<h2 class="nobreak" id="CHAPTER_IV">CHAPTER IV</h2>
<h3>RECORDS LEFT BY RIVERS</h3></div>
<p>When we come to examine more closely the work
which rivers do in removing mineral substances
from the land by washing particles of them
from the surface, we find that the records they leave in
geological history must be plainly marked. Every stream,
large or small, is always busy carrying mud, sand, or
gravel. Rivers are the "navvies" of geology. When they
are swollen by rain they sweep large stones away with
them. If we look at the bed of a mountain torrent we shall
often see huge blocks of stone that have fallen from the
cliffs on either side blocking the pathway of the stream.
To all appearance the stream is quite powerless to remove
these blocks, and has to circumnavigate them. But visit
such a torrent when the snows are melting, or heavy rain
has fallen, and you will hear the stones knocking against
each other or on the rocky bottom as they are driven
downwards by the flood. It is not easy to estimate the
driving power of water. M. Gustave le Bon has furnished
an illustration of its power which is very curious. In the
south of France a stream is led downwards from the
mountains to drive the turbine of some machinery at
<span class="pagenum" id="Page_51">-51-</span>
a manufactory. It comes down several thousand feet. In
the manufactory there is a vent-hole, out of which the
water can be allowed to shoot. The vent-hole is about
an inch in diameter; and the water rushes out with such
swiftness and force that the water-jet becomes as rigid as
steel. It is impossible to cut through this water-jet; and
if any one were to try to do so with a sword, the sword
might break but it could never pierce or pass through
those swiftly moving particles of water. A more commonplace
illustration is the use that is sometimes made of
water-jets to break up the surfaces of rock in quarries;
nor must it be forgotten that horse-power of great value
and extent for electric lighting and other purposes is
always being drawn from waterfalls. Thus as a mechanical
force merely the river can be immensely powerful; and
must leave marks of its power on the rocks.</p>
<p>The aspect of its force with which we are, however,
most concerned is that which is directed to lowering
gradually the surface of the land. In the last chapter we
showed how much mineral might be dissolved in the
waters of rivers. If we are to include also the amount of
mud, sand, and other things classed altogether as silt
which a river carries down, the figures become much more
imposing. Sir Archibald Geikie says that, taking the
Mississippi as a typical river (it is as good an example as
would be found, because in its great length it passes
through many different kinds of land, soil, and climate),
we may assume that the average amount of sediment
carried down by a river is one part of sediment to every
1500 parts of water.</p>
<p><span class="pagenum" id="Page_52">-52-</span></p>
<p>If now, says he, we assume that all over the world this
is the amount carried down, we can see how seriously the
level of the land is lowered by rivers. The Mississippi
carries from the land it drains every year the <sup>1</sup>/<sub>6000</sub>th part
of a foot of rock. If we take the general height of the
land of the whole globe to be 2100 feet, and suppose it to be
continuously wasted at this rate, then the whole dry land
would be carried into the sea in 12,600,000 years. Or if
we assume the average height of the continent of Europe
to be 940 feet, and to be lowered by its rivers at the
same rate, then the last vestige of Europe would have disappeared
in 8,640,000 years. Such figures are of course
not exact; and it must always be remembered that
the rivers are merely robbing Peter to pay Paul, and
whatever they take away are always putting somewhere
else, but we may learn from the foregoing considerations
that the lowering of the land is much more rapid than is
sometimes supposed. Another thing about the excavating
work of rivers has to be remembered. The torrents
carry sand, shingle, and rock with them, and these very
materials act as agents of destruction on the beds of the
water-courses. If we want to polish brass or steel we mix
emery powder (or something finer or coarser) with the
polishing liquid. The torrent or river uses sand or
shingle as its polishing powder. It then wears out the
rock over which it travels, and sometimes carves it
into holes or caverns, gorges or ravines. Sometimes the
process is varied, as when a stream finds its way over a hard
rock which overlies a softer rock. If the arrangement is
like that of a series of steps (there may be only one or two
steps) it is possible for the river as it foams in a waterfall
over the hard step at the top to eat its way into the
lower softer step. The lower softer step will gradually
disappear, and then the waterfall, still eating its way in,
will begin to undermine the hard top step, and when that
has gone on long enough the hard top step will fall down
and the waterfall will have to begin a little farther up the
stream. In this way a waterfall, gorge, or ravine can be
constructed by a river.</p>
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<div class="txtrt"><i>London and New York</i></div>
<div class="figcaption" style="clear: both; padding-top:1em;">
<p class="tdc"><span class="smcap">The Grand Cañon of Arizona</span></p>
<p>The Colorado River at this point is nearly 200 feet wide. The man is
seated about 1200 feet above the river's level. This whole cañon up to the
top of the mountains in the distance has been worn away by prehistoric
current; and the river has gradually cut its bed deeper.</p>
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</div>
<p><span class="pagenum" id="Page_53">-53-</span></p>
<p>The Falls of Niagara are an illustration of this method.
