30
circulating water to be traced? Broadly speaking, we may classify
them as mechanical and chemical. The first involves the
separation of rock masses into smaller fragments of all sizes,
down to the finest dust. The second involves the actual solution
in the water of the rock constituents, which may be regarded as
the final act of disintegration. The rivers bear the burden both
of the comminuted and the dissolved materials to the sea. The mud
and sand carried by their currents, or gradually pushed along
their beds, represent the former; the invisible dissolved matter,
only to be demonstrated to the eye by evaporation of the water or
by chemical precipitation, represents the latter.
The results of these actions, integrated over geological time,
are enormous. The entire bulk of the sedimentary rocks, such as
sandstones, slates, shales, conglomerates, limestones, etc., and
the salt content of the ocean, are due to the combined activity
of mechanical and solvent denudation. We shall, later on, make an
estimate of the magnitude of the quantities actually involved.
In the Swiss valleys we see torrents of muddy water hurrying
along, and if we follow them up, we trace them to glaciers high
among the mountains. From beneath the foot of the glacier, we
find, the torrent has birth. The first debris given to the river
is derived from the wearing of the rocky bed along which the
glacier moves. The river of ice bequeaths to the river of
water--of which it is the parent--the spoils which it has won from
the rocks
31
The work of mechanical disintegration is, however, not restricted
to the glacier's bed. It proceeds everywhere over the surface of
the rocks. It is aided by the most diverse actions. For instance,
the freezing and expansion of water in the chinks and cracks in
those alpine heights where between sunrise and sunset the heat of
summer reigns, and between sunset and sunrise the cold of winter.
Again, under these conditions the mere change of surface
temperature from night to day severely stresses the surface
layers of the rocks, and, on the same principles as we explain
the fracture of an unequally heated glass vessel, the rocks
cleave off in slabs which slip down the steeps of the mountain
and collect as screes in the valley. At lower levels the
expansive force of vegetable growth is not unimportant, as all
will admit who have seen the strong roots of the pines
penetrating the crannies of the rocks. Nor does the river which
flows in the bed of the valley act as a carrier only. Listening
carefully we may detect beneath the roar of the alpine torrent
the crunching and knocking of descending boulders. And in the
potholes scooped by its whirling waters we recognise the abrasive
action of the suspended sand upon the river bed.
A view from an Alpine summit reveals a scene of remarkable
desolation (Pl. V, p. 40). Screes lie piled against the steep
slopes. Cliffs stand shattered and ready to fall in ruins. And
here the forces at work readily reveal themselves. An occasional
wreath of white smoke among
32
the far-off peaks, followed by a rumbling reverberation, marks
the fall of an avalanche. Water everywhere trickles through the
shaly _débris_ scattered around. In the full sunshine the rocks are
almost too hot to bear touching. A few hours later the cold is
deadly, and all becomes a frozen silence. In such scenes of
desolation and destruction, detrital sediments are actively being
generated. As we descend into the valley we hear the deep voice
of the torrents which are continually hurrying the disintegrated
rocks to the ocean.
A remarkable demonstration of the activity of mechanical
denudation is shown by the phenomenon of "earth pillars." The
photograph (Pl. IV.) of the earth pillars of the Val d'Hérens
(Switzerland) shows the peculiar appearance these objects
present. They arise under conditions where large stones or
boulders are scattered in a deep deposit of clay, and where much
of the denudation is due to water scour. The large boulders not
only act as shelter against rain, but they bind and consolidate
by their mere weight the clay upon which they rest. Hence the
materials underlying the boulders become more resistant, and as
the surrounding clays are gradually washed away and carried to
the streams, these compacted parts persist, and, finally, stand
like walls or pillars above the general level. After a time the
great boulders fall off and the underlying clay becomes worn by
the rainwash to fantastic spikes and ridges. In the Val d'Hérens
the earth pillars are formed
33
of the deep moraine stuff which thickly overlies the slopes of
the valley. The wall of pillars runs across the axis of the
valley, down the slope of the hill, and crosses the road, so that
it has to be tunnelled to permit the passage of traffic. It is
not improbable that some additional influence--possibly the
presence of lime--has hardened the material forming the pillars,
and tended to their preservation.
Denudation has, however, other methods of work than purely
mechanical; methods more noiseless and gentle, but not less
effective, as the victories of peace ate no less than those of
war.
