erosive or disaggregating power, may have scooped out the valley in
the bottom of which it at present flows, in a state of feebleness
very different from its original state, we must account for the
disposal of a vast mass of earth and rock, which filled up the valley
before the river had removed it. It is not possible to suppose that
it has been transported into the sea, which is often more than a
hundred leagues from the valley; for we know that when rivers, on
reaching the plains, lose their rapidity, they allow the matters to
be precipitated which they held in suspension. Besides, we have
shown that many rivers, on leaving the mountains, traverse lakes, in
which they deposit all the earthy matters suspended in their waters.
This deposition is particularly striking in all the considerable
rivers, which descend from the ridge of the Alps toward the
north-west and south-east of that chain of mountains. These rivers
meet, at the opening of the valleys they flow through, lakes, which
they traverse, and which seem intended for their purification. Thus,
on the northern side, we see the Rhone traversing the lake of Geneva;
the Aar, the Lakes of Brientz and Thun; the Reuss, the Lake of the
Four Cantons; the Linth, the Lake of Zurich; the Rhine, the Lake of
Constance. On the south side, the Lac Majeur is traversed by the
Tessin, the Lake of Como by the Adda, the Lake Disco by the Oglio;
the Lake of Guarda by the Mincio, &c. Now, these lakes, which are
only themselves deeper parts of the valley, would have been filled up
by the debris conveyed to the valley, if this valley had the origin
attributed to it. Proceeding from one hypothesis to another, it might
perhaps be supposed that these lakes may have been sufficiently deep
to swallow up all the debris of the valley, without being chocked up.
But, rather than admit such suppositions, why not grant that the same
unknown cause which has scooped out the lake, has also scooped out
the valley which is only a continuation of it?
4. But if facts had proved that the waters degrade the rocks, scoop
them out, and perpetually remove their debris, we might perhaps be
induced to admit that unknown causes, of which we are absolutely
ignorant, and of which we can form no idea, have given to the
original rivers the means of surmounting all these obstacles. Now,
observation would seem absolutely to prove the contrary.
We have remarked, that rapid rivers which, in the bottom of valleys,
fall in cascades, from rock to rock, which beat with violence against
the walls which contain them, do not in any degree alter these
rocks, and that, far from corroding their surface, they allow it to
be covered with a rich coating of mosses, confervæ, &c. which could
neither maintain itself, nor be formed at all, were the least portion
of the surface of these rocks continually or even only frequently
removed.
A much more striking fact is that which some of the great rivers
present, such as the Nile, the Orinoco, &c. which flow in the
equatorial regions.
These powerful rivers, when they have arrived at places where they
are contracted, and, as it were, jammed in between two rocky walls,
form impetuous cataracts. Their waters, endowed by the celerity of
this fall with the greatest erosive power that can be attributed to
this fluid, must necessarily have corroded, or at least worn, the
rocks which they have thus beat against since the creation of our
present Continent. Now, so far from removing the surface, they cover
it with a brownish varnish of a peculiar nature.
It appears, therefore, well established, that water _alone_ does not
scoop those rocks, whose aggregation is complete, or which are solid;
and that it does not wear them in any way, whatever be its quantity
of motion.
We say _water alone_; and we must insist on this distinction, in
order to make the preceding facts agree with other facts, which might
seem contradictory.
We often see furrows scooped out on the walls that bound the narrows
of rivers; we also see rocks rounded, and entirely destitute of moss.
But let the facts be examined with attention, and we shall find that
this erosion always takes place in the parts of their course, where,
on account of the nature of the neighbouring soil, the torrents carry
with them, in their risings (or floods), debris and detached stones
from their banks; and it is by means of these stones that they wear
the rocks which are in their bed.
It is very easy to appreciate these circumstances. It is remarked,
that this erosion has never taken place at the sources of powerful
springs. All the pebbles which had to be carried off have been so
long ago, and the mosses which grow abundantly on the rocks at the
level of the water, and in the bed of these torrents, have nothing
more to fear from the destructive action of these solid bodies. The
case is the same with the parts which immediately succeed a lake, or
a great excavation, capable of arresting all the hard bodies carried
off by the river. There the mosses appear in abundance; because they
are not subjected to the action of any other substance than of the
water alone.