The river flows from Lake Erie through a level country
for a few miles, then begins to go faster as the path
becomes steeper, and finally plunges over a hard limestone
precipice. Beneath the hard limestone (the top step) are
softer beds of shale and sandstone. As the water eats
into them and removes them, large portions of the face of
the limestone precipice on the top fall into the stream below.
Thus gradually the Falls of Niagara are eating their way
back to Lake Erie, and have been doing so for hundreds
of thousands of years. In the process of doing so the
Niagara River has cut out below the Falls a gorge which
is not less than seven miles long, from two hundred to
four hundred yards wide, and from two hundred to three
hundred feet deep. There is no reason to doubt that the
Niagara gorge has been entirely cut out in this way, and
that at first the river fell over cliffs seven miles farther
down its course at Queenstown. The amount of rock
thus tunnelled would make a rampart about twelve feet
high and six feet thick going round the world at the
Equator. Still more gigantic are the gorges or caverns of
<span class="pagenum" id="Page_54">-54-</span>
the Colorado and its tributaries in Western America.
The Grand Cañon of the Colorado is three hundred miles
long, and in some places more than six thousand feet deep.
The country traversed by it is a network of deep ravines,
at the bottom of which flow the streams that have dug
themselves down from the top of the Colorado tableland.</p>
<p>Now suppose that the river has dug itself in as far as it
can go. There must be a limit, and the limit is reached
when the slope of the bed has been made so slight that
the current can only go on languidly. In that case it
cannot sweep along stones, or shingle, or even coarse
gravel; and then the river so far from deepening its
channel begins to raise it by allowing more of the transported
sediment to settle down. If a fast stream meets a
slower one deposition of material will take place; and the
same thing will occur when the rivers meet a lake or a
sea. Whatever checks the swiftness of a current weakens
its carrying power and causes it to drop some of its
sediment to the bottom. Therefore accumulations of
sediment occur at the foot of torrent slopes along the
lower and more level ground. These deposits we call
alluvium, and sometimes when the mountain torrent ends
abruptly in the plain they may stand up in cones of silt.
They are sometimes called <i>alluvium cones</i> or <i>fans</i>. Quitting
the steep descents, and reinforced by tributaries on
either side, the stream ceases to be a torrent and becomes
a river. It goes fast enough at first to carry still coarse
gravel; but the big angular blocks of rock have been
dropped, and the stones it now leaves in its bed are
<span class="pagenum" id="Page_55">-55-</span>
smaller, and become rounded and smoothed as it goes
farther and farther across the plain. At many places it
deposits gravel or sand, more especially at the inner side
of the curves which the stream makes as it winds down
the valley. When the stream runs low in summer, strips
of bare sand and shingle are seen at each of these bends;
and the stones are always well smoothed and lie on the
whole regularly. Those that are oblong are so placed
that the greater length of the stone points across the
stream; those that are flat usually slope upstream.
These facts, though apparently insignificant, are really of
importance, because they point to us a method by which
geology can determine, after a river has disappeared, the
slope of the bed and the direction of the curves which
once it had. If we examine the steep banks or cliffs by
the side of a river the layers of gravel or shingle in the
strata may be found to lie not flat on one another but in
sloping planes. That at once will furnish a clue to the
direction of the river. Another thing of great importance
are the terraces which a river forms by the side of
itself. When it overflows in floods it deposits mud on
either side, and when after the flood it subsides the mud
is left. If the reader will imagine the river in the course
of ages sinking lower into its bed he will see that successive
eras of flood-levels will leave their mark in a series of
steps, or <i>river terraces</i> as they are called, on either side of
the channel.</p>
<p>But besides the stones and gravel and mud carried
down by a river, we must also consider the fate of the
remains of plants and animals that are swept along by it,
<span class="pagenum" id="Page_56">-56-</span>
especially in flood-time. In any ordinary flood trees and
shrubs, and the smaller animals like mice and moles and
rabbits, are drowned by the flood. In greater floods
birds and even large animals are drowned, and their
remains are buried in the sediment. If they are quite
covered over they may perhaps be preserved, and their
bones may last for an indefinite period. If, further, the
mud deposit hardens, these remains may be preserved so
well and so long that they become the fossil records of
creatures which lived before man emerged to dwell in the
world and to become the arbiter of many of its destinies.</p>
<p>What we have said of rivers is true also of lakes.
