former times, as low as the present bottom of the river; but there is
often clear evidence, that it has run or stood much lower, because the
alluvial land reaches far below the present level of the river. This is
known to hold in very many instances, where it has happened that pits
have been sunk to considerable depths on the banks of large rivers. By
that means, the depth of the alluvial ground, under the present bed of
the river, has been discovered to be great; and from this arises the
difficulty, so generally experienced, of finding good foundations for
bridges that are built over rivers in large valleys, or open plains,
the ground being composed of travelled materials to an unknown depth,
without any thing like the native or solid strata. In such cases, it
is evident, that formerly the water must have been much lower, as well
as much higher, than its present level, and this is only consistent
with the notion, that the place was once occupied by a deep lake.
319. If, following the light derived from these indications, we go
back to the time when the river ran above the highest of those levels
at which it has left any traces of its operations, we shall see it
composed of a series of lakes and cataracts, from which, by the
filling up of the one, and the wearing down of the other, the waters
have at length worked out to themselves a quiet and uninterrupted
passage to the ocean. We may, indeed, on good evidence, go back still
farther than the succession of such meadows or terraces, as are above
mentioned, will carry us, and may consider the whole valley, or
trough of the river, as produced by its own operations. The original
inequalities of the surface, and the disposition of the strata, must
no doubt have determined the water courses at first; but this does not
hinder us from considering the rivers as having modified and changed
those inequalities, and as the proximate causes of the shape and
configuration which the surface has now assumed.
320. From this gradual change of lakes into rivers, it follows, that
a lake is but a temporary and accidental condition of a river, which
is every day approaching to its termination; and the truth of this is
attested, not only by the lakes that have existed, but also by those
that continue to exist. Where any considerable stream enters a lake,
a flat meadow is usually observed increasing from year to year. The
soil of this meadow is disposed in horizontal strata: the meadow is
terminated by a marsh; which marsh is acquiring solidity, and is soon
to be converted into a meadow, as the meadow will be into an arable
field. All this while the sediment of the river makes its way slowly
into the lake, forming a mound or bank under the surface of the water,
with a pretty rapid slope toward the lake. This mound increases by the
addition of new earth, sand, and gravel, poured in over the slope; and
thus the progress of filling up continually advances.
321. In small lakes, this progress may easily be traced; and will be
found singularly conspicuous in that beautiful assemblage of lakes,
which so highly adorns the mountain scenery of Westmoreland and
Cumberland. Among these a great number of instances appear, in which
lakes are either partially filled up, or have entirely disappeared. In
the Lake of Keswick, we not only discover the marks of filling up at
the upper end, which extend far into Borrowdale, from which valley a
small river flows into the lake; but we have the clearest proof, that
this lake was once united to that of Bassenthwaite, and occupied the
whole valley from Borrowdale to Ouse-Bridge. These two lakes are at
present joined only by a stream, which runs from the former into the
latter, and their continuity is interrupted by a considerable piece of
alluvial land, composed of beds of earth and gravel, without rock, or
any appearance of the native strata. This separation, therefore, seems
no other than a _bar_, formed by the influx of two rivers, that enter
the valley here from opposite sides, the Greata from the east, and
Newland's water from the west. The surface of this meadow is at present
twelve or fifteen feet at least above the level of either lake and a
quantity of water of that depth must therefore have been drawn off by
the deepening of the issue at Ouse-Bridge, through which the water of
both lakes passes, in its way to the ocean.
Many more examples, similar to this, may be collected from the same
lakes; there are indeed few places from which, in this branch of
geology, more information may be collected.
322. The larger lakes exemplify the same progress. Where the Rhone
enters the Lake of Geneva, the beach has been observed to receive an
annual increase; and the Portus Valesiæ, now Port Valais, which is at
present half a league from the lake, was formerly close upon its bank.
Indeed, the sediments of the Rhone appear clearly to have formed the
valley through which it runs, to a distance of about three leagues at
least from the place where the river now discharges itself into the
lake. The ground there is perfectly horizontal, composed of sand and
mud, little raised above the level of the river, and full of marshes.
The deposition made by the Rhone after it enters the lake, is visible
to the eye; and may be seen falling down in clouds to the bottom.
The great lakes of North America are undergoing the same changes, and,
it would seem, even with more rapidity. As the rivers, however, which
supply these vast reservoirs, are none of them very great, the filling
up is much less remarkable than the draining off of the water, by the
deepening of the outlet. An intelligent traveller has remarked, that
in Lake Superior itself the diminution of the waters is apparent,
and that marks can be discovered on the rocks, of the surface having
been six feet higher than it is at present. In the smaller lakes this
diminution is still more evident.[164] In some of those far inland, the
ground all round appeared to the same traveller to be the deposit from
the rivers, of which the lakes themselves may be considered as a mere
expansion.[165]
[Footnote 164: Mackenzie's Voyages through the Continent of North
America to the Frozen and Pacific Oceans, p. xlii. and xxxvi.]
[Footnote 165: _Ibid._ p. 122.]
323. In order to give uniform declivities to the rivers, the lakes
must not only be filled up or drained, but the cataract, wherever
there is one, must be worn away. The latter is an operation in all
cases visible. The stream, as it precipitates itself over the rocks,
hurries along with it, not only sand and gravel, but occasionally large
stones, which grind and wear down the rock with a force proportioned
to their magnitude and acceleration. The smooth surface of the rocks
in all waterfalls, their rounded surface, and curious excavations,
are the most satisfactory proofs of the constant attrition which they
endure; and, where the rocks are deeply intersected, these marks
often reach to a great height above the level on which the water now
flows. The phenomena, in such instances, are among the arguments best
calculated to remove all incredulity respecting the waste which rivers
have produced, and are continuing to produce. They suffer no doubt to
remain, that the height and asperity of every waterfall are continually
diminishing; that innumerable cataracts are entirely obliterated; that
those which remain are verging toward the same end, and that the Falls
of Montmorenci and Niagara must ultimately disappear.
324. Though there can be no doubt of the justness of the preceding
conclusions, when applied to lakes in general, some apparent
exceptions occur, in which the progress of draining and filling up
seems to have been suspended, or even to have gone in a contrary
direction. These exceptions consist of the lakes which appear to
have received a greater quantity of materials than was sufficient to
have filled them up. Such, for example, is the Lake of Geneva, which
receives the Rhone descending from the Valais, one of the deepest and
longest valleys on the surface of the earth. Now, if this valley, or
even a large proportion of it, had been excavated by the Rhone itself,
as our theory leads us to suppose, the lake ought to have been entirely
filled up, because the materials brought down by the river seem to be
much greater than the lake, on any reasonable supposition concerning
its original magnitude, can possibly have received. What, then, it
may be said, has become of all that the Rhone has brought down and
deposited in it? The lake, at this moment, retains, in some places, the
depth of more than 1000 feet; and yet, of all that the Rhone carries
into it, nothing but the pure water issues. If it has been continuing
to diminish, both in superficial extent and in depth, from the time
when the Rhone began to run into it, what must have been its original
dimensions?
I cannot pretend to remove entirely the difficulty which is here
stated; yet I think the following remarks may go some length in doing
so.
325. It is certain, that from the present state of the Lake of Geneva,
and of the ground round it, we can hardly draw any inference as to its
original dimensions. Saussure has traced, with his usual skill, the
marks of the course of the Rhone, on a level greatly above the present;
and, by observations on the side of Mont Saleve, has found proofs of
the running of water, at least 200 toises above the present superficies
of the lake. But, if ever the superficies of the lake stood at this
height, or at this height nearly, though we can conjecture but little
concerning the state of the adjacent country, which no doubt was also
on a higher level, the lake may very well be supposed to have been of
far greater dimensions than it is now. It may have occupied the whole
space from Jura to Saleve, and included the Lake of Neufchâtel; so
that it may have been of magnitude sufficient to receive the spoils of
the Valais, which, as the surface of its waters lowered, may have been
washed away and carried down to the sea. Thus it may have afforded a
temporary receptacle for the _debris_ of the Alps, and may have served
for an _entrepot_, as it were, where those _debris_ were deposited,
before they were carried to the place of their ultimate destination.
326. But the great depth which the lake has at present, still remains
to be explained, because no mud or gravel could be carried beyond the
gulf, of a thousand feet deep, which was here ready to receive it. The
reality of this difficulty must be acknowledged; and some cause seems
to act, if not in the generation, yet certainly in the preservation of
lakes, with which we are but little acquainted We can indeed imagine
some causes of that kind to occur in the course of the degradation
of the land, which may produce new lakes, or increase the dimensions
of the old. The wearing away of a stratum, or body of strata, may
lay bare, and render accessible to the water, some beds of mineral
substances soluble in that fluid. The district, for instance, in
Cheshire, which contains rock-salt, extends over a tract of fourteen or
fifteen miles, and is covered by a thick stratum of clay, more or less
indurated, which defends the salt from the water at the surface, and
preserves the whole mass in a state of dryness. Should this covering be
broke open by any natural convulsion, or should it be worn away, as it
must be in the progress of the general detritus, the water would gain
admission to the saline strata, would gradually dissolve them, and
form of course a very deep and extensive lake, where all was before dry
land. This event is not only possible, but it should seem, that in the
course of things it must necessarily happen.
327. Something of this kind may have taken place in the track of the
Rhone, and may have produced the Leman Lake. It is not impossible,
that, at a very remote period, the Rhone descended from the Alps
without forming any lake, or at least any lake of which the remains are
now existing; and this supposition, which is more probable than that of
§ 325, we shall soon find to be conformable to appearances of another
kind. The river may have wore away the secondary limestone strata over
which it took its course after it left the schistus of the mountains;
and, in doing so, may have reached some stratum of a saline nature, and
this being washed out, may have left behind it a lake, which is but
modern compared with many of the revolutions that have happened on the
surface of the earth.[166]
[Footnote 166: There are salt springs at Bex, near Aigle, about ten
miles from the head of the lake: saline strata, therefore, are probably
at no great distance.]
This explanation is no doubt hypothetical; but it is proposed in one
of those cases, in which hypothetical reasonings are warranted by
the strictest rules of philosophical investigation. It is proposed
in a case, where the causes visible to man seem inadequate to the
effect, and where we must therefore have recourse to an agent that is
invisible. If the operations ascribed to this agent are conformable to
the analogy of nature, it is all that can in reason be required.