The present rivers do not therefore appear to have any erosive power
upon the rocks which are completely aggregated, when they act by
themselves, and when no other cause, such as frost, decomposition,
&c. has disintegrated the rock. The absence of these foreign
circumstances is proved by the vegetation or the enamel which then
cover the rocks exposed to the action of the water.
These rivers, in proportion as they remove from the rocks in the
neighbourhood of the lofty mountains in which they took their rise,
often gain in volume what they have lost in velocity; but the power
dependent upon size rarely compensates that which they owed to
rapidity; and although these large rivers still retain a transporting
power, sufficient to carry along with them the obstacles which oppose
themselves to their progress, they are far from presenting results
of action so striking as those of torrents. They stir up, when
flooded, or when they change place, the earth and mobile sand which
cover their bottom, especially towards their edges, and transport
them to some distance; but they scarcely move pebbles larger than an
egg, which occur in their bed, and which have been brought there in
other times, and under other circumstances. On thus transporting the
comminuted and mobile mineral matters, they deposite them again in
places where their current is relaxed by some cause, and thus raise
the bottom of their bed in these places; they seek a new passage in
the midst of the barriers which they have themselves constructed.
The principal current is then borne, sometimes against one bank, and
sometimes against the other; and when it comes to beat upon the foot
of a steep part, composed of moveable soil, as the banks commonly
are, in such cases, they really erode it, and make it fall into
the river; and transport to another part of its course, the earth
resulting from the destruction of the bank, and give rise to new
obstacles. Hence the new deposites, which border rivers in all points
where their current is slackened, and principally toward their mouth.
It is sufficient for our present purpose to have referred to facts
remarkable for their number, for the importance which they have had
in regard to the modern changes of the configuration of the globe;
and, lastly, in regard to agriculture and civilisation;--facts of
easy observation, and which tend to prove, that the action of rivers,
whose fall is not sufficiently rapid to entitle them to be considered
as torrents, is not to scoop out their bed, either in the valleys or
in the plains through which they flow, but rather to raise them, and
to tend, consequently, rather to level and flatten the earth than to
furrow it, more than it has been since the Continents have assumed
the configuration which they now possess.
But if we have not been able to recognise a real corroding power in
the great rivers falling in the form of cascades or cataracts, let
us inquire elsewhere, in circumstances where the water seems endowed
with a still superior power, what are the effects of this agent?
3. _Action of Waves._
It is in the sea, an enormous mass, sometimes acquiring, from the
action of the winds, an incalculable power, that we must find the
maximum of force of the water of the _present_ times. In fact, in
this case, the power of transportation is so prodigious, that the
strongest barriers, both natural and artificial, are overturned, and
the largest stones, together with enormous fragments of rocks torn
from their place, transported, and even projected to a distance.
But it is to these effects that this immeasurable power is limited.
The water, which displaces and transports to a distance these heavy
masses, does not abrade the surface when it acts by itself. We see
this surface, on rocks and the sides of piers and dikes, perpetually
beaten by the waves, covered with fuci, confervæ, byssi, and other
delicate vegetables, _without roots_, which the waves have not
prevented from contracting a first and feeble adherence, and which
they do not hinder from growing. But, if the waves carry with them
pebbles, or even sand, it is those hard bodies which act; the surface
of the rocks is abraded, and all vegetation ceases.
The same effect takes place, and is even augmented by the real
degradation of the coasts, if the sea acts upon friable rocks,
capable of mixing with water, such as argillaceous or calcareous
marl, or chalk, or upon rocks which are hard, but naturally fissured,
or partly disaggregated, such as certain granites; it then easily
removes the crumbled or previously detached parts, scoops out the
foot of the rock or steep coast, and causes the upper part, which is
deprived of support, to fall. But, in consequence of this fall, it
forms a slope, which, by its inclination, deadens the violence of the
shock, and even protects the foot of the cliff, for some time only,
if it be friable, or capable of disintegration; and for ever, if,
being compact, it does not carry in it the causes of destruction. The
action of the waves ceasing, the slope is covered with vegetation;
and if the coast continues, nevertheless, to be worn, the changes are
then owing to causes unconnected with the action of water.
