Mechanical forces at work in the disintegration of rocks--Rains
Landslips--Effects of frosts--Glaciers and icebergs--Abrading
power of rivers--Suggested volume on the geology of
rivers--Some of its probable contents--Scene in a woody
ravine--First idea of the origin of the ravine one of primeval
cataclysms--Proved to be incorrect--Love of the marvellous
long the bane of geology--More careful examination shows the
operations of Nature to be singularly uniform and gradual--The
doctrine of slow and gradual change not less poetic than that
of sudden paroxysms--The origin of the ravine may be sought
among some of the quieter processes of Nature--Features of
the ravine--Lessons of the waterfall--Course of the stream
through level ground--True history of the ravine--Waves and
currents--What becomes of the waste of the land--The Rhone
and the Leman Lake--Deltas on the sea-margin--Reproductive
effects of currents and waves--Usual belief in the stability
of the land and the mutability of the ocean--The reverse
true--Continual interchange of land and sea part of the
economy of Nature--The continuance of such a condition of
things in future ages rendered probable by its continuance
during the past.
The forms of decomposition noticed in the last chapter were chiefly
of a chemical kind. Their effects were observable alike on the
surface of the earth and below ground; in the latter case we saw them
excavating caverns and long irregular chasms, in the former we noted
the production of debris which if undisturbed went to the formation
of soils. It must be borne in mind however, that in these operations
other forces than simply those of a chemical kind come into play. The
percolation of water and the removal of insoluble particles on the
exposed parts of rocks rank as mechanical processes. So also do those
by which new surfaces of mineral masses are brought within the sphere
of the chemical agencies, such as the action of frosts, rains, rivers,
and waves. In short, as already noticed, any subdivision of the forces
at work in effecting the decomposition of rocks must ever be more or
less arbitrary; but it remains nevertheless useful, if we bear in
mind that the exactly defined boundary lines are of our making, not
Nature's. With this caution we may proceed to examine what are termed
the mechanical agencies in the disintegration of mineral masses, and in
so doing, we shall find that the chemical forces are not less helpful
to the mechanical than the latter to the former.
First, we may notice the effect of rains in washing away the
disintegrated particles to lower levels or into river-courses whereby
fresh portions of rock become exposed to the decomposing forces. Rains
also act powerfully in altering the form of cliff-lines and steep
declivities, especially where these consist more or less of friable
earthy matter. After a long continuance of wet weather, I have seen
the abrupt sides of a river-channel that were formed of a stiff blue
clay completely cut up by rents of various dimensions, whereby large
masses had subsided many feet, while others had rolled down altogether
and lay in the bed of the stream where they were undergoing a rapid
abrasion. The cause of this alteration was obvious. The rains pouring
down from the sloping grounds on either side of the river had excavated
deep channels on the abrupt face of the cliffs, while a considerable
quantity of water finding its way through the soil, had permeated
through joints and crevices in the clay some feet from the edge of the
bank. By the combined operation of these causes, masses of clay several
yards in extent lost their cohesion and either settled down a few
feet, or found their way to the bottom. Such landslips are of frequent
occurrence where large masses of rock of a hard compact nature rest
upon loose shales and clays more or less inclined. Whole hills have
been known to be hurled in this way into the valleys below.
But these results become perhaps still more marked where to the
ordinary operations of water there are added those of intense frost.
The effects of a severe winter (such, for instance, as a Canadian one),
in loosening the particles of rocks and facilitating the breaking-up
of large masses, must be ranked among the most powerful agencies of
nature. In such a season, the percolating water with which nearly every
surface-rock is charged becomes frozen, and in the act of congelation
expands. The result of this dilatation is to exert great pressure on
the particles of the rock, and thereby loosen their cohesion. When thaw
comes the frozen liquid contracts again, but the loosened particles
have no such elastic power, and so, having lost hold of each other,
crumble down. If the season be a changeable one, frost and thaw quickly
alternating, the amount of waste produced becomes very great. Not only
is the outer surface of the stone decomposed, but the water filtering
through the joints of the rock freezes there, and thus on the arrival
of milder weather vast masses become detached from the cliffs, and
roll down, to be worn by the grinding action either of waves or of
rivers, as the case may be. Spring at last sets in with its warmth and
its showers; the snow rapidly melts away; the whole country streams
with water; every valley and hollow has its red turbid rivulet, that
bears a burden of muddy sediment into the nearest river; and thus the
loosened portions of the rocks get washed away down to sea, leaving a
new surface for the action of next winter. We can easily understand,
therefore, that in certain regions the combined effects of frost and
thaw may work in the course of ages changes of almost inconceivable
extent, and that the agency of ice must be not less varied and
important on the land than, in the case of the boulder clay, we found
it to be in the ocean.