328. Another circumstance may also influence the generation and
preservation of lakes; but it is also one with which we are but little
acquainted. The strata, and indeed the whole body of mineral substances
which forms the basis of our land, have been raised up from the bottom
of the sea, by a progress that should seem in general to have been
gradual and slow. Appearances, however, are not wanting, which show,
that this progress is not uniform; and that both rising and sinking in
the surface of the land, or in the rocks which are the base of it, have
happened within a period of time, which is by no means of great extent.
In this progress, the elevations and depressions may not be the same
for every spot. They may be partial, and one part of a stratum, or body
of strata, may rise to a greater height, or be more depressed, than
another. It is not impossible, that this process may affect the depth
of lakes, and change the relative level of their sides and bottom.
329. All lakes, however, do not involve the difficulty which the
preceding conjectures are intended to remove. The great lakes of North
America do not, for instance, receive their supply from very large
rivers. Of course, it is not from a tract great in comparison of
themselves, that the waste and detritus is brought down into them; and
it seems not at all wonderful, that, without being filled up, they have
been able to receive it. The same, in a degree at least, is true of
many other lakes.
It should also be considered, that we may err greatly in the estimate
we make of the materials actually carried down and deposited in any
lake. To judge of their entire amount, we should know the original
form of the inequalities on the earth's surface; of the quantity of
depression which existed, independently of the rivers; and though,
in general, these original inequalities may be overlooked, and the
present considered as made by the running of water, yet, in particular
instances, this may be far from true. The Valais, for example, which
we consider as the work of the Rhone, may, when the Alps rose out of
the sea, have included many depressions of the surface, which the river
joined together, and, from being a series of lakes, formed into one
great valley.
* * * * *
330. The mouths by which rivers on bold rocky coasts discharge their
waters into the sea, afford a very striking confirmation of the
conclusions concerning the general system of waste and degradation
which have been drawn above. At these mouths we usually see, not only
the bed of the river, but frequently a considerable valley, cut out
of the solid rock, while that rock preserves its elevation, and its
precipitous aspect, wherever it is not intersected by a run of water.
No convulsion that can have torn asunder the rocks; no breach that can
have been made in them, antecedent to the running of the waters, will
account for the circumstance of every river finding a corresponding
opening, by which it makes its way to the sea; for that opening being
so nearly proportional to the magnitude of the river, and for such
breaches never occurring but where streams of water are found.
331. The actual survey of any bold and rocky coast, will make this
clearer than any general statement can possibly do. Let us take, for an
example, the coast of the British Channel, from Torbay to the Land's
End, which is faced by a continued rampart of high cliffs, formed
of much indurated and primeval rock. If we consider the breaches in
this rampart, at the mouths of the Dart, of the Plym and Tamer, of
the river at Fowey, of the Fal, the Hel, &c. it will appear perfectly
clear, that they have been produced by their respective streams. Where
there is no stream, there is no breach in the rock, no softening in
the bold and stern aspect which this shore every where presents to the
ocean. If we look at the smaller streams, we find them working their
way through the cliffs at the present moment; and we see the steps by
which the larger valleys of the Dart and the Tamer have been cut down
to the level of the sea. If we would have still clearer evidence, that
no breaches made antecedently to the running of the rivers have opened
a way for them, we need only look to the opposite side, or northern
shore, of the same promontory, where we also find a series of outlets,
all originating in the ridge of the country, and becoming deeper as
they approach the sea, but altogether unconnected with the openings
on the south side; and this could hardly have been the case, had they
been the effects of previous concussions, or of any peculiarity in the
original structure of the rocks.
332. In contemplating such coasts as these, when we go back to the
time when the rivers ran upon a level as high as the highest of the
cliffs on the sea shore, we must suppose, that the land then extended
many miles farther into what is now occupied by the sea. When at
Plymouth, for instance, the Tamer and the Plym flowed on the level of
Mount Edgecombe or of Staten Heights, if the rivers ran with a moderate
declivity into the sea, the coast must have advanced many miles beyond
its present line. Thus the land, when higher, was also more extended,
and the limits of our island in that ancient state, were doubtless very
different from these by which it is at present circumscribed.
If with the same views we consider any other of the bold coasts which
the map of the world presents us with, we shall quickly remark, that
wherever a deep intersection of the sea is made into the land, as on
the western shores of our own island, or on those of Norway, a river
runs in at the head of it, and points out by what means such inlets
are formed, viz. by the united powers of the sea and of the land, the
waters of the latter having opened the way by which those of the former
have penetrated so far into the country.
333. It is not meant assuredly to deny the irregularities of the sea
coast, as it may have originally existed; these irregularities no doubt
determined the initial operations of that waste and decay, by which, in
process of time, they were themselves entirely effaced. The line of
our coasts may be compared to one of those curves, which are sometimes
treated of in the higher geometry, where the ordinates are functions,
not only of, their abscissæ, but also of the time elapsed since a
certain epocha. The form of the curve at that epocha, or when the time
began to flow, corresponds to the original form of the sea coast, on
its emerging from the ocean, and before the powers of wasting and decay
had begun to act upon it. To speak strictly, the original figure, in
both cases, influences all the subsequent; but the farther removed from
it in point of time, the less is that influence; so that, in physical
questions, and for the purpose of such approximations as suit the
imperfection of our knowledge, the consideration of the original figure
may be wholly left out.
NOTE XVII. § 105.
_Remains of Decomposed Rocks._
334. THE plain of Crau was the _Campus Lapideus_ of the ancients; and,
as mythology always seeks to connect itself with the extraordinary
facts in natural history, it was said to be the spot where Hercules,
fighting with the sons of Neptune, and being in want of weapons, was
supplied from heaven by a shower of stones: hence it was called _Campus
Herculeus_.
This plain is on the east side of the Rhone, between Salon and Aries:
it is of a triangular form, about twenty square leagues in extent, and
is covered almost entirely with quartzy gravel. This immense collection
of gravel has been supposed by some to have been brought down by the
Durance from the Alps of Dauphine; by others it has been ascribed to
the Rhone; and by many to the sea, as being a work too great for any
river. The explanation mentioned above, § 105, namely, that the loose
gravel on the plain arises from the decomposition of a great stratum
of pudding-stone, which is the basis of the whole, is the opinion of
Saussure, and is founded on his own observations.[167]
[Footnote 167: See Voyages aux Alpes, tom. iii. § 1592 et 1597. See
also on this subject a Memoir by Lamanon, Journal de Physique, tom.
xxii. p. 477; and another by M. De Servieres, _ibid._ p. 270.]
335. The theories that have been contrived for explaining the
phenomena of the plain of Crau, afford an instance of the necessity
of generalizing our observations before we can explain a particular
appearance: in other words, they prove the truth of Lord Bacon's
maxim, That the explanation of a phenomenon should not be sought for
from the study of that phenomenon alone, but from the comparison of it
with others. One of the theories of this plain is, that the breccia,
which is the base of it, is formed from the consolidation of the loose
gravel of the plain, by water percolating through it, and carrying some
cementing substance along with it, or some _lapidific juice_, as it
is called. And indeed, whether the gravel is formed from the breccia,
or the breccia from the gravel, is a question which probably could
never be resolved by the mere examination of the plain itself. But the
question is very soon decided, when we compare what is observed here
with other appearances in the natural history of the earth's surface,
and consider how much more frequent the decomposition of solids is,
than their reconsolidation, in any place above the level of the sea.
336. The argument for the decomposition of stony substances which is
afforded by the state of this singular plain, may be confirmed by
the appearances observed in many extensive tracts of land all over
the world, and especially in some parts of Great Britain. The road
to Exeter from Taunton Dean, between the latter and Honiton, passes
over a large heath or down, considerably elevated above the plain of
Taunton. The rock which is the base of this heath, as far as can be
discovered, is limestone, and over the surface of it large flints, in
the form of gravel, are very thickly spread. There is no higher ground
in the neighbourhood from which this gravel can be supposed to have
come, nor any stream that can have carried it, so that no explanation
of it remains, but that it is formed of the flints contained in beds
of limestone, which are now worn away. The flints on the heath are
precisely of the kind found in limestone; many of them are not much
worn, and cannot have travelled far from the rock in which they were
originally contained. It seems certain, therefore, that they are the
_debris_ of limestone strata, now entirely decomposed, that once lay
above the strata which at present form the base of this elevated plain,
and probably covered them to a considerable height. This explanation
carries the greater probability with it, that any other way of
accounting for the fact in question, as the travelling of the gravel
from higher grounds, or the immersion of the surface under the sea,
will imply changes in the face of the country, incomparably greater
than are here supposed. Our hypothesis seems to give the _minimum_ of
all the kinds of change that can possibly account for the phenomenon.
337. The same remarks may be made on the high plain of Blackdown, which
the road passes over in going from Exeter to the westward. The flints
there are disseminated over the surface as thickly as in the other
instance, and can be explained only on the same supposition.
Again, in the interior of England, beginning from about Worcester
and Birmingham, and proceeding north-east through Warwickshire,
Leicestershire, Nottinghamshire, as far as the south of Yorkshire, a
particular species of highly indurated gravel, formed of granulated
quartz, is found every where in great abundance. This same gravel
extends to the west and north-west, as far as Ashburn in Derbyshire,
and perhaps still farther to the north. The quantity of it about
Birmingham is very remarkable, as well as in many other places; and the
phenomenon is the more surprising, that no rock of the same sort is
seen in its native place. It is such gravel as might be expected in a
mountainous country, in Scotland, for instance, or in Switzerland, but
not at all in the fertile and secondary plains of England.
This enigma is explained, however, when it is observed, that the basis
of the whole tract just described is a red sandstone, often containing
in it a hard quartzy gravel, perfectly similar to that which has just
been mentioned. From the dissolution of beds of this sandstone, which
formerly covered the present, there can be no doubt that this gravel
is derived. But, as the gravel is in general thinly dispersed through
the sandstone, and abounds only in some of its layers, it should
therefore seem, that a vast body of strata must have been worn away and
decomposed, before such quantities of gravel as now exist in the soil
could have been let loose.
338. I have said, that a rock capable of affording such gravel as
this, is not to be found in the tract of country just mentioned.