Over the immense tracts of the continents chemical work proceeds
relentlessly. The rock in general, more especially the primary
igneous rock, is not stable in presence of the atmosphere and of
water. Some of the minerals, such as certain silicates and
carbonates, dissolve relatively fast, others with extreme
slowness. In the process of solution chemical actions are
involved; oxidation in presence of the free oxygen of the
atmosphere; attack by the feeble acid arising from the solution
of carbon dioxide in water; or, again, by the activity of certain
acids--humous acids--which originate in the decomposition of
vegetable remains. These chemical agents may in some instances,
_e.g._ in the case of carbonates such as limestone or
dolomite--bring practically the whole rock into solution. In other
instances--_e.g._ granites, basalts, etc.--they may remove some of
the
34
constituent minerals completely or partially, such as felspar,
olivine, augite, and leave more resistant substances to be
ultimately washed down as fine sand or mud into the river.
It is often difficult or impossible to appraise the relative
efficiency of mechanical and chemical denudation in removing the
materials from a certain area. There can be, indeed, little doubt
that in mountainous regions the mechanical effects are largely
predominant. The silts of glacial rivers are little different
from freshly-powdered rock. The water which carries them but
little different from the pure rain or snow which falls from the
sky. There has not been time for the chemical or solvent actions
to take place. Now while gravitational forces favour sudden shock
and violent motions in the hills, the effect of these on solvent
and chemical denudation is but small. Nor is good drainage
favourable to chemical actions, for water is the primary factor
in every case. Water takes up and removes soluble combinations of
molecules, and penetrates beneath residual insoluble substances.
It carries the oxygen and acids downwards through the soils, and
finally conveys the results of its own work to the rivers and
streams. The lower mean temperature of the mountains as well as
the perfect drainage diminishes chemical activities.
Hence we conclude that the heights are not generally favourable
to the purely solvent and chemical actions. It is on the
lower-lying land that soils tend to accumulate,
35
and in these the chief solvent and the chief chemical denudation
of the Earth are effected.
The solvent and chemical effects which go on in the
finely-divided materials of the soils may be observed in the
laboratory. They proceed faster than would be anticipated. The
observation is made by passing a measured quantity of water
backwards and forwards for some months through a tube containing
a few grammes of powdered rock. Finally the water is analysed,
and in this manner the amount of dissolved matter it has taken up
is estimated. The rock powder is examined under the microscope in
order to determine the size of the grains, and so to calculate
the total surface exposed to the action of the water. We must be
careful in such experiments to permit free oxidation by the
atmosphere. Results obtained in this way of course take no
account of the chemical effects of organic acids such as exist in
the soils. The quantities obtained in the laboratory will,
therefore, be deficient as compared with the natural results.
In this manner it has been found that fresh basalt exposed to
continually moving water will lose about 0.20 gramme per square
metre of surface per year. The mineral orthoclase, which enters
largely into the constitution of many granites, was found to lose
under the same conditions 0.025 gramme. A glassy lava (obsidian)
rich in silica and in the chemical constituents of an average
granite, was more resistant still; losing but 0.013 gramme per
square metre per year. Hornblende, a mineral
36
abundant in many rocks, lost 0.075 gramme. The mean of the
results showed that 0.08 gramme was washed in a year from each
square metre. Such results give us some indication of the rate at
which the work of solution goes on in the finely divided
soils.[1]
It might be urged that, as the mechanical break up of rocks, and
the production in this way of large surfaces, must be at the
basis of solvent and chemical denudation, these latter activities
should be predominant in the mountains. The answer to this is
that the soils rarely owe their existence to mechanical actions.
The alluvium of the valleys constitutes only narrow margins to
the rivers; the finer _débris_ from the mountains is rapidly
brought into the ocean. The soils which cover the greater part of
continental areas have had a very different origin.
In any quarry where a section of the soil and of the underlying
rock is visible, we may study the mode of formation of soils. Our
observations are, we will suppose, pursued in a granite quarry.
We first note that the material of the soil nearest the surface
is intermixed with the roots of grasses, trees, or shrubs.
Examining a handful of this soil, we see glistening flakes of
mica which plainly are derived from the original granite. Washing
off the finer particles, we find the largest remaining grains are
composed of the all but indestructible quartz.
[1] Proc. Roy. Irish Acad., VIII., Ser. A, p. 21.