Besides this action in winter, which goes on more or less in every
country wherever the temperature sinks sufficiently low to permit of
the freezing of water, ice effects many changes on the surfaces of
rocks when it takes the form of glaciers and icebergs. We have already
noted the operation of a glacier during its slow progress in crushing
down large fragments of stone, scratching and abrading the rocks over
which it passes, and eventually producing a vast quantity of mud, which
is carried down by streams to form new accumulations either in lakes
or seas. We have also marked the effects of the drifting iceberg in
materially modifying the contour of submarine hills, and depositing
over the ocean-bottom mud, gravel, and boulders. Nothing further,
therefore, need be done here than simply to keep these agencies in
view, as playing an important part in the disintegration of rocks.
Another highly interesting aqueous action is that of streams and
rivers, in scooping out for themselves channels through sometimes
the hardest and most solid rock. Such effects may be seen all over
the globe, in the old world and in the new, in the bed of the
tiniest rivulet, as well as in the course of the mightiest river.
And accordingly, in all the long list of geological agents, we find
none so well known and so often described alike by poets, historians,
and scientific writers, as well in ancient as in modern times. What
a delightful volume might be written about the geology of rivers! It
would, perhaps, begin with that "great river," the Euphrates, along
whose green banks lay the birthplace of the human race, tracing out the
features of its progress from the ravines and cataracts of Armenia,
with all their surrounding relics of ancient art, down into the plains
of Assyria, amid date-palms and Arab villages, onwards to the mounds
of Nineveh and Babylon, and thence to the waters of the Persian Gulf.
Well-nigh as remote, and perhaps still more interesting in its human
history, would be the story of the Nile. We should have to follow that
river from the mystic region of its birth,[53] marking the character
of the rocks through which winds its earlier channel, and the effects
upon them of the floods of untold centuries; it would be needful, too,
to note the influence of the waters on the lower grounds, from where
the stream flows over the cataracts of Syene, down through the alluvial
plains of Egypt; and lastly, the concluding and perhaps most onerous
part of our labour would be the investigation of the delta, marking its
origin and progress, its features in ancient times, as made known to us
in the graphic chapters of Herodotus, and the changes which the lapse
of more than twenty centuries has since wrought in its configuration.
The rivers of Europe would detain us long, not less perhaps by their
historic interest than by the variety and attractiveness of their
physical phenomena. One could scarce help lingering over the Rhine,
with its source among Alpine glaciers, its lakes and gorges, its
castles and antique towns; and when once the narrative entered the
classic ground of Italy, it would perhaps become more antiquarian than
geological. The ravine of Tivoli, for instance, would certainly lay
claim to a whole chapter for itself, with its long-continued river
action, its ancient travertin, its beautiful calcareous incrustations,
and above all its exquisite scenery.
[Footnote 53: "Fontium qui celat origines Nilus" a description not less
true now than in the days of the Sabine bard.]
"Domus Albuneæ resonantis,
Et præceps Anio, ac Tiburni lucus, et uda
Mobilibus pomaria rivis."[54]
[Footnote 54:
"Albuna's grey re-echoing home,
And Anio, headlong in his foam,
And grove of Tivoli,
And orchards with their golden gleam,
Whose boughs are dipping in the stream
That hurries to the sea."
Hor. _Carm._ L vii. 12.]
And when could we exhaust all, that might be said about the rivers of
our own land?
"Of utmost Tweed, or Ouse, or gulfy Dun,
Or Trent, who, like some Earth-born giant, spreads
His thirty arms along the indented meads
Or sullen Mole, that runneth underneath;
Or Severn swift, guilty of maidens' death;
Or rocky Avon, or of sedgy Lee;
Or coaly Tyne, or ancient hallow'd Dee;
Or Humber loud, that keeps the Scythian's name;
Or Medway smooth, or royal-towered Thame."