This however, is not strictly true; for in Worcestershire, between
Bromesgrove and Birmingham, about seven miles from the latter, a rock
is found consisting of indurated strata, greatly elevated, and without
doubt primitive, from the detritus of which such gravel as we are
now speaking of might be produced. These strata seem to rise up from
under the secondary, where they are intersected by the road; and, for
as much as appears, are not of great thickness, so that they cannot
have afforded the materials of this gravel directly, though they may
have done so indirectly, or through the medium of the red sandstone;
that is to say, a primary rock of which they are the remains, may have
afforded materials for the gravel in the sandstone; and this sandstone
may in its turn have afforded the materials of the present soil, and
particularly the gravel contained in it.
339. Pudding-stones being very liable to decomposition, have probably,
in most countries, afforded large proportion of the loose gravel now
found in the soil The mountains, or at least hills, of this rock, which
are found in many places, prove the great extent of such decomposition.
Mount Rigi, for instance, on the side of the Lake of Lucerne, is
entirely of pudding-stone, and is 742 toises in height, measured from
the level of the lake. By the descriptions given of it, as well as
of other hills of the same kind in Switzerland, we may, without due
attention, be led to suppose that they are entirely formed of loose
gravel. Even M. Saussure's description is chargeable with this fault,
though, when attended to, it will be found to contain a sufficient
proof, that this hill is composed of real pudding-stone.[168] The
nature of the thing also, would be sufficient to convince us, that a
hill, more than 4000 feet in height, could not consist of loose and
unconsolidated materials.
If, then, we regard Mount Rigi as the remains of a body of
pudding-stone strata, we must conclude, that these strata were
originally more extensive, and the adjacent valleys and plains will
serve, in some degree, to measure the quantity of them which time has
destroyed.
[Footnote 168: Voyages aux Alpes, tom. iv. § 1941.]
340. If the theory of unstratified mountains, namely those of
whinstone, porphyry, and granite, be admitted as laid down above, it
will furnish a measure of the destruction which has taken place in the
stratified rocks, and of the vast depredations which have been made
upon them since they were raised up from the bottom of the sea. Like
every other measure, however, of wasting, by a thing that is itself
subject to waste, it can only give a _minimum_, or a limit which the
quantity wasted must necessarily exceed.
The abrupt face of a whinstone rock must be understood as an evidence,
that some body of strata which supported it when fluid, remained
in contact with it, when it was become solid; and if this part of
the mould in which the whinstone was cast, has disappeared, it must
generally be ascribed to the operation of waste and decomposition.
Such a face, for instance, as that which Salisbury _Craig_ presents to
the west, viz. a perpendicular wall of whinstone, about ninety feet
high, raised on a body of sandstone strata of the height of about
300 feet, can have been produced only by having been abutted against
some stratified rock, equally abrupt, and of the same elevation with
itself. Of this rock no part remains.
The basaltic rock of Edinburgh Castle is nearly in the same state. Its
perpendicular sides on the south, west, and north, are now disengaged
from the strata by which they were once encompassed.
341. The granite mountains also, where they are quite unstratified,
give rise to the same conclusion. Those central chains which we find
in so many instances towering above the schistus which cover their
sides, have probably been once completely enveloped by the latter;
and, on this supposition, an estimate may sometimes be formed of the
original height of such mountains. In these estimations, however, some
uncertainty must arise, from our being unable to distinguish between
the effects which are to be ascribed to the fracture and dislocation
that took place when the compound body of stratified and unstratified
rocks was raised up from the bottom of the sea, and the effects
produced by the subsequent waste and decomposition at the surface. In
this, as in many other instances, we are not always able to separate
between the original inequalities of the surface, and those which
wearing has produced.
342. It would be important to ascertain the rate at which the elevation
of mountains decreases, and this is what we may perhaps expect to
be accomplished, by the progress of geological science, and the
multiplying of accurate observations. It has been supposed, that the
Pyrenees diminish about ten inches in a century; but what confidence is
to be put in this estimate, I am unable to determine.[169]
[Footnote 169: Essai sur la Mineralogie des Pyrenées, p. 87.]
A very unequivocal mark of the degradation of mountains is often to
be met with in the heaps of loose stones found on their tops. These
stones, it is obvious, cannot have come from any other place by natural
means, and they are accordingly always sharp and angular, and have none
of the characters of transported rocks. They are said sometimes to
have been brought by men's hands; but this is highly improbable, their
quantity is often so considerable, and the difficulty of transportation
so great. Where any purpose was to be served by heaping them together,
men have availed themselves of the stones that they found ready
prepared on the summit, and have constructed from them cairns, which
have served as signals, useful in their pastoral, and sometimes in
their military occupations.
NOTE XVIII. § 112.
_Transportation of Stones, &c._
343. NATURE supplies the means of tracing with considerable certainty
the migration of fossil bodies on the surface of the earth, as only
the more indurated stones, and those most strongly characterized, can
endure the accidents that must befal them in travelling to a distance
from their native place.
It is a fact very generally observed, that where the valleys among
primitive mountains open into huge plains, the gravel of those plains
consists, of stones, evidently derived from the mountains. The nearer
that any spot is to the mountains, the larger are the gravel stones,
and the less rounded is their figure; and, as the distance increases,
this gravel, which often forms a stratum nearly level, is covered
with a thicker bed of earth or vegetable soil. This progression has
particularly been observed in the valleys of Piémont and the plains of
Lombardy, where a bed of gravel forms the basis of the soil, from the
foot of the Alps to the shores of the Hadriatic.[170] We may collect
from GUETTARD, that a similar gradation is found in the gravel and
earth which cover the great plain of Poland, from Mount Krapack to the
Baltic.[171] The reason of this gradation is evident; the farther the
stones have travelled, and the more rubbing they have endured, the
smaller they grow, the more regular is the figure they assume, and the
greater the quantity of that finer detritus which constitutes the soil.
The washing of the rains and rivers is here obvious; and each of the
three quantities just mentioned, if not directly proportional to the
distance which the stones have migrated from their native place, may
be said, in the language of geometry, to be at least proportional to a
certain function of that distance.
[Footnote 170: Voyages aux Alpes, tom. iii. § 1315.]
[Footnote 171: Mém. Acad. des Sciences, 1762, p. 234, 293, &c.]
344. The immense quantity of _cailloux roulés_, or rounded gravel,
collected in the immediate vicinity of mountainous tracts, has led
some geologists to suppose the existence of ancient currents, which
descended from the mountains, in a quantity, and with a _momentum_,
of which there is no example in the present state of the world. Thus
Saussure imagines, that the hill of Supergue, near Turin, which is
formed of gravel, can only be explained by supposing such currents as
are just mentioned, or what he terms a _debacle_, to have taken place
at some former period.[172] If, however, we ascribe to the mountains
a magnitude and elevation vastly greater than that which they now
possess; if we regard the valleys between them as cut out by the rivers
and torrents from an immense rampart of solid rock, neither materials
sufficiently great, nor agents sufficiently powerful, will appear to
be wanting, for collecting bodies of gravel and other loose materials,
equal to any that are found on the surface of the earth. The necessity
of introducing a _debacle_, or any other unknown agent, to account for
the transportation of fossils, seems to arise from underrating the
effects of action long continued, and not limited by such short periods
as circumscribe the works, and even the observations, of men.
[Footnote 172: Voyages aux Alpes, tom. iii. § 1303.]
345. The supply of gravel and _cailloux roulés_, for the plains
extended at the feet of primitive mountains, is doubtless in many cases
much increased by the pudding-stone, interposed between the secondary
and the primary strata. The beds of pudding-stone contain gravel
already formed on the shores of continents, that ceased to exist before
the present were produced; and the cement of this gravel, yielding
easily to the weather, allows the stones included in it to be washed
down by the torrents, and scattered over the plains. I know not if
the hill of Supergue above mentioned, is not in reality a mass of the
pudding-stone which forms the border of the Alps, and of which the
materials have suffered no transportation since the time of their last
consolidation. This at least is certain, that Saussure, notwithstanding
his accuracy, has sometimes confounded the loose gravel on the surface
with that which is consolidated into rock; an inaccuracy which is to be
charged, as I have elsewhere observed, rather against his system than
himself.
346. The loose stones found on the sides of hills, and the bottoms
of valleys, when traced back to their original place, point out with
demonstrative evidence the great changes which have happened since the
commencement of their journey; and in particular serve to show, that
many valleys which now deeply intersect the surface, had not begun to
be cut out when these stones were first detached from their native
rocks. We know, for instance, that stones under the influence of such
forces as we are now considering, cannot have first descended from one
ridge, and then ascended on the side of an opposite ridge. But the
granite of Mont Blanc has been found, as mentioned above, on the sides
of Jura, and even on the side of it farthest from the Alps. Now, in
the present state of the earth's surface, between the central chain of
the Alps, from which these pieces of granite must have come, and the
ridge of Mont Jura, besides many smaller valleys, there is the great
valley of the Rhone, from the bottom of which, to the place where they
now lie, is a height of not less than 3000 feet. Stones could not, by
any force that we know of, be made to ascend over this height. We must
therefore suppose, that when they travelled from Mont Blanc to Jura,
this deep valley did not exist, but that such an uniform declivity, as
water can run on with rapidity, extended from the one summit to the
other. This supposition accords well with what has been already said
concerning the recent formation of the Leman Lake, and of the present
valley of the Rhone.
347. We can derive, in a matter of this sort, but little and from
calculation; yet we may discover by it, whether our hypothesis
transgresses materially against the laws of probability, and is
inconsistent with physical principles already established. The
horizontal distance from Mont Jura to the granite mountains, at the
head of the Arve, may be accounted fifty geographic miles. Though
we suppose Mont Blanc, and the rest of those mountains, to have been
originally much higher than they are at present, the ridge of Jura must
have been so likewise; and though probably not by an equal quantity,
yet it is the fairest way to suppose the difference of their height to
have been nearly the same in former ages that it is at present, and
it may therefore be taken at 10,000 feet. The declivity of a plane
from the top of Mont Jura to the top of Mont Blanc, would therefore be
about one mile and three quarters in fifty, or one foot in thirty; an
inclination much greater than is necessary for water to run on, even
with extreme rapidity, and more than sufficient to enable a river or
a torrent to carry with it stones or fragments of rock, almost to any
distance.