37
This also is from the granite. Some few of the grains are of
chalky-looking felspar; again a granitic mineral. What is the
finer silt we have washed off? It, too, is composed of mineral
particles to a great extent; rock dust stained with iron oxide
and intermixed with organic remains, both animal and vegetable.
But if we make a chemical analysis of the finer silt we find that
the composition is by no means that of the granite beneath. The
chemist is able to say, from a study of his results, that there
has been, in the first place, a large loss of material attending
the conversion of the granite to the soil. He finds a
concentration of certain of the more resistant substances of the
granite arising from the loss of the less resistant. Thus the
percentage amount of alumina is increased. The percentage of iron
is also increased. But silica and most other substances show a
diminished percentage. Notably lime has nearly disappeared. Soda
is much reduced; so is magnesia. Potash is not so completely
abstracted. Finally, owing to hydration, there is much more
combined water in the soil than in the rock. This is a typical
result for rocks of this kind.
Deeper in the soil we often observe a change of texture. It has
become finer, and at the same time the clay is paler in colour.
This subsoil represents the finer particles carried by rain from
above. The change of colour is due to the state of the iron which
is less oxidised low down in the soil. Beneath the subsoil the
soil grows
38
again coarser. Finally, we recognise in it fragments of granite
which ever grow larger as we descend, till the soil has become
replaced by the loose and shattered rock. Beneath this the only
sign of weathering apparent in the rock is the rusty hue imparted
by the oxidised iron which the percolating rain has leached from
iron-bearing minerals.
The soil we have examined has plainly been derived in situ from
the underlying rock. It represents the more insoluble residue
after water and acids have done their work. Each year there must
be a very slow sinking of the surface, but the ablation is
infinitesimal.
The depth of such a soil may be considerable. The total surface
exposed by the countless grains of which it is composed is
enormous. In a cubic foot of average soil the surface area of the
grains may be 50,000 square feet or more. Hence a soil only two
feet deep may expose 100,000 square feet for each square foot of
surface area.
It is true that soils formed in this manner by atmospheric and
organic actions take a very long time to grow. It must be
remembered, however, that the process is throughout attended by
the removal in solution: of chemically altered materials.
Considerations such as the foregoing must convince us that while
the accumulation of the detrital sediments around the continents
is largely the result of activities progressing on the steeper
slopes of the land, that is,
39
among the mountainous regions, the feeding of the salts to the
ocean arises from the slower work of meteorological and organic
agencies attacking the molecular constitution of the rocks;
processes which best proceed where the drainage is sluggish and
the quiescent conditions permit of the development of abundant
organic growth and decay.
Statistics of the solvent denudation of the continents support
this view. Within recent years a very large amount of work has
been expended on the chemical investigation of river waters of
America and of Europe. F. W. Clarke has, at the expense of much
labour, collected and compared these results. They are expressed
as so many tonnes removed in solution per square mile per annum.
For North America the result shows 79 tonnes so removed; for
Europe 100 tonnes. Now there is a notable difference between the
mean elevations of these two continents. North America has a mean
elevation of 700 metres over sea level, whereas the mean
elevation of Europe is but 300 metres. We see in these figures
that the more mountainous land supplies less dissolved matter to
the ocean than the land of lower elevation, as our study has led
us to expect.
We have now considered the source of the detrital sediments, as
well as of the dissolved matter which has given to the ocean, in
the course of geological time, its present gigantic load of
salts. It is true there are further solvent and chemical effects
exerted by the sea water
40
upon the sediments discharged into it; but we are justified in
concluding that, relatively to the similar actions taking place
in the soils, the solvent and chemical work of the ocean is
small. The fact is, the deposited detrital sediments around the
continents occupy an area small when contrasted with the vast
stretches of the land. The area of deposition is much less than
that of denudation; probably hardly as much as one twentieth.
And, again, the conditions of aeration and circulation which
largely promote chemical and solvent denudation in the soils are
relatively limited and ineffective in the detrital oceanic
deposits.
The summation of the amounts of dissolved and detrital materials
which denudation has brought into the ocean during the long
denudative history of the Earth, as we might anticipate, reveals
quantities of almost unrealisable greatness. The facts are among
the most impressive which geological science has brought to
light. Elsewhere in this volume they have been mentioned when
discussing the age of the Earth. In the present connection,
however, they are deserving of separate consideration.