Passing to the new world, a vast field would spread out before us: the
Mississippi, the Atchafalaya, the Ohio, the St. Lawrence, the Amazon,
and many other rivers that in some cases rise high among the regions
of perpetual snow, and after traversing large areas of country in the
temperate zone, fall into the waters of tropical seas. By studying such
examples of river-action and delta-formation as are presented by these
gigantic streams, we should arrive at some conception of the conditions
anciently at work in producing our present coal-fields. Nor would our
researches assume aught like completion until after a scrutiny of all
the larger and more important rivers of the globe. Such a work could
be undertaken, perhaps, only by another Humboldt. Its successful
accomplishment would certainly insure the highest renown to its author,
and incalculable benefits to science.
From what we have seen of the wide waste and decay everywhere in
progress on the solid lands of our planet, it becomes no difficult
matter to perceive what a number of agencies must be at work in the
formation of a river channel. Let the reader take his stand in some
wooded ravine, where the shelving rocks on either side are hung all
over with verdure, and a tiny streamlet murmurs on beneath with a
flow so quiet and gentle as scarcely to shake the long pendant willow
branches that dip into its surface, while the polished pebbles that
strew its bed lie unmoved by the rippling current that glides over
them. If in the midst of such a scene the question were to arise in
his mind, How came this deep, narrow ravine into existence? what
answer would in all likelihood be the first to suggest itself? His eye
would scan the precipitous walls of the dell, with their rocks cleft
through to a depth of perchance fifty feet. It would require no great
scrutiny to assure him that the beds on the one side formed the onward
prolongations of those on the other, and that consequently there must
have been a time ere yet the ravine existed, when these beds stretched
along unbroken. Satisfied with these results, his first impulse might
be to bethink him of some primeval earthquake, when the solid land
rocked to and fro like a tempested sea, and broke up into great rents
and yawning chasms. Into one of these clefts he might suppose the
little streamlet had eventually found its way, moistening the bare and
barren rocks, until at length their surface put on a livery of moss, or
lichen, or liver-wort, and the birch, the alder, and the willow, found
a nestling-place in their crevices. Such a view of the origin of the
woody dell would be certainly a very natural one, and in some instances
might be sufficiently correct, but in the present case it will not
explain the phenomena. If the reader will kindly permit me to visit the
locality in his company, perhaps we may be able to light upon the true
explanation, and see a few appearances worthy our attention.
First, then, how can we make sure that no convulsion of nature has
produced a rent in the rocks, and so helped the streamlet to a channel?
a simple question that may be well-nigh as simply answered. We stand
in the centre of the dell on a broad ledge of stone, round whose
well-worn sides the rivulet is ever eddying onwards. The block consists
of a pale sandstone lying in a bed about three feet thick, that dips
gently down the stream and underlies a seam of dull, soft, blue shale,
full of small shells. We trace the edge of this sandstone bed across
to the left-hand side of the ravine, and away up into the precipitous
cliff, till it is lost amid the ferns and brushwood. There can be no
doubt, therefore, that the ledge on which we were but now standing is a
continuous portion of the rocks that form the left side of the ravine.
Returning again to the centre of the stream, we proceed to trace out
the course of the other end of the same bed, and find that it, too,
strikes across to the rocks on the right-hand side without a break
or fissure, and passes up into the cliff, of which it forms a part.
Clearly, then, the sandstone bed runs in an unbroken, unfissured line,
from the one side to the other, and the rocks of either cliff form
one continuous series. There occurs no break or dislocation, which,
of course, there must have been had the ravine owed its origin to any
subterranean agency. And so we come to conclude that no great cataclysm
in primeval times, no yawning abyss, or gaping chasm, has had anything
whatever to do with the formation of our deep sequestered dell. What
then? "Whither shall we turn," you ask, "to find another agency equally
grand and powerful in its operation and mighty in its results?"
Stay, gentle reader. That craving for the grand and the sublime, that
hungering after cataclysms and convulsions, that insatiable appetite
for upheavals, and Titanic earth-throes, and all the mightier machinery
of Nature, has done no little mischief to geology. Men have reasoned
that gigantic results in the physical structure of the earth must
have had equally gigantic causes operating in sublime conflict and
in periodic paroxysms, now heaving a mountain chain to the clouds
of heaven, now swallowing up a continent in the depths of the sea.