Saussure, in relating the fact that pieces of granite are found among
the high passes near the summits of Mont Jura, alleges, that they are
only found in spots from which the central chain of the Alps may be
seen. But it should seem that this coincidence is accidental, because,
from whatever cause the transportation of these blocks has proceeded,
the form of the mountains, especially of Mont Jura, must be too much
changed to admit of the supposition, that the places of it from which
Mont Blanc is now visible, are the same from which that mountain was
visible when these stones were transported hither. It may be, however,
that the passes which now exist in Mont Jura are the remains of valleys
or beds of torrents, which once flowed westward from the Alps; and it
is natural, that the fragments from the latter mountains should be
found in the neighbourhood of those ancient water-tracks.
348. Saussure observed in another part of the Alps, that where the
Drance descends from the sides of Mont Velan and the Great St Bernard,
to join the Rhone in the Valais, the valley it runs in lies between
mountains of primary schistus, in which no granite appears, and yet
that the bottom of this valley, toward its lower extremity, is for
a considerable way covered with loose blocks of granite.[173] His
familiar acquaintance with all the rocks of those mountains, led him
immediately to suspect, that these stones came from the granite chain
of Mont Blanc, which is westward of the Drance, and considerably higher
than the intervening mountains. This conjecture was verified by the
observations of one of his friends, who found the stones in question to
agree exactly with a rock at the point of Ornes, the nearest part of
the granite chain.
[Footnote 173: Voyages aux Alpes, tom. ii. § 1022.]
In the present state of the surface, however, the valley of Orsiere
lies between the rocks of Ornex and the valley of the Drance, and would
certainly have intercepted the granite blocks in their way from the one
of these points to the other, if it had existed at the time when they
were passing over that tract. The valley of Orsiere, therefore, was not
formed, when the torrents, or the glaciers transported these fragments
from their native place.
Mountainous countries, when carefully examined, afford so many facts
similar to the preceding, that we should never have done were we
to enumerate all the instances in which they occur. They lead to
conclusions of great use, if we would compare the machinery which
nature actually employs in the transportation of rocks, with the
largest fragments of rock which appear to have been removed, at some
former period, from their native place.
349. For the moving of large masses of rock, the most powerful engines
without doubt which nature employs are the glaciers, those lakes or
rivers of ice which are formed in the highest valleys of the Alps,
and other mountains of the first order. These great masses are in
perpetual motion, undermined by the influx of heat from the earth, and
impelled down the declivities on which they rest by their own enormous
weight, together with that of the innumerable fragments of rock with
which they are loaded. These fragments they gradually transport to
their utmost boundaries, where a formidable wall ascertains the
magnitude, and attests the force, of the great engine by which it was
erected. The immense quantity and size of the rocks thus transported,
have been remarked with astonishment by every observer,[174] and
explain sufficiently how fragments of rock may be put in motion, even
where there is but little declivity, and where the actual surface of
the ground is considerably uneven. In this manner, before the valleys
were cut out in the form they now are, and when the mountains were
still more elevated, huge fragments of rock may have been carried to a
great distance; and it is not wonderful, if these same masses, greatly
diminished in size, and reduced to gravel or sand, have reached the
shores, or even the bottom, of the ocean.
[Footnote 174: The stones collected on the _Glacier de Miage_, when
Saussure visited it, were in such quantity as to conceal the ice
entirely. Voyages aux Alpes, tom. ii. § 854.]
350. Next in force to the glaciers, the torrents are the most powerful
instruments employed in the transportation of stones. These, when
they descend from the sides of mountains, and even where the declivity
of their course is not very great, produce effects which nothing but
direct experience could render credible. The fragments of rock which
oppose the torrent, are rendered specifically lighter by the fluid
in which they are immersed, and lose by that means at least a third
part of their weight: they are, at the same time, impelled by a force
proportional to the square of the velocity with which the water rushes
against them, and proportional also to the quantity of gravel and
stones which it has already put in motion. Perhaps, after taking all
these circumstances into computation, in the midst of a scene perfectly
quiet and undisturbed, a philosopher might remain in doubt as to the
power of torrents to move the enormous bodies of rock which are seen
in the bottom of the narrow valleys or deep glens of a mountainous
country; but his incredulity, says an experienced traveller, will
cease altogether, if he has been surprised by a storm in the midst of
some Alpine region; if he has seen the number and impetuosity of the
cataracts which rushed down the sides of the mountains, and beheld the
ruin which accompanied them; and if, when the tempest was passed, he
has viewed those meadows, which a few hours before were covered with
verdure, now buried under heaps of stones, or overwhelmed by masses of
liquid mud, and the sides of the mountains cut by deep ravines, where
the track of the smallest rivulet was not before to be discovered.[175]
[Footnote 175: See an account of a thunder storm near Bareges, in the
Essai sur la Mineralogie des Pyrenées, p. 134.]
It is but rarely, however, even on occasions like these, that such
vast masses of rock can be seen actually in motion, as are often found
on the surface, apparently removed to a great distance from their
native place. The magnitude of these is so great, in many instances,
that their transportation cannot be explained without supposing,
that the surface was very different when these transportations took
place from what it is at present; that the elevation of the mountains
was greater, and the ground smoother and more uniform, at least in
some directions. If these suppositions are admitted, and they are
countenanced, as we have already seen, by almost every phenomenon in
geology, the difficulties which present themselves here will not appear
insurmountable.
351. One of the largest blocks of granite that we know of, is on
the east side of the lake of Geneva, called _Pierre de Gouté_,
about ten feet in height, with a horizontal section of fifteen by
twenty.[176] Another block not far from it, and nearly of the same
size, has some remains of schistus attached to it. These stones very
much resemble those which have fallen from the _Aiguilles_, in the
valley of Chamouni. The distance from their present situation to those
_Aiguilles_ is about thirty English miles, with many mountains and
valleys at present interposed. By whatever means, therefore, these
blocks were transported, their motion must have been over a surface
of much more uniform declivity than the present. If the surface was
without great inequalities, and its general declivity about one foot
in thirty, as already computed, the glaciers, in the first place, and
the torrents afterwards, may have served for the transportation even of
these rocks.
[Footnote 176: Voyages aux Alpes, tom. i. § 308.]
352. Again, in the narrow vale or glen which separates the Great from
the Little Saleve, the strata are all calcareous, but a great number
of loose blocks of granite and primary schistus are scattered over the
surface. A block of the former, near the lower end of the valley, is
about the size of 1200 cubic feet. Two other large blocks of the same
kind of stone rest on a base of horizontal limestone, elevated two
or three feet above the rest of the surface. This elevation arises
no doubt from the protection which the stones have afforded to the
calcareous beds on which they lie, so that these beds do not wear away
so fast as those which are fully exposed to the weather. But it is
surely to take a very limited view of the operations on the surface,
to suppose, with Saussure, that the parts of the calcareous rock under
these stones has suffered no waste whatsoever, so that the stones
remain now in the identical spot where they were placed by the great
_debacle_ which brought them down from the high Alps.[177] For my part,
I have no doubt that the Arve, which is still at no great distance,
when it ran on a higher level, and in a line different from the
present, aided by the glaciers and superior elevation of the mountains,
was an engine sufficiently powerful for effecting the transportation of
these stones.
[Footnote 177: _Ibid._ § 227.]
353. These phenomena are not peculiar to the Alps, but prevail, in
a greater or less degree, in the vicinity of all primary or granite
mountains. In the island of Arran, a fragment of the same kind with
that which constitutes the upper part of Goatfield, is found on the sea
shore, at least three miles from the nearest granite rock, and with a
bay of the sea intervening. Its dimensions are not far from those of
the _pierre de gouté_. In some former state of the granitic mountains
in that island, the declivity from the top of Goatfield may have been
very uniform, and more rapid than it is at present.
354. Besides glaciers and torrents, which have no doubt been the
principal instruments in producing these changes, other causes
may have occasionally operated. Large stones, when once detached,
and resting on an inclined plane, from the effects of waste and
decomposition, may advance horizontally, at the same time that they
descend perpendicularly, and this will happen though they be not urged
by any torrent, or any thing but their own weight; for the surface
of the ground, as it wastes, remains higher under the stone, and for
a little way round it, than at a greater distance, on account of the
protection which it receives from the stone, as in the instances at
Saleve, just mentioned. The stone itself also becomes rounded at the
bottom; and thus the surface in contact with the ground is diminished
in extent, and the two surfaces rendered convex towards one another. It
must therefore happen, that the support, continually weakening, will at
length give way, and the stone incline or roll toward the lower side,
and may even roll considerably, if its centre of gravity has been high
above its point of support, and if its surface has had much convexity:
Thus the horizontal may very far exceed the perpendicular motion;
and, in the course of ages, the stone may travel to a great distance.
A stone, however, which travels in this manner, must diminish as it
proceeds, and must have been much greater in the beginning than it is
at present.
355. This kind of motion may be aided by particular circumstances.
When a stone rests on an inclined plane, so as to be in a state
not very remote from equilibrium, if a part be taken away from the
upper side, the equilibrium will be lost, and the stone will thereby
be put in motion. That stones which lie on other stones, may, by
wearing, be brought very near an equilibrium, is proved by what are
called _rocking-stones_, or in Cornwall _Logan stones_, which have
sometimes been mistaken for works of art; but are certainly nothing
else than stones, which have been subjected to the universal law
of wasting and decay, in such peculiar circumstances, as nearly to
bring about an equilibrium of that stable kind, which, when slightly
disturbed, re-establishes itself.[178] The Logan stone at the Land's
End, is a mass of granite, weighing more than sixty tons, resting
on a rock of granite, of considerable height, and close on the sea
shore. The two stones touch but in a small spot, their surfaces being
considerably convex towards one another. The uppermost is so nearly
in an equilibrium, that it can be made to vibrate by the strength
of a man, though to overset it entirely would require a vast force.
This arises from the centre of gravity of the stone being somewhat
lower than the centre of curvature of that part of it on which it
has a tendency to roll; the consequence of which is, that any motion
impressed on the stone, forces its centre of gravity to rise, (though
not very considerably,) by which means it returns whenever the force
is removed, and vibrates backward and forward, till it is reduced to
rest. Were it required to remove the stone from its place, it might be
most easily done, by cutting off a part from one side, or blowing it
away by gunpowder; the stone would then lose its balance, would tumble
from its pedestal, and might roll to a considerable distance. Now, what
art is here supposed to perform, nature herself in time will probably
effect. If the waste on one side of this great mass shall exceed that
on the opposite in more than a certain proportion, and it is not likely
that that proportion will be always maintained, the equilibrium of the
Logan stone will be subverted, never to return. Thus we perceive how
motion may be produced by the combined action of the decomposition and
gravitation of large masses of rock.