The basis of our reasoning is that the ocean owes its saltness
mainly if not entirely to the denudative activities we have been
considering. We must establish this.
We may, in the first place, say that any other view at once
raises the greatest difficulties. The chemical composition of the
detrital sediments which are spread over
41
the continents and which build up the mountains, differs on the
average very considerably from that of the igneous rocks. We know
the former have been derived from the latter, and we know that
the difference in the composition of the two classes of materials
is due to the removal in solution of certain of the constituents
of the igneous rocks. But the ocean alone can have received this
dissolved matter. We know of no other place in which to look for
it. It is true that some part of this dissolved matter has been
again rejected by the ocean; thus the formation of limestone is
largely due to the abstraction of lime from sea water by organic
and other agencies. This, however, in no way relieves us of the
necessity of tracing to the ocean the substances dissolved from
the igneous rocks. It follows that we have here a very causa for
the saltness of the ocean. The view that the ocean "was salt from
the first" is without one known fact to support it, and leaves us
with the burden of the entire dissolved salts of geological time
to dispose of--Where and how?
The argument we have outlined above becomes convincingly strong
when examined more closely. For this purpose we first compare the
average chemical composition of the sedimentary and the igneous
rocks. The following table gives the percentages of the chief
chemical constituents: [1]
[1] F. W. Clarke: _A Preliminary Study of Chemical Denudation_,
p. 13
42
Igneous. Sedimentary.
Silica (SiO2) - 59.99 58.51
Alumina (Al2O3) - 15.04 13.07
Ferric oxide (F2O3) - 2.59 3.40
Ferrous oxide (FeO) - 3.34 2.00
Magnesia (MgO) - 3.89 2.52
Lime (CaO) - 4.81 5.42
Soda (Na2O) - 3.41 1.12
Potash (K2O) - 2.95 2.80
Water (H2O) - 1.92 4.28
Carbon dioxide (CO2) - -- 4.93
Minor constituents - 2.06 1.95
100.00 100.00
In the derivation of the sediments from the igneous rocks there
is a loss by solution of about 33 per cent; _i.e._ 100 tons of
igneous rock yields rather less than 70 tons of sedimentary rock.
This involves a concentration in the sediments of the more
insoluble constituents. To this rule the lime-content appears to
be an exception. It is not so in reality. Its high value in the
sediments is due to its restoration from the ocean to the land.
The magnesia and potash are, also, largely restored from the
ocean; the former in dolomites and magnesian limestones; the
latter in glauconite sands. The iron of the sediments shows
increased oxidation. The most notable difference in the two
analyses appears, however, in the soda percentages. This falls
from 3.41 in the igneous rock to 1.12 in the average sediment.
Indeed, this
43
deficiency of soda in sedimentary rocks is so characteristic of
secondary rocks that it may with some safety be applied to
discriminate between the two classes of substances in cases where
petrological distinctions of other kinds break down.
To what is this so marked deficiency of soda to be ascribed? It
is a result of the extreme solubility of the salts of sodium in
water. This has not only rendered its deposition by evaporation a
relatively rare and unimportant incident of geological history,
but also has protected it from abstraction from the ocean by
organic agencies. The element sodium has, in fact, accumulated in
the ocean during the whole of geological time.
We can use the facts associated with the accumulation of sodium
salts in the ocean as a means of obtaining additional support to
the view, that the processes of solvent denudation are
responsible for the saltness of the ocean. The new evidence may
be stated as follows: Estimates of the amounts of sedimentary
rock on the continents have repeatedly been made. It is true that
these estimates are no more than approximations. But they
undoubtedly _are_ approximations, and as such may legitimately be
used in our argument; more especially as final agreement tends to
check and to support the several estimates which enter into
them.
The most recent and probable estimates of the sediments on the
land assign an average thickness of one mile of
44
secondary rocks over the land area of the world. To this some
increase must be made to allow for similar materials concealed in
the ocean, principally around the continental margins. If we add
10 per cent. and assign a specific gravity of 2.5 we get as the
mass of the sediments 64 x 1016 tonnes. But as this is about 67
per cent. of the parent igneous rock--_i.e._ the average igneous
rock from which the sediments are derived--we conclude that the
primary denuded rock amounted to a mass of about 95 x 1016
tonnes.