Happily such extreme notions are fast passing away, though the old
tendency in a modified form still abounds. A closer scrutiny of Nature
as she actually shows herself, not as theorists fancy she should be,
has revealed to us that her operations are for the most part slow,
gradual, and uniform, and that she oftentimes produces the mightiest
results by combinations of forces that to us might seem the very
emblems of feebleness and inactivity. In place of sudden paroxysms
she demands only an unlimited duration of time, and with the aid of
but a few of these simple, tardy agents, she will eventually effect
results perchance yet more gigantic than could be accomplished even
by the grandest catastrophe. Nor in thus seeking to explain the past
by defining what seems the usual mode of Nature's operations in the
present, do we, as is sometimes alleged, deprive them of their high
poetic element. Assuredly there is something thrilling to even the
calmest imagination in contemplating the results of vast and sudden
upheavals, in picturing the solid crust of the earth heaving like a
ground-swell upon the ocean, in tracing amid
"Crags, knolls, and mounds, confusedly hurl'd,
The fragments of an earlier world;"
and in conjuring up visions of earthquakes, and frightful abysses from
which there ever rose a lurid glare as hill after hill of molten rock
came belching up from the fires below. But while far from denying that
such appearances may have been sometimes seen during the long lapse
of the geological ages, and that they give no little vividness and
sublimity to a geological picture, we claim for the doctrine of the
tranquil and uniform operation during past time of existing laws and
forces, an element not less poetic. In the former case the pervading
idea is that of unlimited expenditure of power, in the latter that of
unlimited lapse of time. In the one case the action is Titanic but
transient, in the other it is tranquil but immensely protracted. The
two doctrines in this way counterbalance each other; yet I cannot but
think that however impressive it may be to stand in some lone glen, and
while gazing at its dark jagged precipitous cliffs, to dream about the
paroxysmal convulsions of some hour far back in the distant past, the
scene becomes yet more impressive when we look on its nakedness and
sublimity not as the sudden and capricious creation of a day, but as
the gradual result of a thousand centuries. These cliffs may once have
been low-browed rocks rising but a little way out of a broad grassy
plain, and serving as a noon-tide haunt for animals of long extinct
races. Thousands of years pass away and we see these same rocks higher
and steeper in their outline, brown with algæ and ever wet with surf,
while around them stretches a shoreless sea. Ages again roll on, and
we mark still the same rocks shooting up as bleak crags covered with
ice and snow. Another interval of untold extent elapses, and rock,
snow, and ice have all disappeared beneath a broad ocean cumbered
with ice-floes and wandering bergs. Again the curtain drops upon the
scene, and when once more it rises, the cliffs stand out in much the
same abrupt precipitous aspect with that which they now present, save
that their bald foreheads look less seamed and scarred than now, and
their dark sides show no trace of bush or tree. The white cascades that
to-day pour down from their summits and sides--seeming in the distance
like the white hairs of age--are insensibly deepening the scars and
furrows on these ancient hills, and thus slowly but yet surely carrying
on the process of degradation and decay. Musing on all this long
series of stages in the formation of one single cliff-line, is there
not something more sublime, something yet more impressive than if we
pictured but the chance random result of the gigantic paroxysm of an
hour?
Let us not be deterred then from seeking an explanation of the origin
of the ravine among some of the quieter and more unobtrusive forces
of Nature. Give them but an unlimited period to work in and they will
abundantly satisfy all our demands.
We return again to the rocky ledge in mid-channel, and proceed to
ascend the course of the stream, marking as we go the changes in the
character and features of the stone that forms the cliff on either
hand. We come to a bare part of the ravine where brushwood and herbage
find but a scanty footing and where accordingly the rocks can be
attentively studied. The face of the escarpment shows a number of beds
of pale grey sandstone alternating with courses of a dark crumbly
shale. The sandstones being harder and firmer in texture stand out
in prominent relief while the shales between have been wasted away,
covering the bottom of the slope with loose debris. We can mark too
that, as this decay goes on, the harder beds continually lose their
support, cracking across chiefly along the lines of joint, and rolling
down in huge angular blocks into the stream. In truth we cannot doubt
that every year adds to this decay and thus slowly widens the dell, for
the broken fragments do not form in heaps over the solid rock below
so as to protect it from the weather, but are evidently carried away
by the stream and hurried down the ravine onwards to the sea. From
what has been said above relative to the disintegration of rocks by
percolating water, frosts, and other causes, the reader will easily
see how this rotting away of the sides of the ravine must be carried
on; and he will not fail to mark that here we have at work an agency
not yet considered, that of running water. The effects of the weather
are seen in the crumbling, ruinous cliffs overhead; the effects of
the streamlet are observable in the continual removal of the rubbish
whereby a fresh surface is ever exposed to the decomposing forces,
while at some points we can mark the water actually undermining an
overhanging part of the cliff from which there are ever and anon vast
masses precipitated into the channel where eventually they get worn
down and carried away out to sea. "Still," you may remark, "these
forces are at work only in widening a channel already made. How was the
ravine formed at first?"