[Footnote 178: I do not presume so far as to say, that all
rocking-stones are produced by natural means: I have not sufficient
information to justify that assertion; but the great size of that
at the Land's End, its elevated position, and the approaches toward
something of the same kind which are to be seen in other parts of that
shore, prove that it is no work of art. They who ascribe it to the
Druids, do not consider the rapidity with which the Cornish granite
wastes, nor think how improbable it is, that the conditions necessary
to a rocking-stone, whether produced by nature or art, should have
remained the same for sixteen or seventeen hundred years.]
356. Besides the gradual waste to which stones exposed to the
atmosphere are necessarily subject, those of a great size appear to be
liable to splitting, and dividing into large portions, no doubt from
their weight. This may be observed in almost all stones that happen to
be in such circumstances as we are now considering; and from this cause
the subversion of their balance may be more sudden, and of greater
amount, than could be expected from their gradual decay.
Thus, if to the wasting of a stone at the bottom, we add the accidents
that may befal it in the wasting of its sides, we see at least the
physical possibility of detached stones being put in motion, merely
by their own weight. It is indeed remarkable, that some of the largest
of these stones rest on very narrow bases. Those at the foot of
Saleve touch the ground only in a few points: The Boulder stone of
Borrowdale is supported on a narrow ridge like the keel of a ship, and
is prevented from tumbling by a stone or two, that serve as a kind of
shores to prop it up. Very unexpected accidents sometimes happen to
disturb the rest of such fragments of rock as have once migrated from
their own place. Saussure mentions a great mass of _lapis ollaris_[179]
that lies detached on the side of a declivity in the valley of Urseren,
in the canton of Uri. The people use this stone as a quarry, and are
working it away on the upper side, in consequence of which it will
probably be soon overset, and will roll to the bottom of the valley.
[Footnote 179: Voyages aux Alpes, tom. iv. § 1851.]
357. In many instances it cannot be doubted, that stones of the kind
here referred to are the remains of masses or veins of whinstone or
granite, now worn away, and that they have travelled but a very short
way, or perhaps not at all, from their original place. Many of the
large blocks of whinstone which we find in this country, sometimes
single, and sometimes scattered in considerable abundance over a
particular spot, are certainly to be referred to this cause. But the
most remarkable examples of this sort are the stones found at the Cape
of Good Hope, on the hill called _Paarlberg_, which takes its name from
a chain of large round stones, like the pearls of a necklace, that
passes over the summit. Two of these, placed near the highest point,
are called the Pearl and the Diamond, and were mentioned several years
ago in the Philosophical Transactions.[180] From a more recent account,
these stones appear to be a species of granite, though the hill on
which they lie is composed of sandstone strata.[181] The Pearl is a
naked rock, that rises to the height of 400 feet above the summit of
the hill; the Diamond is higher, but its base is less, and it is more
inaccessible.
[Footnote 180: Vol. lxviii. p. 102.]
[Footnote 181: Barrow's Travels into Southern Africa, p. 60.]
From the above stones forming a regular chain, as well as from the
immense size of the two largest, it is impossible to suppose that they
have been moved; and it is infinitely more probable, that they are
parts of a granite vein, which runs across the sandstone strata, and
of which some parts have resisted the action of the weather, while the
rest have yielded to it. The whole geological history of this part of
Africa seems highly interesting, since, as far as can be collected from
the accounts of the ingenious traveller just mentioned, it consists
of horizontal beds of sandstone or limestone, resting immediately on
granite, or on primary schistus. Loose blocks of granite are seen in
great abundance at the foot of the Table Mountain, and along the sea
shore.
* * * * *
358. The system which accounts for such phenomena as have been
considered in this and some of the preceding notes, by the operation
of a great deluge, or _debacle_, as it is called, has been already
mentioned. In Dr Hutton's theory, nothing whatever is ascribed to such
accidental and unknown causes; and, though their existence is not
absolutely denied, their effects, whatever they may have been, are
alleged to be entirely obliterated, so that they can be referred to
no other class but that of mere possibilities. A minute discussion,
however, of the question, Whether there are, on the surface of the
earth, any effects that require the interposition of an extraordinary
cause, would lead into a longer digression than is suited to this
place. I shall briefly state what appear to be the principal
objections to all such explanations of the phenomena of geology.
359. The general structure of valleys among mountains, is highly
unfavourable to the notion that they were produced by any single great
torrent, which swept over the surface of the earth. In some instances,
valleys diverge, as it were from a centre, in all directions. In
others, they originate from a ridge, and proceed with equal depth and
extent on both sides of it, plainly indicating, that the force which
produced them was _nothing_, or evanescent at the summit of that ridge,
and increased on both sides, as the distance from the ridge increased.
The working of water collected from the rains and the snows, and
seeking its way from a higher to a lower level, is the only cause we
know of, which is subject to this law.
360. Again, if we consider a valley as a space, which perhaps with many
windings and irregularities, has been hollowed out of the solid rock,
it is plain, that no force of water, suddenly applied, could loosen
and remove the great mass of stone which has actually disappeared.
The greatest column of water that could be brought to act against
such a mass, whatever be the velocity we ascribe to it, could not
break asunder and displace beds of rock many leagues in length, and
in continuity with the rock on either side of them. The slow working
of water, on the other hand, or the powers that we see every day in
action, are quite sufficient for this effect, if time only is allowed
them.
361. Some valleys are so particularly constructed, as to carry with
them a still stronger refutation of the existence of a _debacle_. These
are the longitudinal valleys, which have the openings by which the
water is discharged, not at one extremity, but at the broadside Such is
that on the east side of Mont Blanc, deeply excavated on the confines
of the granite and schistus rock, and extending parallel to the beds of
the latter, from the Col de la Segne to the Col de Ferret; its opening
is nearly in the middle, from which the Dora issues, and takes its
course through a great valley, nearly at right angles to the chain of
the Alps, and to the valley just mentioned. From the structure of these
valleys, Saussure has argued very justly against Buffon's hypothesis,
concerning the formation of valleys by currents at the bottom of the
sea.[182] It affords indeed a complete refutation of that hypothesis:
and it affords one no less complete of the system which Saussure
himself seems on some occasions so much inclined to support. For if it
be said, that this valley was cut out by the current of a _debacle_,
that current must either have run in the direction of the valley of
Ferret, or in that of the Dora, which issues from it. If it had the
direction of the first, it could not cut out the second; and if it had
the direction of the second, it could not cut out the first. Besides,
the force which excavated this valley must have been _nothing_ at the
two extreme points, viz. at the Col de la Segne and the Col de Ferret,
and must have increased with the distance from each. It can have been
produced, therefore, only by the running of two streams in opposite
directions, on a surface that was but slightly uneven, these streams at
meeting taking a new direction, nearly at right angles to the former. A
clearer proof could hardly be required than is afforded in this case,
that what is now a deep valley was formerly solid rock, which the
running of the waters has gradually worn away; and that the waters,
when they began to run, were on a level as high, at least, as the tops
of those mountains by which the valley is bounded toward the lower side.
[Footnote 182: Voyage aux Alpes, tom. ii. § 920.]
362. Longitudinal valleys, with the water bursting out transversely
from their sides, like the preceding, are by no means confined to
mountains of the first order. We have a very good example, though
on a small scale, of a valley of this sort, within a few miles of
Edinburgh. The Pentland Hills form a double ridge, separated by a
small longitudinal valley, that runs from N. E. to S. W., the water of
which issues from an opening almost in the middle, and directed towards
the south. This, therefore, is not the work of any great torrent, which
overwhelmed the country; for no one direction, which it is possible
to assign to such a torrent, will afford an explanation, both of the
valley and its outlet.[183]
[Footnote 183: In Scotland there is one valley, of a kind that I
believe is extremely rare in any part of the world, in accounting for
which, the hypothesis of a torrent or _debacle_ might, if any where,
be employed to advantage. This is the valley which extends across the
island, from Inverness to Fort William, or from sea to sea, being open
at both ends, and very little elevated in the middle. It is nearly
straight, and of a very uniform breadth, except that towards each end
it widens considerably. The bottom, reckoning transversely, is flat,
without any gradual slope from the sides towards the middle. From the
sides the mountains rise immediately, and form two continued ridges
of great height, like ramparts or embankments on each side of a large
fossé. A great part of the bottom of this singular valley is occupied
by lakes, namely, Loch Ness, Loch Oich, and Loch Lochy. Its length is
about sixty-two miles, and the point of partition from which the waters
run different ways, viz. north-east to the German Ocean, and south-west
to the Atlantic, is between Loch Oich and Loch Lochy; and, by the
estimation of the eye, I should hardly think that it is elevated more
than ten or fifteen feet above the surface of either lake. The country
on both sides is rugged and mountainous, and the streams which descend
from thence into the valley, either fall directly into the lakes, or
turn off almost at right angles when they enter the valley. Though
the bottom of this valley, therefore, is every where alluvial, with
the exception, perhaps, of a few rocks which appear at the surface,
it is certainly not excavated by the rivers which now flow in it. The
direction of the valley, it is to be observed, is the same with that of
the vertical strata which compose the mountain on either side.
Here, then, we have a valley, not cut out by the working of any streams
which now appear; and we may therefore make trial of the hypothesis
of a _debacle_. This, however, will afford us no assistance; because,
if we suppose what is now hollow to have been once occupied by the
same kind of rock which is on either side, no force of torrents can
have suddenly loosened and removed from its place a body of such vast
magnitude. A greater column of water, than one having for its base a
transverse section of the valley, could not act against it, and this
would have to overcome the cohesion and inertia of a column of rock
of the same section, and of the length of sixty-two miles. It is not
hazarding much to affirm, that no velocity which could be communicated
to water, not even that which it could acquire by falling from an
infinite height, could give to it a force in any degree adequate to
this great effect.