Now from the mean chemical composition of the secondary rocks we
calculate that the mass of sediments as above determined contains
0.72 x1016 tonnes of the sodium oxide, Na2O. If to this amount we
add the quantity of sodium oxide which must have been given to
the ocean in order to account for the sodium salts contained
therein, we arrive at a total quantity of oxide of sodium which
must be that possessed by the primary rock before denudation
began its work upon it. The mass of the ocean being well
ascertained, we easily calculate that the sodium in the ocean
converted to sodium oxide amounts to 2.1 x 1016 tonnes. Hence
between the estimated sediments and the waters of the ocean we
can account for 2.82 x 1016 tonnes of soda. When now we put this
quantity back into the estimated mass of primary rock we find
that it assigns to the primary rock a soda percentage of 3.0. On
the average analysis given above this should be 3.41 per cent.
The agreement,
45
all things considered, more especially the uncertainty in the
estimate of the sediments, is plainly in support of the view that
oceanic salts are derived from the rocks; if, indeed, it does not
render it a certainty.
A leading and fundamental inference in the denudative history of
the Earth thus finds support: indeed, we may say, verification.
In the light of this fact the whole work of denudation stands
revealed. That the ocean began its history as a vast fresh-water
envelope of the Globe is a view which accords with the evidence
for the primitive high temperature of the Earth. Geological
history opened with the condensation of an atmosphere of immense
extent, which, after long fluctuations between the states of
steam and water, finally settled upon the surface, almost free of
matter in solution: an ocean of distilled water. The epoch of
denudation then began. It will, probably, continue till the
waters, undergoing further loss of thermal energy, suffer yet
another change of state, when their circulation will cease and
their attack upon the rocks come to an end.
From what has been reviewed above it is evident that the sodium
in the ocean is an index of the total activity of denudation
integrated over geological time. From this the broad facts of the
results of denudation admit of determination with considerable
accuracy. We can estimate the amount of rock which has been
degraded by solvent and chemical actions, and the amount of
sediments which has been derived from it. We are,
46
thus, able to amend our estimate of the sediments which, as
determined by direct observation, served to support the basis of
our argument.
We now go straight to the ocean for the amount of sodium of
denudative origin. There may, indeed, have been some primitive
sodium dissolved by a more rapid denudation while the Earth's
surface was still falling in temperature. It can be shown,
however, that this amount was relatively small. Neglecting it we
may say with safety that the quantity of sodium carried into the
ocean by the rivers must be between 14,000 and 15,000 million
million tonnes: _i.e._ 14,500 x 1012 tonnes, say.
Keeping the figures to round numbers we find that this amount of
sodium involves the denudation of about 80 x 1016 tonnes of
average igneous rock to 53 x 1016 tonnes of average sediment.
From these vast quantities we know that the parent rock denuded
during geological time amounted to some 300 million cubic
kilometres or about seventy million cubic miles. The sediments
derived therefrom possessed a bulk of 220 million cubic
kilometres or fifty million cubic miles. The area of the land
surface of the Globe is 144 million square kilometres. The parent
rock would have covered this to a uniform depth of rather more
than two kilometres, and the derived sediment to more than 1.5
kilometres, or about one mile deep.
The slow accomplishment of results so vast conveys some idea of
the great duration of geological time.
47
The foregoing method of investigating the statistics of solvent
denudation is capable of affording information not only as to the
amount of sediments upon the land, but also as to the quantity
which is spread over the floor of the ocean.
We see this when we follow the fate of the 33 per cent. of
dissolved salts which has been leached from the parent igneous
rock, and the mass of which we calculate from the ascertained
mass of the latter, to be 27 x 1016 tonnes. This quantity was at
one time or another all in the ocean. But, as we saw above, a
certain part of it has been again abstracted from solution,
chiefly by organic agencies. Now the abstracted solids have not
been altogether retained beneath the ocean. Movements of the land
during geological time have resulted in some portion being
uplifted along with other sediments. These substances constitute,
mainly, the limestones.
We see, then, that the 27 x 1016 tonnes of substances leached
from the parent igneous rocks have had a threefold destination.
One part is still in solution; a second part has been
precipitated to the bottom of the ocean; a third part exists on
the land in the form of calcareous rocks.
Observation on the land sediments shows that the calcareous rocks
amount to about 5 per cent. of the whole. From this we find that
3 x 1016 tonnes, approximately, of such rocks have been taken
from the ocean. This accounts for one of the three classes of
material
48
into which the original dissolved matter has been divided.