We continue our ascent. A scrambling walk through briars and
hazel-bushes, sometimes on rocky ledges high among the cliffs,
sometimes among the prostrate blocks that dam up the stream, brings
us at last full in front of a sparkling waterfall that dashes over
a precipitous face of rock some twenty feet high. The appearances
observable here deserve a careful attention. Our eyes have not been
long employed noting the more picturesque features of the scene ere
they discover that the dark-brown band of rock forming the summit of
the ledge over which the water tumbles is continuous all round the
sides of the dell. There is consequently no break or dislocation here.
Approaching the cascade we note the rock behind it so hollowed out that
its upper bars project several feet beyond the under ones. In this
way the body of water is shot clear over the top of the cliff without
touching rock till it comes splashing down among the blocks in the
channel. And yet this hollowed surface is never dry; the spray of the
fall constantly striking on it keeps it always dank and dripping. In
some parts the rock stands out bare and worn, while on the less exposed
portions there gathers a thick green scum which is replaced on the
drier ledges by the soft cellular leaves of the liver-wort. Now our
examination of the influence of percolating water upon even the hardest
rocks teaches us that this moist soaked surface is just the very best
condition for favouring the decay of the rock. Nay more, the green
vegetation that mantles over the stone serves to prevent the water from
running off too rapidly, and keeps the rock in a still more moist state
than would otherwise happen. So that the portion of sandstone behind
the cascade comes to be in a still more favourable situation for speedy
decay than the ledge over which the water is rapidly driven. We can
see, therefore, how in the lapse of years the corrosion may go on until
the upper projecting part of the cliff loses its support and falls with
a crash into the rocky pool below, while the form of the waterfall
becomes thus greatly altered, and new surfaces are exposed to the wear
and tear of the stream.
But we have not yet exhausted all that the rocks at the cascade can
teach us. By dint of some exertion we climb the cliff and gain the
upper edge of the fall. The rocks that form the bed of the stream are
now seen to be deeply grooved and worn, every exposed surface having
a smoothed blunted aspect. We can mark how the stone has split up
along the natural lines of joint, whereby great facility is given to
the removing power of the current, and how large irregular angulated
blocks become detached and are swept down the stream. In not a few
parts, too, we may notice circular holes of greater or less depth, in
the bottom of each of which lie perhaps a pebble or two, that with a
constant gyratory movement, caused by the eddying water, have eaten
their way downwards into the solid rock. When the stream is in flood
and comes roaring down the rocky gorge bearing along with it a vast
amount of mud, gravel, and stones, one can easily see how the friction
of the transported material must wear down the hard bed and sides of
the channel, and how this process repeated month after month and year
after year, must aid the decomposing forces in scooping out a deep
ravine. From the cascade the ascent of the stream becomes steeper and
the run of water is consequently more rapid. Soon however we emerge
from the woody copse, and find ourselves on a flat alluvial cultivated
plain through which the rivulet winds in a tortuous meandering course,
bending back upon itself into loops that almost meet and well-nigh
form broad flat islets. Strolling along this winding route we can mark
the effects of the stream in eating away the soft clay and sand at one
part of the bend and piling them up at another. Such loose material
can present but little resistance to a stream swollen with rains, and
consequently a large quantity of the mud and gravel along with the
interspersed boulders must be swept away down into the dell at every
season of flood. The matter thus removed will of course be still
further comminuted in its passage, and at the same time will help to
grind down the hard rock surfaces over which it is driven.
Here then may be found the whole history of the ravine. Originally the
streamlet wound its devious course through a flat alluvial country with
a channel sunk but a foot or two below the level of the plain. Such
continued its character till it reached a low bluff, down which the
water flowed more rapidly to gain a second level undulating region.