The explanation of this valley, which appears to me the most probable,
is the following. It will be shown hereafter, that there is good reason
to suppose, that, in most parts of our island, the relative level of
the sea and land has been in past ages considerably higher than it is
at present. In such circumstances, this valley may have been under
the surface of the sea, the highest part of it being scarcely 100
feet above that level at present. It may have been a kind of sound,
therefore, or strait, which connected the German Sea with the Atlantic;
and the strong currents, which, on account of the different times
of high water in these two seas, must have run alternately up and
down this strait, may have produced that flatness of the bottom, and
straightness of the sides, and that widening at the extremities, which
are mentioned above. In this way, too, some difficulties are removed
relative to Loch Ness, which is so deep as hardly to be consistent with
the indefinite length of the period of waste that must be ascribed to
the mountains on each side of it. Its depth is said, where greatest,
not to be less than 180 fathoms. According to this hypothesis, it may,
at no very distant period, have been a part of the bottom of the sea.]
363. They who maintain the existence of the _debacle_, will no doubt
allege, that though these valleys were not cut out by means of it, yet
others may. But it must be recollected, that if some of the greatest
and deepest valleys on the face of the earth, such as that just
mentioned, on the east side of Mont Blanc, are thus shown to be the
work of the daily wasting of the surface, what other inequalities can
be great enough to require the interposition of a more powerful cause?
If a _dignus vindice nodus_ does not exist here, in what part of the
natural history of the earth is it likely to be found?
364. The large masses of rock so often met with at a distance from
their original place, are one of the arguments used for the _debacle_.
It has, however, been shown, that, supposing a form of the earth's
surface considerably different from the present, especially, supposing
the absence of the valleys which the rivers have gradually cut out, the
transportation of such stones is not impossible, even by such powers
as nature employs at present. Now, without the supposition that the
surface was more continuous, and that its present inequalities did not
exist, no force of torrents, whatever their velocity and magnitude may
have been, could have produced this transportation. No force of water
could raise a stone like the _pierre de goutté_ from the bottom of
a valley, to the top of a steep hail. Indeed, if we suppose a great
fragment of rock to be hurried along on a horizontal or an inclined
plane, by the force of water, the moment it comes to a deep valley, and
has to rise up over an ascent of a certain steepness, it will remain
at rest; the water itself will lose its velocity, and the heavy bodies
which it carried with it will proceed no farther. Thus, therefore, we
have the following dilemma. If the surface is not supposed to have
had a certain degree of uniformity in past times, a _debacle_ is
insufficient for the transportation of stones: If it is supposed to
have had that uniformity, a _debacle_ is unnecessary.
365. Another fact, which has been supposed favourable to the opinion
of the action of great torrents at some former period, is, that in
countries like that round Edinburgh, where whinstone hills rise up from
among secondary strata, a remarkable uniformity is observed in the
direction of their abrupt faces. Thus, in the country just mentioned,
the steep faces generally front the west, whiles in the opposite
direction, the slope is gentle, and the hills decline gradually into
the plain. Hence it is supposed, that a torrent, sweeping from west to
east, has carried off the strata from the west side of these hills,
but, being obstructed by the whinstone rock, has left the strata on
the east side in their natural place.
But, besides that no force which can ever be ascribed to a torrent
could have removed at once bodies of strata 300 or 400 feet, nay even
800 or 1000 in thickness, which must have been the case if this were
the true explanation of the fact, there is a circumstance which may
perhaps enable us to explain these phenomena without the assistance of
any extraordinary cause. The secondary strata in which the whinstone
hills are found in this part of Scotland, are not horizontal, but
rise or _head_ towards the west, dipping towards the east. The side,
therefore, of the whinstone hills which is precipitous, is the same
with that towards which the strata rise. Now, from the manner in which
these hills are supposed to have been elevated, the strata are likely
to have been most broken and shattered towards that side, while, on the
opposite, they had the support of the whinstone rock. They would become
a prey, therefore, more easily to the common causes of erosion and
waste on the upper side than on the lower. The streams that flowed from
the higher grounds would wear them on the former most readily; and the
action of these streams would be resisted by the superior hardness of
the whinstone, just as the great torrent of the _debacle_ is supposed
to have been.
It should also be observed, that this fact of the uniform direction of
the abrupt faces of mountains, is often too hastily generalized. In
primitive countries, it is no farther observed than by the steep faces
of the mountains being most frequently turned toward the central chain.
In Scotland, as soon as you leave the flat country, and enter the
Highlands, the scarps of the hills face indiscriminately all the points
of the compass, and are directed as often to the east as to the west.
366. Where the strata are nearly horizontal, they afford the most
distinct information concerning the direction and progress of the
wasting of the land. The inclined position of the strata, which in all
other cases must enter for so much into our estimate of the causes
which have produced the present inequality of the earth's surface,
disappears there entirely; and the whole of that inequality is to be
ascribed to the operations at the surface, whether they have been
sudden or gradual. A very important fact from a country of this
sort, is related by BARROW, in his Travels into Southern Africa. The
mountains about the Cape of Good Hope, and as far to the north as that
ingenious traveller prosecuted his journey, are chiefly of horizontal
strata of sandstone and limestone, exhibiting the appearance, on their
abrupt sides, of regular layers of masonry, of towers, fortifications,
&c. Now, among all these mountains, he observed, that the high or
steep sides look constantly down the rivers, while the sloping or
inclined sides have just the opposite direction. When, in travelling
northward, he passed the line of partition, where the waters from
running south take their direction to the north, he found, that the
gradual slope, which had hitherto been turned to the north, was now
turned to the south: The abrupt aspect of the mountains, in like
manner, from facing the south, was directed to the north; so that,
in both cases, the hills turned their backs on the line of greatest
elevation.[184]
[Footnote 184: Barrow's Travels into Southern Africa, p. 245.]
It is evident, therefore, that the form of this land has been
determined by the slow working of the streams. The causes which
produced the effects here described, began their action from the line
of greatest elevation, and extended it from thence on both sides, in
opposite directions. This is the most precise character that can mark
the alluvial operations, and distinguish them from the overwhelming
power of a great _debacle_.
367. Lastly, if there were any where a hill, or any large mass composed
of broken and shapeless stones, thrown together like rubbish, and
neither worked into gravel nor disposed with any regularity, we must
ascribe it to some other cause than the ordinary _detritus_ and
wasting of the land. This, however, has never yet occurred; and it
seems best to wait till the phenomenon is observed, before we seek for
the explanation of it.
368. These arguments appear to me conclusive against the necessity of
supposing the action of sudden and irregular causes on the surface of
the earth. In this, however, I am perhaps deceived: neither Pallas,
nor Saussure, nor Dolomieu, nor any other author who has espoused
the hypothesis of such causes, has explained his notions with any
precision; on the contrary, they have all spoken with such reserve and
mystery, as seemed to betray the weakness, but may have concealed the
strength of their cause. I have therefore been combating an enemy, that
was in some respects unknown; and I may have supposed him dislodged,
only because I could not penetrate to his strongholds. The question,
however, is likely soon to assume a more determinate form. A zealous
friend of Dr Hutton's theory, has lately[185] declared his approbation
of the hypothesis which has here been represented as so adverse to that
theory; and, from his ability and vigour of research, it is likely to
receive every improvement of which it is susceptible.
[Footnote 185: Trans. Royal Society Edin. vol. v. p 68.]
NOTE XIX. § 117.
_Transportation of Materials by the Sea._
369. THE existence of the great and extensive operations, by which the
spoils of the land are carried all over the ocean, and spread out on
the bottom of it, may be supposed to require some further elucidation.
We must attend, therefore, to the following circumstances.
When the detritus of the land is delivered by the rivers into the sea,
the heaviest parts are deposited first, and the lighter are carried
to a greater distance from the shore. The accumulation of matter
which would be made in this manner on the coast, is prevented by the
farther operation of the tides and currents, in consequence of which
the substances deposited continue to be worn away, and are gradually
removed farther from the land. The reality of this operation is
certain; for otherwise we should have on the sea shore a constant and
unlimited accumulation of sand and gravel, which, being perpetually
brought down from the land, would continually increase on the shore, if
nature did not employ some machinery for removing the advanced part
into the sea, in proportion to the supply from behind.
The constant agitation of the waters, and the declivity of the
bottom, are no doubt the causes of this gradual and widely extended
deposition. A soft mass of alluvial deposit, having its pores filled
with water, and being subject to the vibrations of a superincumbent
fluid, will yield to the pressure of that fluid on the side of the
least resistance, that is, on the side toward the sea, and thus will
be gradually extended more and more over the bottom. This will happen
not only to the finer parts of the detritus, but even to the grosser,
such as sand and gravel. For suppose that a body of gravel rests on
a plane somewhat inclined, at the same time that it is covered with
water to a considerable depth, that water being subject not only to
moderate reciprocations, but also to such violent agitation as we see
occasionally communicated to the waters of the ocean; the gravel,
being rendered lighter by its immersion in the water, and on that
account more moveable, will, when the undulations are considerable,
be alternately heaved up and let down again. Now, at each time that
it is heaved up, however small the space may be, it must be somewhat
accelerated in its descent, and will hardly settle on the same point
where it rested before. Thus it will gain a little ground at each
undulation, and will slowly make its way towards the depths of the
ocean, or to the lowest situation it can reach. This, as far as we may
presume to follow a progress which is not the subject of immediate
observation, is one of the great means by which loose materials of
every kind are transported to a great distance, and spread out in beds
at the bottom of the ocean.
370. The lighter parts are more easily carried to great distances,
being actually suspended in the water, by which they are very gradually
and slowly deposited. A remarkable proof of this is furnished from an
observation made by Lord Mulgrave, in his voyage to the North Pole.
In the latitude of 65° nearly, and about 250 miles distant from the
nearest land, which was the coast of Norway, he sounded with a line of
688 fathoms, or 4098 feet; and the lead, when it struck the ground,
sunk in a soft blue clay to the depth of ten feet.[186] The tenuity
and fineness of the mud, which allowed the lead to sink so deep into
it, must have resulted from a deposition of the lighter kinds of
earth, which being suspended in the water, had been carried to a great
distance, and were now without doubt forming a regular stratum at the
bottom of the sea.
[Footnote 186: Phipps's Voyage, p. 74, 141.]
371. The quantity of detritus brought down by the rivers, and
distributed in this manner over the bottom of the sea, is so great,
that several narrow seas have been thereby rendered sensibly shallower.