Another of the three quantities is easily estimated: the amount
of matter still in solution in the ocean. The volume of the ocean
is 1,414 million cubic kilometres and its mass is 145 x 1016
tonnes. The dissolved salts in it constitute 3.4 per cent. of its
mass; or, rather more than 5 x 1016 tonnes. The limestones on the
land and the salts in the sea water together make up about 8 x
1016 tonnes. If we, now, deduct this from the total of 27 x 1016
tonnes, we find that about 19 x 1016 tonnes must exist as
precipitated matter on the floor of the ocean.
The area of the ocean is 367 x 1012 square metres, so that if the
precipitated sediment possesses an average specific gravity of
2.5, it would cover the entire floor to a uniform depth of 218
metres; that is 715 feet. This assumes that there was uniform
deposition of the abstracted matter over the floor of the ocean.
Of course, this assumption is not justifiable. It is certain that
the rate of deposition on the floor of the sea has varied
enormously with various conditions--principally with the depth.
Again, it must be remembered that this estimate takes no account
of solid materials otherwise brought into the oceanic deposits;
_e.g._, by wind-transported dust from the land or volcanic
ejectamenta in the ocean depths. It is not probable, however,
that any considerable addition to the estimated mean depth of
deposit from such sources would be allowable.
49
The greatness of the quantities involved in these determinations
is almost awe inspiring. Take the case of the dissolved salts in
the ocean. They are but a fraction, as we have seen, of the total
results of solvent denudation and represent the integration of
the minute traces contributed by the river water. Yet the common
salt (chloride of sodium) alone, contained in the ocean, would,
if abstracted and spread over the dry land as a layer of rock
salt having a specific gravity of 2.2, cover the whole to a depth
of 107 metres or 354 feet. The total salts in solution in the
ocean similarly spread over the land would increase the depth of
the layer to 460 feet. After considering what this means we have
to remember that this amount of matter now in solution in the
seas is, in point of fact, less than a fifth part of the total
dissolved from the rocks during geological time.
The transport by denudation of detrital and dissolved matter from
the land to the ocean has had a most important influence on the
events of geological history. The existing surface features of
the earth must have been largely conditioned by the dynamical
effects arising therefrom. In dealing with the subject of
mountain genesis we will, elsewhere, see that all the great
mountain ranges have originated in the accumulation of the
detrital sediments near the shore in areas which, in consequence
of the load, gradually became depressed and developed into
synclines of many thousands of feet in depth. The most impressive
surface features of the Globe originated
50
in this manner. We will see too that these events were of a
rhythmic character; the upraising of the mountains involving
intensified mechanical denudation over the elevated area and in
this way an accelerated transport of detritus to the sea; the
formation of fresh deposits; renewed synclinal sinking of the sea
floor, and, finally, the upheaval of a younger mountain range.
This extraordinary sequence of events has been determined by the
events of detrital denudation acting along with certain general
conditions which have all along involved the growth of
compressive stresses in the surface crust of the Earth.
The effects of purely solvent denudation are less easily traced,
but, very probably, they have been of not less importance. I
refer here to the transport from the land to the sea of matter in
solution.
Solvent denudation, as observed above, takes place mainly in the
soils and in this way over the more level continental areas. It
has resulted in the removal from the land and transfer to the
ocean of an amount of matter which represents a uniform layer of
one half a kilometre; that is of more than 1,600 feet of rock.
The continents have, during geological time, been lightened to
this extent. On the other hand all this matter has for the
greater part escaped the geosynclines and become uniformly
diffused throughout the ocean or precipitated over its floor
principally on the continental slopes before the great depths are
reached. Of this material the ocean
51
waters contain in solution an amount sufficient to increase their
specific gravity by 2.7 per cent.
Taking the last point first, it is interesting to note the
effects upon the bulk of the ocean which has resulted from the
matter dissolved in it. From the known density of average sea
water we find that 100 ccs. of it weigh just 102.7 grammes. Of
this 3.5 per cent. by weight are solids in solution. That is to
say, 3.594 grammes. Hence the weight of water present is 99.1
grammes, or a volume of 99.1 ccs. From this we see that the salts
present have increased the volume by 0.9 ccs. or 0.9 per cent.
The average depth of the ocean is 2,000 fathoms or 3,700 metres.