Rivers pour into lakes, bringing with them, especially
in flood-time, enormous freights of gravel, sand, and mud,
and mingled with them the remains of vegetation and of
animal life. Hundreds of thousands of tons may be
swept down by one storm. To the Lake of Lucerne, for
example, the River Reuss, which comes down from the
St. Gothard, brings seven million cubic feet of sediment
every year with it. Since the time of the Romans the
Rhone has so filled up part of the Lake of Geneva that
the Roman harbour, Port Valais, is now nearly two miles
from the edge of the lake. The ground between it and
the Lake first became marsh. It is now farm land.
And though these accumulations are most marked where
the rivers drain into the lake, there are deposits always
taking place from the hills all round the lake. Thus
lake bottoms become most interesting and valuable
receptacles of the life that has for ages lived by or
near their shores. These deposits are in many ways
peculiar. The snails that live in lake waters are distinct
from the land snails of the adjoining shores. Their dead
shells gather at the bottom of some lakes in such numbers
as to form there a deposit of white crumbling <i>marl</i>, sometimes
many yards in thickness. On the sites of lakes that
have been gradually filled up, or artificially drained, this
marl shows at once where the lake borders were, and,
roughly, the period of the lake. In some lakes also are
found concretions of iron-oxide, which are formed by the
chemical action of the water on some of the rocks by the
lake-side; and in several Swedish lakes this ironstone
forms so fast that the lakes are regularly dredged for it.</p>
<div class="figcenter"> <ANTIMG class="w100" src="images/fpage56.png" width-obs="418" height-obs="655" alt="" /> <div class="txtlf"><i>Stereo Copyright, Underwood & U.</i></div>
<div class="txtrt"><i>London and New York</i></div>
<div class="figcaption" style="clear: both; padding-top:1em;">
<p class="tdc"><span class="smcap">Cleopatra Terrace, with its Mirror-like Pools,
Yellowstone Park, U.S.A.</span></p>
<p>These beautiful basins are formed of incrustations of volcanic limestone.
They are of all colours: pink, orange-yellow, green, and blue. The water
in them is of a brilliant blue, caused by the growth of water plants
(algæ), which live in water with as high a temperature as 150 F.</p>
</div>
</div>
<p><span class="pagenum" id="Page_57">-57-</span></p>
<p>Thus among the rocks which form the dry lands of the
globe there occur masses of limestone, sand, marl, and
other materials which we know were deposited in lakes,
because they contain types of plants and animals like
those found in the lakes of our own time. From this
kind of evidence we can mark out the places of great
lakes that have long ago vanished from the face of
Europe and North America.</p>
<p>There are also the so-called Salt Lakes to consider.
These are generally the lakes that have no outlet and
into which a small amount of water now flows, but never
enough to cause the lake now to overflow, whatever
it may have done in past times. The water that now
runs in escapes merely by evaporation. But just as the
bottom of a kettle in which hard water is constantly
boiled gradually becomes furred, so a lake bottom into
which water is continually pouring, bringing dissolved in
it all sorts of mineral salts, becomes coated with sediment.
<span class="pagenum" id="Page_58">-58-</span>
The mineral salts are not evaporated, consequently the
lakes become gradually more mineral—or, for convenience,
let us say, become salter. Among the mineral salts
common salt and gypsum are most important; but
some bitter lakes contain sodium carbonate or magnesium
chloride. The Dead Sea and the Great Salt Lake of
Utah show by the deposits round them how they have
changed their shape and depth. In the upper terraces
of the Great Salt Lake, 1000 feet above the present
level of the water, fresh-water shells occur, showing that
the basin was at first fresh. The valley bottoms around
salt lakes are now crusted with gypsum, salt, or other
deposits, and their waters are without sign of life. Such
conditions help us to understand how great deposits of
salt or gypsum were once laid down in England, Poland,
and Germany, and in many other places where now the
climate would not permit of the necessary evaporation
and condensation of the water.</p>
<div class="figcenter"> <ANTIMG class="w100" src="images/fpage58.png" width-obs="421" height-obs="646" alt="" /> <div class="txtlf"><i>Stereo Copyright, Underwood & U.</i></div>
<div class="txtrt"><i>London and New York</i></div>
<div class="figcaption" style="clear: both; padding-top:1em;">
<p class="tdc"><span class="smcap">A Petrified Tree</span></p>
<p>This magnificent fossil is in the Petrified Forest of Arizona; and it
affords one of the most striking examples known of the solidification and
petrifaction of material by the infiltration of mineral salts. The trunk is
now not merely encrusted with stone: it is permeated by silica, and is, in
fact, itself a stone as hard as flint.</p>
</div>
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<hr class="chap x-ebookmaker-drop" />
<p><span class="pagenum" id="Page_59">-59-</span></p>
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