The Baltic has been computed to decrease in depth at the rate of
forty inches in a hundred years. The Yellow Sea, which is a large
gulf contained between the coast of China and the peninsula of Corea,
receives so much mud from the great rivers that run into it, that it
takes its colour, as well as its name, from that circumstance; and the
European mariners who have lately navigated it, observed, that the
mud was drawn up by the ships, so as to be visible in their wake to a
considerable distance.[187] Computations have been made of the time
that it will require to fill up this gulf, and to withdraw it entirely
from the dominion of the ocean: but the data are not sufficiently exact
to afford any precise result, and are no doubt particularly defective
from this cause, that much of the earth carried into the gulf by the
rivers, must be carried out of it by the currents and tides, and
the finer parts wafted probably to great distances in the Pacific
Ocean.[188] The mere attempt, however, towards such a computation,
shows how evident the progress of filling up is to every attentive
observer; and, though it may not ascertain the measure, it sufficiently
declares the reality of the operations, by which the waste of the
present continents is made subservient to the formation of new land.
[Footnote 187: Staunton's Account of the Embassy to China, vol. i. p.
448.]
[Footnote 188: Perouse, in sailing along the coast of China, from
Formosa to the strait between Corea and Japan, though generally
fifty or sixty leagues from the land, had soundings at the depth of
forty-five fathoms, and sometimes at that of twenty-two. Atlas du
Voyage de la Perouse, No. 43.]
372. Sandbanks, such as abound in the German Ocean, to whatever they
owe their origin, are certainly modified, and their form determined,
by the tides and currents. Without the operation of these last, banks
of loose sand and mud could hardly preserve their form, and remain
intersected by many narrow channels. The formation of the banks on the
coast of Holland, and even of the Dogger Bank itself, has been ascribed
to the meeting of tides, by which a state of tranquillity is produced
in the waters, and of consequence a more copious deposition of their
mud. Even the great bank of Newfoundland seems to be determined in its
extent by the action of the Gulf stream. In the North Sea, the current
which sets out of the Baltic, has evidently determined the shape of
the sandbanks opposite to the coast of Norway, and produced a circular
sweep in them, of which it is impossible to mistake the cause.
In proof of the action here ascribed to the waters of the sea, in
transporting materials to an unlimited extent, we may add the well
known observation, that the stones brought up by the lead from the
bottom of the sea, are generally round and polished, hardly ever sharp
and angular. This could never happen to stones that were not subject to
perpetual attrition.
373. Currents are no doubt the great agents in diffusing the detritus
of the land over the bottom of the sea. These have been long known to
exist; but it is only since the later improvements in navigation, that
they have been understood to constitute a system of great permanence,
regularity, and extent, connected with the trade winds, and other
circumstances in the natural history of the globe. The Gulf stream was
many years since observed to transport the water, and the temperature
of the tropical regions into the climates of the north; and we are
indebted to the researches of Major RENNELL, for the knowledge of a
great system of currents, of which it is only a part. That geographer,
who is so eminent for enriching the details of his science with the
most interesting facts in history or in physics, has shown, that
along the eastern coast of Africa, from about the mouth of the Red
Sea, a current fifty leagues in breadth sets continually towards the
south-west.[189] It doubles the Cape of Good Hope, runs from thence
north-west, preserving on the whole the direction of the coast, but
reaching so far into the ocean, that, about the parallel of St Helena,
its breadth exceeds 1000 miles. From thence, as it approaches the line,
its direction is more nearly east; and meeting in the parallel of 3°
north, with a current which has come along the western coast of Africa
from the north, the two united stretch across the Atlantic, in a line
somewhat south of west, and in a very wide and rapid stream. This
stream meets the American land at Cape St Roque, where it is joined
by another coming up along the eastern shore of that continent, and
directed towards the north. They proceed northward together till they
enter the Gulf of Florida, from which being as it were reflected, they
form the Gulf stream, passing along the coast of North America, and
stretching across the Atlantic to the British Isles. From thence the
current turns to the south, and, proceeding down the coast of Spain
and Africa, meets the stream ascending from the south, as already
described, and thus continues in perpetual circulation. The velocity of
these currents is not less remarkable than their extent. At the Cape
of Good Hope, the rate is thirty nautical miles in twenty four hours;
in some places forty five; and under the line seventy seven. When the
Gulf stream issues from the Straits of Bahama, it runs at the rate of
four miles an hour, and proceeds to the distance of 1800 miles, before
its velocity is reduced to half that quantity. In the parallel of 38°,
near 1000 miles from the above strait, the water of the stream has been
found ten degrees warmer than the air.
[Footnote 189: Geography of Herodotus, p. 672.]
374. The course of the Gulf stream is so fixed and regular, that
nuts and plants from the West Indies are annually thrown ashore on
the Western Islands of Scotland. The mast of a man of war, burnt at
Jamaica, was driven several months afterwards on the Hebrides,[190]
after performing a voyage of more than 4000 miles, under the direction
of a current, which, in the midst of the ocean, maintains its course as
steadily as a river does upon the land.
[Footnote 190: Pennant's Arctic Zoology, Introd. p. 70.]
The great system of currents thus traced through the Atlantic, has no
doubt phenomena corresponding to it in the Indian and Pacific Oceans,
which the industry of future navigators may discover. The whole appears
to be connected with the trade winds, the figure of our continents, the
temperature of the seas themselves, and perhaps with some inequalities
in the structure of the globe. The disturbance produced by these causes
in the equilibrium of the sea, probably reaches to the very bottom of
it, and gives rise to those counter currents, which have sometimes been
discovered at great depths under the surface.[191]
[Footnote 191: Histoire Naturelle de Buffon, Supplément, tom. ix. p.
479. 8vo.]
The great transportation of materials that must result from the
action of these combined currents is obvious, and serves not a little
to diminish our wonder, at finding the productions of one climate
so frequently included among the fossils of another. Amid all the
revolutions of the globe, the economy of nature has been uniform, in
this respect, as well as in so many others, and her laws are the only
thing that have resisted the general movement. The rivers and the
rocks, the seas and the continents, have been changed in all their
parts; but the laws which direct those changes, and the rules to which
they are subject, have remained invariably the same.
375. Objections have been made to that translation of materials by the
waters of the ocean which is supposed in this theory, particularly
by Mr Kirwan, in his Geological Essays; and, though I might perhaps
content myself with the remark already made, that the Neptunian system
involves suppositions concerning the transportation of solid bodies by
the sea, in the early ages of the world, as wonderful as those which,
according to our theory, are common to all ages, I am unwilling to
remain satisfied with a mere _argumentum ad hominem_, where the fallacy
of the reasoning is so easily detected.
376. One of Mr Kirwan's objections to the deposition of materials at
the bottom of the sea, is thus stated: "FRISI has remarked, in his
mathematical discourses, that if any considerable mass of matter were
accumulated in the interior of the ocean, the diurnal motion of the
globe would be disturbed, and consequently it would be perceptible;
a phenomenon, however, of which no history or tradition gives any
account."[192]
[Footnote 192: Geol. Essays, p. 441.]
The appeal made here to Frisi is singularly unfortunate, as that
philosopher has demonstrated the very contrary of Mr Kirwan's
position, and has proved, that the disturbance given to the diurnal
motion by the causes here referred to may be real, but cannot be
perceptible. Having investigated a formula expressing the law which
all such disturbances must necessarily observe, he concludes, "Hàc
autem formulâ manifestum fiet, ex iis omnibus variationibus quæ in
terrestri superficie observari solent, montium et collium abrasione,
dilapsu corporum ponderosiorum in inferiores telluris sinus, nullam
oriri posse variationem _sensibilem_ diurni motûs. Nam si statuamus
data aliqua annorum periodo terrestrem superficiem ad duos usque pedes
abradi undique, eam vero materiæ quantitatem ad profunditatem pedum
1000 dilabi; erit omne quod inde orietur incrementum velocitatis diurni
motûs 30000/(19638051)^2 = 1/12855068184."[193]
[Footnote 193: Frisii Opera, tom. iii. p. 269.]
Here, it is evident, that Frisi admits those very changes on the
surface which we are contending for, and shows, that their tendency is
to accelerate the earth's diurnal motion, but, by a quantity so small,
that, in a space of time amounting at least to 200 years, the increase
of the diurnal motion would only be such a part of the whole as the
preceding fraction is of unity.[194]
[Footnote 194: The time requisite for taking away by waste and erosion
two feet from the surface of all our continents, and depositing it at
the bottom of the sea, cannot be reckoned less than 200 years. The
fraction 1/12855068184, reduced to parts of a day, is 1/148554 of a
second; so that it would require 200 years to shorten the length of
the day, by the above fraction of a second; and therefore it would
require 148554 times 200 years, or 29710800 years, to diminish it an
entire second. The accumulated effect, however, of all the diminutions
during that period, would amount to much more: and if we had any
perfectly uniform standard to compare the motion of the earth with,
its difference from that standard would increase as the squares of the
time, and the total acceleration would amount to one second in 77080
years. Whatever relation this bears to the age of the globe itself, it
exceeds more than ten times the age of any historical record.
Though Frisius concludes, as is stated here, that the acceleration
produced in the diurnal motion of the earth, is far too inconsiderable
to become the object of astronomical observation, he makes a
supposition difficult to be reconciled with this conclusion, namely,
that the acceleration has had a sensible effect on the figure of the
earth, or rather of the sea, having increased the centrifugal force,
and thereby accumulated the waters under the equator, in the present,
more than in former ages. Such an accumulation, he thinks agreeable
to certain appearances that have been observed respecting the ancient
level of the sea. These appearances will be afterwards considered: it
is sufficient to remark here, that though the fraction, expressing the
increment of the centrifugal force, must be double that which expresses
the acceleration, it must be too small to have any perceptible effect
in elevating the sea, except after an immense interval of time; and the
compensations which arise from other causes, probably must prevent it
from becoming sensible in any length of time whatsoever.]
377. The instance just given may serve as one of many, to shew what
confidence is to be placed in that indigested mass of facts and
quotations which Mr Kirwan, without discrimination, and without
discussion, has brought together from all quarters. He has no
intention, I believe, to deceive his readers; but we may judge, from
this specimen, of the precautions he has taken against being deceived
himself.