The part of this bluff crossed by the stream was ere long bared of
its covering of soil and clay, and the rock below came to be washed
by a group of little cascades. Once exposed to the decomposing and
disintegrating forces, the stone soon began to decay and the cascades
ere long merged into one. By slow degrees the rock gave way and the
waterfall retreated from the bluff. For perchance thousands of years
the same process has been going on, now with greater, now with less
rapidity, according to the nature of the rocks encountered and other
modifying causes, until the fall has eaten its way back for well-nigh
three miles and scooped out a wild rocky gorge some fifty or sixty feet
deep. This is but a solitary and insignificant instance of what may be
seen all over the world, for the process remains the same whether we
stand beside a tiny rivulet in some lone Highland glen or listen to the
roar of the falls of Niagara.
There is but one other principal agency at work in the demolition of
rock-masses, the waves and currents of the ocean. But we have already
noted the effects thus produced, and need not now retrace our steps
further than to recall the vast amount of devastation which can be
shown to have been effected in our own country by marine causes, both
in breaching the existing shores and in scooping out valleys and
grinding down hills at former periods when the land was either rising
above or sinking below the level of the sea.
Having now satisfied ourselves that there goes on all over the world an
incessant waste of the solid lands, that the disintegrated debris is
washed down by rains and transported seawards by rivers, and that the
waves are ever eating their way into the iron-bound coast-line as well
as into the low alluvial shore, we naturally come to ask the result
and end of all this decay. What becomes of that vast amount of mineral
matter annually removed from the land? To be able to answer this
question clearly and distinctly, let us look for a little at what takes
place in lakes, at river-mouths, and in open sea.
The river Rhone rises among the Bernese Alps, and after a course of
about 100 miles through the Canton of Valais, it enters the upper end
of the Lake of Geneva. Its waters, where they mingle with those of the
lake, are muddy and discoloured, but where they pass out at the town of
Geneva are limpid and clear. The mud, therefore, which they bring into
the lake must be deposited there, and as the stream may have continued
to flow for thousands of years, we may reasonably expect to find some
trace of the large amount of sediment necessarily deposited during the
whole or part of that long period. Accordingly, careful examination of
the Lake of Geneva has shown that such accumulations have really been
formed, and that their progress and amount during part of the historic
period can be approximately calculated. Where the turbid current of the
Rhone enters the still water of the lake, the mud slowly sinks to the
bottom. In the lapse of centuries layer after layer has been thrown
down, rendering the lake at this part sensibly shallower, until a large
area or delta has been filled up and converted into a flat alluvial
plain. Thus, a town which in the time of the Romans formed a harbour
on the water's edge, now stands more than a mile and a half inland.
This new-formed land is entirely the work of the stream, and if we
could obtain a complete section of it from the surface to the bottom,
"we should see a great series of strata, probably from 600 to 900 feet
thick (the supposed original depth of the head of the lake), and nearly
two miles in length, inclined at a very slight angle." These strata,
which are said to have taken about eight centuries to form, "probably
consist of alternations of finer and coarser particles; for, during the
hotter months, from April to August, when the snows melt, the volume
and velocity of the river are greatest, and large quantities of sand,
mud, vegetable matter, and drift-wood, are introduced; but, during the
rest of the year, the influx is comparatively feeble, so much so that
the whole lake, according to Saussure, stands six feet lower."[55] If
the present conditions continue for a sufficient length of time, the
lake may be eventually filled up with mud, sand, and gravel, deposits
that would eventually harden by pressure into shale and sandstone. So
that the day may yet arrive when the blue waters of the Leman lake
shall have passed away, when the Rhone perchance may have ceased to
flow or found its way by some other channel, when the peasant may guide
the plough where now the boatman plies the oar, and when the geologist
shall trace out in quarries and excavations the successive deposits of
hardened sediment with their lacustrine shells and drift-wood, and,
musing on the changes of which they are the silent yet impressive
witnesses, may sit down to pen a record of the gradual extinction of
the Leman lake on that classic ground where an immortal historian
described the decline and fall of the empire of Rome.
[Footnote 55: Lyell's _Principles of Geology_. Ninth edition, p. 252.]