The increase of depth due to salts dissolved in the ocean has
been, therefore, 108 feet or 33.24 metres. This result assumes
that there has been no increased elastic compression due to the
increased pressure, and no change of compressional elastic
properties. We may be sure that the rise on the shore line of the
land has not been less than 100 feet.
We see then that as the result of solvent denudation we have to
do with a heavier and a deeper ocean, expanded in volume by
nearly one per cent. and the floor of which has become raised, on
an average, about 700 feet by precipitated sediment.
One of the first conceptions, which the student of geology has to
dismiss from his mind, is that of the immobility or rigidity of
the Earth's crust. The lane, we live on sways even to the gentle
rise and fail of ocean tides
52
around the coasts. It suffers its own tidal oscillations due to
the moon's attractions. Large tracts of semi-liquid matter
underlie it. There is every evidence that the raised features of
the Globe are sustained by such pressures acting over other and
adjacent areas as serve to keep them in equilibrium against the
force of gravity. This state of equilibrium, which was first
recognised by Pratt, as part of the dynamics of the Earth's
crust, has been named isostasy. The state of the crust is that of
"mobile equilibrium."
The transfer of matter from the exposed land surfaces to the
sub-oceanic slopes of the continents and the increase in the
density of the ocean, must all along have been attended by
isostatic readjustment. We cannot take any other view. On the one
hand the land was being lightened; on the other the sea was
increasing in mass and depth and the flanks of the continents
were being loaded with the matter removed from the land and borne
in solution to the ocean. How important the resulting movements
must have been may be gathered from the fact that the existing
land of the Globe stands at a mean elevation of no more than
2,000 feet above sea level. We have seen that solvent denudation
removed over 1,600 feet of rock. But we have no evidence that on
the whole the elevation of land in the past was ever very
different from what it now is.
We have, then, presented to our view the remarkable fact that
throughout the past, and acting with extreme
53
slowness, the land has steadily been melted down into the sea and
as steadily been upraised from the waters. It is possible that
the increased bulk of the ocean has led to a certain diminution
of the exposed land area. The point is a difficult one. One thing
we may without much risk assume. The sub-aereal current of
dissolved matter from the land to the ocean was accompanied by a
sub-crustal flux from the ocean areas to the land areas; the
heated viscous materials creeping from depths far beneath the
ocean floor to depths beneath the roots of the mountains which
arose around the oceans. Such movements took ages for their
accomplishment. Indeed, they have been, probably, continuous all
along and are still proceeding. A low degree of viscosity will
suffice to permit of movements so slow. Superimposed upon these
movements the rhythmic alternations of depression and elevation
of the geosynclines probably resulted in releasing the crust from
local accumulation of strains arising in the more rigid surface
materials. The whole sequence of movements presents an
extraordinary picture of pseudo-vitality--reminding us of the
circulatory and respiratory systems of a vast organism.
All great results in our universe are founded in motions and
forces the most minute. In contemplating the Cause or the Effect
we stand equally impressed with the spectacle presented to us. We
shall now turn from the great effects of denudation upon the
history and evolution of a world and consider for a moment
activities
54
so minute in detail that their operations will probably for ever
elude our bodily senses, but which nevertheless have necessarily
affected and modified the great results we have been
considering.
The ocean a little way from the land is generally so free from
suspended sediments that it has a blackness as of ink. This
blackness is due to its absolute freedom from particles
reflecting the sun's light. The beautiful blue of the Swiss and
Italian lakes is due to the presence of very fine particles
carried into them by the rivers; the finest flour of the
glaciers, which remain almost indefinitely suspended in the
water. But in the ocean it is only in those places where rapid
currents running over shallows stir continually the sediments or
where the fresh water of a great river is carried far from the
land, that the presence of silt is to be observed. The beautiful
phenomenon of the coal-black sea is familiar to every yachtsman
who has sailed to the west of our Islands.[1]
There is, in fact, a very remarkable difference in the manner of
settlement of fine sediments in salt and in fresh water. We are
here brought into contact with one of those subtle yet
influential natural actions the explanation of which involves
scientific advance along many apparently unconnected lines of
investigation.
[1] See Tyndall's Voyage to Algeria in _Fragments of Science._ The
cause of the blue colour of the lakes has been discussed by
various observers, not always with agreement.
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It is easy to observe in the laboratory the fact of the different
behaviour of salt and fresh water towards finely divided
substances. The nature of the insoluble substance is not
important.