Such is, in few words, the ordinary action of the water of the sea
upon steep coasts, and even that of great masses of water in a
state of agitation. M. De Luc, in his various works, has estimated
this action with a correctness of observation and of reasoning,
which is remarkable only, because it has not been adopted by all
naturalists; and few have bestowed the unremitting attention upon
the subject which this respectable geologist has done. He has shewn,
that the destructive action of the waters upon steep shores, and
other coasts or abrupt cliffs, was considerably restrained by the
very consequences of this action; that the debris which accumulated
protected the lower parts of these coasts from the action of the
water, or gradually reduced an abrupt coast to a very inclined and
permanent slope.
Next, to _torrents_, to rapid and large _rivers_, and to _waves_,
it is to _currents_ that a great influence on the earth’s surface
has been attributed,--an influence which a highly gifted naturalist,
Buffon, has employed to explain all the inequalities of the earth’s
surface.
Our knowledge of the action of currents is less precise than that
which we possess of rivers. But if we cannot so visibly demonstrate
that, in no circumstance similar to those which we have specified,
do they scoop out the bottom of the sea into valleys, nor form any
mountains, we can, at least, conjecture with much probability, and
maintain, that we have no direct and constant proof of that action.
4. _Action of Currents._
No one doubts that currents, near coasts, heap up upon the beach,
at the mouth of rivers and harbours, pebbles, sand, gravel, mud, or
other transportable matters, whether these currents constantly exist,
or simply result from the momentary action of a predominating wind;
but this action, although already limited to the mobile matters which
form the bottom of the sea only in some parts, whether this action,
I say, extends to a great depth, that is to several hundred yards,
is a question not yet resolved. In the first place, the observation
made by mariners, that, in the most violent tempests, the sea is only
agitated towards the coasts, or on shallows, and that bodies, sunk
to a great depth, (and still what is this depth in comparison with
that of the sea,) do not feel the motions of its surface, nor that of
currents; and, secondly, reasoning, and even calculation, according
to Messieurs La Place and Poisson, concur to shew, that the violent
motions of the waters of the sea are not propagated to a great depth.
It is therefore probable, that all the transportable matters, which
are at this depth, must remain nearly in the position in which they
are, since our Continents have assumed their present configuration,
unless phenomena and motions of the sea take place at the bottom, of
which we are ignorant, and which are foreign to the subject which at
present occupies our attention.
But if we have no perfectly certain ideas regarding the propagation
of the motions of the sea in depth, we can assert, that, whatever
that extent and that power may be, the submarine currents no more
abrade the rocks than rivers do the surface of the land. This
proof is always derived from the same kind of fact, namely, from
the vegetable and animal bodies which constantly cover the rocks,
and which are found, at all times, by means of various sorts of
dredge-fishing. In fact, no one has remarked, that the places in
which oysters, mussels, corals and sponges are fished, are more
sheltered from currents than others; nor that these places, after
violent tempests, have been deprived, and consequently, as it were,
despoiled of those productions, which, by covering the rocks,
demonstrate that they preserve the integrity of their surface. Many
of these bodies, however, as sponges, fuci and confervæ, contract but
a feeble adherence to the bodies upon which they are placed.
* * * * *
It therefore appears, if not completely proved, at least extremely
probable, from the facts and reasonings which we have related,
1. That the _presently existing_ waters, that is to say, in the state
of purity in which we are acquainted with them, have no erosive
action upon rocks, whatever be the nature of these rocks, when, 1st,
The rocks are completely solid, and when they are neither friable
nor disintegrated; 2d, When these waters act by themselves, that is
to say, when their action is not complicated with the really erosive
action of solid bodies, such as pebbles, sand, and perhaps even
pieces of ice.