The alluvial matter deposited by the Rhone at its entrance into the
Lake of Geneva suffers perhaps no change when it once reaches the
bottom. Layer after layer accumulates tranquilly, without disturbance
from surface currents or other causes, so that the renovating effects
of the stream have here every advantage. It is otherwise, however,
where a delta gathers at the mouth of a river upon the sea-margin.
There tides and currents are ever demolishing what the stream has
piled up. Often, too, owing to the prevalence of high winds from
seawards, the river is dammed up for leagues, and the waters of
the ocean encroach far on the delta, mingling in this way marine
remains with those that are fluviatile or terrestrial. But with these
modifications the process of delta-formation remains essentially the
same, both in lakes and at the sea. The vast quantities of sand and
gravel transported by rivers during the flood-season sink to the bottom
as soon as the motion of the water will permit. This takes place at
the shore, where eventually wide tracts of low alluvial land encroach
upon the sea, covered with marshes and overgrown with vegetation. A
section of any of these deltas, obtained in boring for water, shows a
succession of sands and clays, with occasionally a few calcareous beds
and quantities of peaty matter formed of vegetation either drifted
or that grew on the spot.[56] If, now, a sufficient amount of matter
were piled over these loose incoherent strata, they would eventually
become as hard and compact as any of our ordinary building stones. The
sand would subside into a firm compact sandstone; the clay, in like
manner, would consolidate into fissile shale; the peat would become
chemically altered into coal; the calcareous seams would take the form
of layers of limestone; while the leaves, twigs, branches, and trunks,
dispersed through all the beds, would get black and carbonized, so as
precisely to resemble the lepidodendra, calamites, stigmariæ, &c., of
the carboniferous rocks. And thus might a mass of fossiliferous strata,
thousands of feet deep and thousands of square miles in extent, be
amassed by the prolonged operation of a single river.
[Footnote 56: The structure of maritime deltas, especially their
relation to the growth and entombment of forests, will be more fully
alluded to in a subsequent chapter, when we come to inquire into the
origin of a coal-field.]
It often happens that a delta is prevented from extending further
seawards owing to the prevalence of some marine current that comes
sweeping along the coast-line and cuts away the accumulations thrown
down by the river. The sediment thus removed is often carried to great
distances, and eventually settles down as a fine mud along the floor of
the sea, entombing any fucoids, infusoria, shells, corals, fish-bones,
or other relics that may lie at the bottom.
He who has witnessed a storm along a rocky coast-line, has marked the
breakers battering against the weather-bleached cliffs, and heard the
thunder-like rattle of the shingle at the recoil of every wave, needs
not to be told how vast an amount of sediment must in this way be
formed. The pebbles of the beach are ground down still smaller, the
sand produced by their friction finds its way to a lower level, while
the finer particles taken up by the water are borne out to sea, and if
a current traverse the locality may be transported for leagues, till
they at last settle to the bottom. The floor of the sea is consequently
always receiving additions in the form of fine mud--the waste of the
land--derived either from breaker-action, rivers, or icebergs, so
that a series of marine deposits exactly similar to those we find
among the rocks of our hills and valleys, must be constantly in the
course of formation. If circumstances be favourable, the shingle of
the beach may eventually either be covered over or reach a part of
the sea undisturbed by currents or waves, and then consolidate into
what we call conglomerate or pudding-stone. The sand, as before,
becomes sandstone, and the mud laminated shale or hardened clay. These
deposits may go on forming for thousands of years, until at last some
slow elevation or some sudden upheaval of the ocean bed brings them to
the light of day as part of a new continent. Thus exposed they would
differ in no respect from rocks of a similar kind now visible, and
the geologist, in tracing out their origin and history, would have no
hesitation in ranking them among the ordinary marine formations of the
globe.
In fine, we cannot quit the subject without being convinced that
these ceaseless changes afford one of the grandest examples of that
continuous series of mutations--cycle and epicycle--which has been
already alluded to as a distinguishing feature in all the operations
of Nature. We are accustomed to think and speak of "the everlasting
hills." We look on the solid lands whereon we dwell as the emblem of
all that is stable and steadfast, and on the boundless ocean as the
type of all that is unsteady and changeful. The traveller who stands
on those plains where the human race was cradled, marks still the same
valleys with their winding rivers, still the same rocks and hills,
still the same blue sky overhead. The dust of centuries has gathered
over the graves and the dwellings of the early races, yet the covering
is but thin, and if we could conjure from their resting-place some
of these venerable patriarchs, they might perhaps see little or no
change on the haunts of their boyhood. We feel it otherwise, however,
when we contemplate the ocean. In sunshine and in storm its surface
never rests. The wave that now breaks against some bald headland of
our western shores may have come sweeping across from the coast of
America, and the broad swell that rolls into surf along the shores
of Newfoundland may have travelled from the frozen seas of the North
Pole. And so it has ever been; the "far resounding sea" of Homer is the
"far resounding sea" still; and the "countless dimpling of the waves,"
invoked in his agony by the chained Prometheus, remains restless and
playful as ever.