We place, in a good light, two glass vessels of equal dimensions;
the one filled with sea water, the other with fresh water. Into
each we stir the same weight of very finely powdered slate: just
so much as will produce a cloudiness. In a few hours we find the
sea water limpid. The fresh water is still cloudy, however; and,
indeed, may be hardly different in appearance from what it was at
starting. In itself this is a most extraordinary experiment. We
would have anticipated quite the opposite result owing to the
greater density of the sea water.
But a still more interesting experiment remains to be carried
out. In the sea water we have many different salts in solution.
Let us see if these salts are equally responsible for the result
we have obtained. For this purpose we measure out quantities of
sodium chloride and magnesium chloride in the proportion in which
they exist in sea water: that is about as seven to one. We add
such an equal amount of water to each as represents the dilution
of these salts in sea water. Then finally we stir a little of the
finely powdered slate into each. It will be found that the
magnesium chloride, although so much more dilute than the sodium
chloride, is considerably more active in clearing out the
suspension. We may now try such marine salts as magnesium
sulphate,
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or calcium sulphate against sodium chloride; keeping the marine
proportions. Again we find that the magnesium and calcium salts
are the most effective, although so much more dilute than the
sodium salt.
There is no visible clue to the explanation of these results. But
we must conclude as most probable that some action is at work in
the sea water and in the salt solutions which clumps or
flocculates the sediment. For only by the gathering of the
particles together in little aggregates can we explain their
rapid fall to the bottom. It is not a question of viscosity
(_i.e._ of resistance to the motion of the particles), for the
salt solutions are rather more viscous than the fresh water.
Still more remarkable is the fact that every dissolved substance
will not bring about the result. Thus if we dissolve sugar in
water we find that, if anything, the silt settles more slowly in
the sugar solution than in fresh water.
Now there is one effect produced by the solution of such salts as
we have dealt with which is not produced by such bodies as sugar.
The water is rendered a conductor of electricity. Long ago
Faraday explained this as due to the presence of free atoms of
the dissolved salt in the solution, carrying electric charges. We
now speak of the salt as "ionised." That is it is partly split up
into ions or free electrified atoms of chlorine, sodium,
magnesium, etc., according to the particular salt in solution.
This fact leads us to think that these electrified
57
atoms moving about in the solution may be the cause of the
clumping or flocculation. Such electrified atoms are absent from
the sugar solution: sugar does not become "ionised" when it is
dissolved.
The suspicion that the free electrified atoms play a part in the
phenomenon is strengthened when we recall the remarkable
difference in the action of sodium chloride and magnesium
chloride. In each of the solutions of these substances there are
free chlorine atoms each of which carries a single charge of
negative electricity. As these atoms are alike in both solutions
the different behaviour of the solutions cannot be due to the
chlorine. But the metallic atom is very different in the two
cases. The ionised sodium atom is known to be _monad_ or carries
but _one_ positive charge; whereas the magnesium atom is _diad_ and
carries _two_ positive charges. If, then, we assume that the
metallic, positively electrified atom is in each case
responsible, we have something to go on. It may be now stated
that it has been found by experiment and supported by theory that
the clumping power of an ion rises very rapidly with its valency;
that is with the number of unit charges associated with it. Thus
diads such as magnesium, calcium, barium, etc., are very much
more efficient than monads such as sodium, potassium, etc., and
again, triads such as aluminium are, similarly, very much more
powerful than diad atoms. Here, in short, we have arrived at the
active cause of the phenomenon. Its inner mechanism
58
is, however, harder to fathom. A plausible explanation can be
offered, but a study of it would take us too far. Sufficient has
been said to show the very subtile nature of the forces at work.
We have here an effect due to the sea salts derived by denudation
from the land which has been slowly augmenting during geological
time. It is certain that the ocean was practically fresh water in
remote ages. During those times the silt from the great rivers
would have been carried very far from the land. A Mississippi of
those ages would have sent its finer suspensions far abroad on a
contemporary Gulf stream: not improbably right across the
Atlantic. The earlier sediments of argillaceous type were not
collected in the geosynclines and the genesis of the mountains
was delayed proportionately. But it was, probably, not for very
long that such conditions prevailed. For the accumulation of
calcium salts must have been rapid, and although the great
salinity due to sodium salts was of slow growth the salts of the
diad element calcium must have soon introduced the cooperation of
the ion in the work of building the mountain.
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