2. That water, sometimes acquiring, on account of its quality and
velocity, a great transporting power, may remove masses, already
detached, and of great size, according to its degree of velocity, and
the bulk of its mass, and so far as it preserves this same power.
3. That the presently existing waters may have attacked, undermined,
and caused to fall down, portions of solid and steep rocks, by mixing
with beds of clay, marl, and sand, interposed between their solid
strata; that they may also, in their rapid falls, have scooped pretty
deep ravines in very inclined deposites, consisting of disintegrated
rocks; but that these waters could not have scooped out, either
by a violent action, or by a slow one, however long continued, any
of those long and broad longitudinal depressions, which are named
valleys, or of those narrow openings, with almost vertical walls,
which are named _gorges_ or ravines.
4. That, even when the deposites, which border these valleys or
these ravines, are composed of transportable matter, the waters
which at present flow in them could not have scooped them out, even
supposing them to have been much larger in some than they now are;
the declivity of the present deposite not being sufficiently great to
give to these masses of water the rapidity necessary for producing
this effect, and a power sufficient for carrying off the moveable
matters which filled the valley or gorge.
5. Lastly, that the present running waters, so far from having
contributed to form the numerous valleys, glens, gorges and ravines,
continually tend to fill them up, and rather to level the surface of
the globe than to furrow it, more deeply than it is.
_Vid._ Brongniart _sur l’Eau_.
NOTE
_On the Connection of Geology with Agriculture and Planting[410]._
That all sorts of soils are not equally adapted to all productions,
is a remark of Virgil’s, the truth of which becomes obvious, when
we consider many facts ascertained in Agriculture and Forestry. If,
therefore, as the poet advises, our object be to determine what each
particular region can produce, and what it cannot, our attention
ought in the first place to be directed to the physical circumstances
which exert their influence over vegetation.
All plants that are the subject of cultivation are fixed in the
ground. By one of their parts, through which they derive their
principal nourishment, they penetrate into the soil, which serves
them as a basis, and affords them the means of procuring subsistence;
by the other part they raise themselves into the atmosphere, which
is not only necessary in itself for their existence, but is also the
medium through which they derive the warming and vivifying influence
of the solar rays. Hence we can understand how much the existence of
plants must be influenced by differences in the condition of the soil
and air.
The superficial crust of the globe is formed of soil capable
of producing vegetables. This productive soil, however, is not
everywhere continuous, being interrupted on the one hand by the
watery covering of the earth, and on the other by perennial snow and
bare rock. Where soil does occur, it separates the solid mass of the
earth from the atmosphere, and is the porous medium through which the
gaseous and watery parts of the latter may act in a greater or less
degree upon the former. It is very seldom that strata of vegetable
soil lie beneath strata of other matters; and where they occur in
this position, the overlying strata are either of volcanic or of
alluvial origin. Of the former case, a very remarkable example occurs
in the Isle of Bourbon, in which large tracts covered with vegetables
and even trees, have been laid waste and overwhelmed by streams of
lava; and large rivers in their overflowings occasionally leave
deposits of various characters, over the productive soil containing
remains of formerly existing plants.
Productive soil, as well in regard to its _situation_ as to its
_constitution_, depends upon the nature and condition of the rocks
which form the solid mass of the earth. It is always of secondary
formation, compared with the rock on which it rests, its principal
parts usually originating from the decomposition of this rock. While
the forms of the surface of the solid mass of the earth, have much
influence upon the action of the atmosphere, they also in some degree
modify that of climate. From these circumstances it would appear that
the solid substrata of productive soil exert an influence in various
ways upon vegetables; whence it follows that, in order to obtain a
more intimate knowledge of the conditions which operate upon their
existence, it is necessary to call in geology to our assistance.