"Firm as a rock," and "fickle as the sea," have therefore become
proverbs of universal acceptance. Yet when we investigate the matter
as we have done in this and the preceding chapter, it appears that an
exactly opposite arrangement would be nearer the truth. It is the sea
that remains constant--
"Time writes no wrinkle on its azure brow;"
while the land undergoes a continual change. Hills are insensibly
mouldering away, valleys are ever being widened and deepened, rocky
coasts and low alluvial shores suffer a constant abrasion, while
even within the bowels of the earth the process of decomposition
uninterruptedly proceeds. And thus, in place of remaining unchanged
from the beginning, we know of nothing more mutable than the land on
which we dwell, so that if the waste everywhere so apparent were to
go on unchecked or unmodified, island and continent would eventually
disappear beneath the waves. Here, however, another principle comes
into operation. The debris removed from the land, as we have seen,
is not annihilated. Slowly borne seawards, it settles down at river
mouths or on the floor of the ocean as an ever-thickening deposit,
which eventually hardens into rock, as solid and enduring as that
whence it was derived. But it does not always remain there. Owing to
the action of subterranean agencies with which we are but slightly
acquainted, different parts of the sea-bottom are continually rising.
Sometimes this process goes on very slowly, as along the shores
of Sweden, where the coast has been ascertained to emerge in some
localities at the rate of about thirty inches in a century; sometimes
with prodigious rapidity, as on the coast of Chili, where the land
was upheaved from two to seven feet in a single night. There can thus
be no doubt that the mysterious agency which produces earthquakes and
volcanoes on the land affects equally that portion of the earth's crust
covered by the waters of the ocean, and must be ceaselessly employed
in elevating large areas of sea-bottom into new continents, that will
ere long become clothed with vegetation and peopled with animals.
In contemplating, therefore, the constant decay in progress on the
surface of the land, we see not a mere isolated process of waste, but a
provision for future renovation. The sandstone cliffs of the shore are
battered down and their debris carried out to sea, but when sea-bottom
comes to be land-surface, they may be sandstone cliffs again, lashed
once more by the breakers, and once more borne as sediment to the
depths of the sea. And thus, in what may seem to us sublime antagonism,
land is ever rising in the domain of ocean, and ocean ever encroaching
on the regions of land. No sooner does a new island, or mountain peak,
or wide area of continent, appear above the waves, than the abrading
agencies are at work again. Rain, air, frost, rivers, currents,
breakers, all begin anew the process of destruction, and cease not
until the land has utterly disappeared, and its worn debris has sunk
in mid-ocean to be in process of time once more dry land, and suffer
another slow process of obliteration.
Such is the economy of nature around us now, and that such will
continue to remain the condition of things in the future, we can affirm
with probability from a consideration of the history of the past. The
geologist can point to masses of rock several miles in thickness, and
occupying a large area of the globe, formed entirely of the worn debris
of pre-existing formations. The very oldest rocks with which he is
acquainted are made up of hardened sediment, pointing to the existence
of some land, even at that early period, worn down by rivers or
wasted by the sea. During all the subsequent ages the same principles
were at work, and now well-nigh the only evidence of the geological
periods is to be gathered from the layers of sediment that successively
settled down at the sea-bottom. The records which it is the task of
the geologist to decipher, are for the most part written in sand and
mud--the deposits of the ocean, for in by far the larger number of
formations into which the stratified part of the earth's crust has been
divided, and which form his only guide to the history of the past, he
can detect no trace of land. Hill and valley have alike disappeared,
and the character of their scenery and inhabitants he can often but
dimly conjecture from the nature of the sediment and of the drifted
terrestrial relics that may chance to be found among strata wholly
marine.