Continental, 66.
Sea-bottom deposits, 62.

Distribution of, 67, 71, 93, 95,
98.

Sea-bottom deposits — cont.

Description of, 63.

Extra-terrestrial, 64.

Grading of, 68.

Levigation of, 71.

Littoral, 65.

Neritic, 74.

Oceanic, 77.

Organic, 64, 74.

Original nature, 63.

Pelagic, 65.

Radio-active, 167.

Rate of formation, 61.

Sampling of, 38.

Shallow water, 65.

Shore materials, 64.

Siliceous, 64.

Terrigenous, 65.

Terrestrial, 64.

Transport of, 63.

Solution of, 98.

Vertical range of, 98.
Sea-bottom life, 77.
Sea temperature.

Atlantic,

Currents, 178, 180.

Epicontinental seas, 182.

Bottom, 191.

Indian Ocean.

Latitude, 176.

Marine life, 182.

Micro-measurement, 189.

Pacific, 178.

Polar Seas, 182.

Southern ocean, 178.

Surface, 189.

Tides, 181.

Temperate seas, 182.

Tropics, 182.
Sea temperature variations,
170, 181.

Annual, 171, 181.

Seasonal, 171.

at bottom, 192.

Long periodic, 172.

Vertical, 188, 190.
Sea water.

Alkalinity, 148.

Analyses, 136.

Boiling point, 198.

Carbon dioxide, 158.

Chlorine and density, 194.

Colour, 196.

Compressibility, 166, 195.

Density, 186, 192.

Electric conductivity, 196.

Elements in, 134.

350 AN INTRODUCTION TO OCEANOGRAPHY

Sea water — cont.

Food substances in, 156.

Freezing, 185.

Gases, 146, 154.

Heat conductivity, 171.

lonisation, 134.

Optical characters, 196.

Osmotic pressure, 186.

Oxygen, 155.

Salinity, 137.

Titration, 137, 193.

Transparency, 196.

Upwelling, 180.

Vapour pressure, 196.

Viscosity, 196.
Sediments, agglutination of, 72.
Seiches, 200.
Semper (corals), 82, 332.
Shallow water deposits, 68.

Composition of, 70.
Shark's teeth, 97.
Shoal, 45.

Shore line changes, 102.
Shell gravels, 76.
Shield lands, 120, 301, 319, 321,

326.
Siberia, currents at, 284.
Silica in sea, 160.
Sierras, 125.
Seychelles, 59.
Solar radiation, 172.
Solenoids, 260.
Somali current, 282.
Soundings.

On charts, 41.

By electric oscillators, 39.

Lines, 34, 39.

Machines, 36.

Si'ikers. 36.

Tides, 46.

Sound waves, 39.

Technique of, 35.
South Georgia, 319.
Southern ocean currents, 275,
283.

Yield-tracts, 25.
South Orkneys, 54.
South Slietlacds, 54.
South Polar seas, 47.
Spitzbergen currents, 273, 284.
Stars, new, 19.
Stellar collisions, 15.
Spring tides, 202.
Strand, 103.

Submarine volcanoes, 64.
Symbiotic algea, 85.

Tanganyika Lake, 110.
Telegraphic plateau, 53.
Temperature of sea, 167.

Estimation of, 168.
Temperature of earth's interior,

10.
Terrigenous muds, 71.
Tethys, Sea of, 326, 333.
Tetrahedral earth theory, 10,

31.
Tides, 199.

Tides and astronomy, 217, 228.
Age of tides, 222.
Declination of sun arid

moon, 214.
Diurnal and semi-
diurnal, 204.
Equinoxes, 203, 216.
Moon's transits, 227.
Rotation of earth, 223.
Luni-solar, 232.
Lunitidal intervals, 22D,
233.
Tides.

Atmospheric circulation, 252.
Baltic, 252.
Datum marks, 201.
Ebb and flow, 200.
English coastal, 205.
Evolution, 9.
Foreshore, 103.
Gauges, 200.
Generating force, 210.

Variations of, 212, 217,
251.

and volcanic eruptions,
253.
Tides.

Land contours, 223.
Marks. 103.
Marine agencies, 250.
Prediction, 225.

Constituents, 242, 244.
Corrections, 239.
Empirical methods, 226,

234.
Establishment of port,

230.
Harmonic methods, 239.
Harmonic analysis, 245.
Harmonic synthesis, 249.
Inequalities, 234.
Non-harmonic, 226.
Machines, 250.
Partial tides, 240.
Meteorological effects,
236.

INDEX

351

Tides, Prediction — cont.

Kesultant tides, 249.

Satellites (tidal), 241.
Eange, 102, 202.
Rise and fall, 199.
Streams, 206.

Bores (tidal), 209.

(and) deposits, 69.

(and) heat transfer, 251.

North Sea, 208.

Overfalls, 209.

Eaces, 209.

Kesultant, 251, 285, 297.

Velocities, 207.
Tides.

Skagerak, 252.

Springs and neaps, 202, 213.
Submarine, 252, 292.
Tables of, 226, 239.
Theories of, 219.

Equilibrium, 220.

Forced wave, 220, 223.

Progressive wave, 220.

Stationary wave, 222.
Types of, 204.
(and) Topography, 240.
Periodicity of, 202.
(at) Victoria, B.C., 205.
Thermometers, sea-water, 168.
Deep sea, 37, 169.
Reversing, 168.
Thomson (age of earth), 161.
Thrusts, 307.

Trawl net, 77.

Trench, 45.

Triassic land and sea, 328.

Tristan d'Acunha, 53.

Valdivia expedition, 295.
Vaughan (corals), 332.
Vital production, 156.
Vertical currents, 293.
Volcanoes, submarine, 74.
Volcanic dusts, 64.

Eruptions, 172.

Muds, 73.

Waterbottles. 37, 168.

Water on ancient earth, 326.

Wave action and depo.sits, 69.

Weddell Sea, 48.

Wegener (continental displace-
ments), 128.

West Indies. 54, 318.

Whewell (tides). 2.39.

White Sea, 108, 284.

Winds and currents, 179.

World ocean, 1.

Wyville-Thompson ridge, 45,
50, 269.

Yellow Sea, 73, 108.

Currents in, 277.
Yield-tracts on earth, 24-29.

Zuider Zee, 252.

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