Although the scientific study of agriculture has made great progress
in our times, the relations which exist between the constitution
of the solid crust of the earth, and the formation and nature of
vegetable soil, present a wide field for investigation. Geologists
have hitherto too much neglected the examination of the productive
covering of the earth, and those who have treated scientifically
of agriculture and forestry have usually looked upon the vegetable
soil in its own simple capacity, without regard to its foundation
and origin. To point out the way by which we are to proceed in our
investigation of the relations which exist between the solid crust of
the earth and the productive soil which covers it, is the principal
object of the following observations.
Bare rocks cannot be made subservient to the purposes of agriculture.
Lichens indeed, cover the surface of rocks, deriving their chief
nutriment from the atmosphere; mosses draw the water necessary for
their subsistence from the fissures of stones; the roots of grasses
seek in the chinks of rocks for particles of earth sufficient for
their sustenance; various shrubs and trees penetrate here and there
into rocky masses by their roots (having the powerful and continued
action of living wedges), where the cohesion of the parts is
smallest, in order to prepare a fixed seat for themselves, and be
secure from the pernicious effects of the atmosphere. The surface of
the earth is always sterile, however, when it shows a continuity of
naked rock, uncovered by vegetable mould. The cultivation of fields
and woods, and even the rearing of cattle, cannot therefore find
scope in regions which are entirely rocky. Abrupt and precipitous
mountains being generally in this condition are usually barren; but
in plains and on declivities, a bare rocky surface is much less
frequently the cause of sterility than an unfavourable proportion
of mould. Some rocky and moderately elevated regions also occur,
more or less destitute of vegetable mould, whose sterility depends
upon volcanic causes. Iceland, for example, affords cases of this
description. In many parts of Sweden, as in Westrogothia, in
Scotland, &c., there occur many elevated regions, in which gneiss
and granite predominating, exclude to a great extent all kinds of
vegetation excepting lichens. In the same districts we sometimes meet
with pastures and corn-fields interrupted here and there by bare
rocks rising but little above the surface, by which the value of
the ground is much diminished, and great impediments opposed to its
cultivation.
As bare rocks are incapable of all cultivation, their distance
from the under surface of vegetable mould must also be of great
importance. In the plains of the north of Germany, for example, this
distance is often so great that a rocky surface is never found,
while, on the contrary, in other countries, especially such as are
mountainous, the roots of plants not unfrequently touch the subjacent
rock; the variation between these extremes being of all degrees. The
effect of the distance of the surface of the solid rock from the
under surface of productive soil may be both _direct_ and _indirect_,
and may vary much, not only with reference to the species of rock,
but also to the vegetables.
The surface of the solid strata of the earth has a _direct_ influence
upon the cultivation of plants, because it terminates the extension
of their roots, and limits the volume of the soil necessary for their
sustenance. As the length and direction of the roots vary exceedingly
in different species, the difference of effect with regard to their
growth, and the approximation of the rock to the under surface of the
soil, must in general be so much the less prejudicial in proportion
as the roots decline from the perpendicular; whence it follows, that
certain grasses, and some small pasture plants, may grow in very
thin layers of soil, where the larger grasses and pasture plants
with longer roots, could not find subsistence; and that shrubs and
trees, with long perpendicular roots, cannot survive in many places,
where others with more horizontal roots may thrive. These inferences
are proved to be correct by observations in agriculture and forestry
generally known.
Mountainous regions, which are not so elevated but that corn might
grow sufficiently well in them, in so far as depends upon the
conditions of the air or climate, are yet frequently not adapted for
its cultivation, on account of the too near approach of the rock to
the surface, or shallowness of the soil, and produce nothing but
grasses, and some other pasture plants, among which, however, there
is the greatest difference in this respect. _Trifolium montanum_, for
example, can support itself on rocky mountains, where _T. pratense_
could not grow. _Hedysarum onobrychis_ grows luxuriantly on the sunny
declivities of calcareous mountains, where _Medicago sativa_ (Lucern)
does not find a suitable station. The cultivation of this excellent
pasture plant in some mountainous regions, especially where the rocks
are calcareous, has not proved so advantageous as might have been
expected, because the plants have died out in the course of a few
years; whereas, in proper places, where its very long roots find a
sufficient depth of soil, they usually last for a great length of
time.