In some respects, the result of Frisi's investigation must be
considered as imperfect. If there were no relative motion in the parts
of our globe, but that by which things descend from a higher to a lower
level, a continual acceleration of its rotation, though extremely slow,
would take place, as above computed. But as, in the interior of the
earth, there are undoubtedly motions of a tendency opposite to those on
the surface, and directed from the centre towards the circumference,
they must produce a retardation in the diurnal revolution; and from
this must arise an inequality, not uniformly progressive in the same
direction, but periodical, and confined within certain limits, as the
causes are by which it is produced.[195]
[Footnote 195: Even in the descent of bodies from a higher to a lower
level at the surface of the earth, the whole tendency is not to
increase the velocity of the earth's rotation, and many compensations
take place, which, when the matter is considered only in general,
are necessarily overlooked. This will appear evident, if we reflect,
that it is not simply the approach of a body towards the centre of
the earth, or its removal from that centre, which tends to disturb
the rotation of the earth; but its approach to the axis of the earth,
or its removal from that axis. The velocity with which a particle of
matter revolves, whether on the surface, or in the interior of the
globe, is proportional to its distance from the axis of rotation;
and therefore, when a body comes nearer to the axis, it loses a part
of the motion which it had before; which part, of consequence, is
communicated to the whole mass of the earth, and therefore tends to
increase the velocity with which it revolves. The contrary happens when
a body recedes from the axis; for it then receives an addition to its
velocity, which, of course, is taken away from the rotatory motion of
the earth.
Hence, bodies moving in a horizontal plane, may increase or diminish
the swiftness of the diurnal motion, according as they move towards the
poles or towards the equator; and those which descend from a higher to
a lower level, disturb the earth's rotation, much more in consequence
of their horizontal, than of their perpendicular motion. The Ganges,
for instance, though its source is probably elevated no less than
7000 feet above the level of the sea, tends to retard the earth's
rotation, by bringing its waters, and the mud contained in them, from
the parallel of 31° to that of 22°, and so increasing their distance
from the earth's axis by more than 1/12 th part. Had the Ganges flowed
towards the north, as the Nile does, its effect would have been just
the contrary.
In the same manner, a stone descending from the top of a mountain, may
accelerate or retard the earth's rotation, according to the direction
in which it descends. If it descend on the side of the elevated pole,
it will then produce acceleration, because its distance from the axis
will be diminished; but if it descend on the side of the depressed
pole, and if the direction in which it is moved, be over a line less
inclined, than a line drawn from the same point to the depressed pole,
it will then produce a retardation, because its distance from the axis
will be increased.
Let us suppose, for example, that the top of Mount Blanc is in latitude
45° 49′, and that its height is 2450 toises above the level of the sea.
The point at which a line drawn from the top of this mountain, parallel
to the earth's axis, will meet the superficies of the sea, (supposing
that superficies continued inland from the Mediterranean), must be
about 2382 toises in horizontal distance, or about 2-1/2 minutes south
of the summit, that is, in the parallel of 45° 46-1/2′; and if this
parallel be continued all round the globe, the points of the earth's
surface between it and the equator, are all more distant from the
earth's axis than the top of Mount Blanc is; whereas all the points
to the north of it are nearer to that axis. A stone, therefore, from
the top of Mount Blanc, if carried any where to the south of the above
parallel, will retard the earth's diurnal motion; but if carried any
where to the north of the same line, will accelerate that motion.
The same quantity of matter, however, carried an equal distance
toward the pole, and toward the equator, from any point, will lose
more velocity in the former case than it will gain in the latter, as
easily follows from the nature of circle. Therefore, supposing an equal
dispersion of the detritus of a mountain in all directions, the parts
that go toward the pole will most disturb the diurnal motion; and hence
a balance on their side, or in favour of acceleration, as already
observed.]
378. Mr Kirwan's second objection is founded on the misapprehension
of a well-known fact in the natural history of the earth. "Rivers,"
says this author, "do not carry into the sea the spoils which they
bring from the land, but employ them in the formation of deltas of low
alluvial land at their mouths, according to what Major Rennell has
proved." The fact of the formation of _deltas_ from the spoils which
the rivers carry from the higher grounds, is perfectly ascertained;
and the detail into which Major Rennell has entered in the passage
referred to by Mr Kirwan, does credit to the acuteness and accuracy
of that excellent geographer. But it is not there asserted, that
rivers employ _all_ the materials which they carry with them, in the
formation of those deltas, and deliver none of them into the sea. On
the contrary, they carry from the _delta_ itself mud and earth, which
they can deposit no where but in the sea; and it is this circumstance
chiefly that limits the increase of those alluvial lands, and makes
them either cease to increase, or makes them increase very slowly after
a certain period, though the supply of earth from the higher grounds
remains nearly the same. To make Mr Kirwan's argument conclusive, it
would be necessary to prove, that _all_ the mud carried down by the
Nile or the Ganges, was deposited on the low lands before these rivers
enter the sea; a thing so obviously absurd, that nothing but his haste
to obtain a conclusion unfavourable to the Plutonic system, could have
prevented him from perceiving it[196].
[Footnote 196: The instance mentioned in the Geological Essays, from
the travels of the Abbé Fortis, concerning urns thrown into the
Adriatic, upwards of 1400 years ago, and not yet covered with mud,
must be explained from peculiar circumstances, or local causes, with
which we are unacquainted, as it makes against the deposition of earth
near the shore, and in narrow seas; a general fact which, I think,
every body admits.]
379. A remark which Major Rennell has made concerning the mouths of
rivers, in his Geography of Herodotus, deserves Mr Kirwan's attention,
though perhaps he may not be able to put on it an interpretation quite
so favourable to his system. The remark is, that the mouths of great
rivers are often formed on principles quite opposite to one another, so
that some of them have a real delta or triangle of flat land at their
mouths, while others have an estuary, or what may not improperly be
called a _negative_ delta. Of the latter kind are some of the greatest
rivers in the world, the Plata, the Oroonoko and the Maranon, and by
far the greatest number of our European rivers. Nobody can doubt, that
the three rivers just named carry with them as much earth as the Nile,
or the Euphrates, or any other river in the world. All this they have
deposited in the sea, and committed to the currents, which sweep along
the shore of the American continent, and by these they have been spread
out over the unlimited tracts of the ocean.
Indeed, nothing can be more just than Dr Hutton's observation, that
where low land is formed at the mouths of rivers, there the rivers
bring down more than the sea is able to carry away; but that where
such land is not formed, it is because the sea is able to carry off
immediately all the deposit which it receives.
380. Mr Kirwan has denied on another principle the power of the sea to
carry to a distance the materials delivered into it: "Notwithstanding,"
says he, "many particles of earth are by rivers conduced to the sea,
yet _none are conveyed in any distance_, but are either deposited at
their mouths, or rejected by currents or by tides; and the reason
is, because the tide of flood is always more impetuous and forcible
than the tide of ebb, the advancing waves being pressed forward by
the countless number behind them, whereas the retreating are pressed
backward by a far smaller number, as must be evident to an attentive
spectator; and hence it is that all floating things cast into the sea,
are at last thrown on shore, and not conveyed into the mid regions of
the sea, as they should be if the reciprocal undulations of the tides
were equally powerful "[197]
[Footnote 197: Kirwan's Geol. Essays, p. 439.]
381. But if the _attentive spectator_, instead of trusting to a vague
impression, or listening to some crude theory of undulations, reflects
on one of the most simple facts respecting the ebbing and flowing of
the tides, he will be very little disposed to acquiesce in the above
conclusion. He has only to consider, that the flowing of the tide
requires just six hours, and the ebbing of it likewise six hours; so
that the same body of water flows in upon the shore, and retreats from
it, in the same time. The quantity of matter moved, therefore, and the
velocity with which it is moved, are in both cases the same; and it
remains for Mr Kirwan to show in what the difference of their force can
possibly consist.
The force with which the waves usually break upon our shores, does not
arise from the velocity of the tide being greater in one direction
than in another. In the main ocean, the waves have no progressive
motion, and the columns of water alternately rise and fall, without
any other than a reciprocating motion: a kind of equilibrium takes
place among the undulations, and each wave being equally acted upon by
those on opposite sides, remains fixed in its place. Near the shore
this cannot happen; the water on the land side from its shallowness
being incapable of rising to the height necessary to balance the
great undulations which are without. The water runs, therefore, as it
were, from a higher to a lower level, spreading itself towards the
land side. This produces the breakers on our shores, and the surf
of the tropical seas. A rock or a sandbank coming within a certain
distance of the surface, is sufficient, in any part of the ocean, to
obstruct the natural succession of undulations; and, by destroying the
mutual reaction of the waves, to give them a progressive instead of a
reciprocating motion.
382. It is, however, but from a small distance, that the waves are
impelled against the shore with a progressive motion. The border of
breakers that surrounds any coast is narrow, compared with the distance
to which the _detritus_ from the land is confessedly carried; the
water, while it advances at the surface, flows back at the bottom;
and these contrary motions are so nearly equal, that it is but a very
momentary accumulation of the water that is ever produced on any shore.
If it were otherwise, and if it were true that the sea throws out every
thing, and carries away nothing, we should have a constant accumulation
of earth and sand along all shores whatsoever, at least wherever a
stream ran into the sea. This, as is abundantly evident, is quite
contrary to the fact.
So, also, the bars formed at the mouths of rivers, after having
attained a certain magnitude, increase no farther, not because they
cease to receive augmentations from the land, but because their
diminution from the sea, increasing with their magnitude, becomes at
length so great, as completely to balance those augmentations. When
properly examined, therefore, the phenomena, which have been proposed
as most inconsistent with the indefinite transportation of stony
bodies, afford very satisfactory proofs of that operation.
383. It is true, that bodies which float in the water, when carried
along on the tops of the waves towards a shelving beach, having
acquired a certain velocity, are thrown farther in upon the land than
the distance they would have floated to, if they had been simply
sustained by the water. The depth of water, therefore, at the place
where they take the ground, is not likely to be such as to float them
again, and to carry them out towards the sea. They are, therefore, left
behind; and this produces an appearance of a force impelling floating
bodies towards the land, much greater and more general than really
takes place.
These observations may serve to show, how unsound the principles are
from which Mr Kirwan's conclusions are deduced: they are perhaps more
than is necessary for that purpose: it might have been sufficient to
observe, that the increase of land on the sea shore is limited, though
the augmentation from the land is certainly indefinite, a proof that
the diminution from the sea is constant and equal to the increase.
384. "Mariners," says Mr Kirwan, "were accustomed, for some centuries
back, to discover their situation, by the kind of earth or sand brought
up by their sounding plummets; a method which would prove fallacious,
if the surface of the bottom did not continue invariably the same."[198]
[Footnote 198: Geol. Essays, p. 440.]
The fact here stated, that mariners, when navigation was more imperfect
than it is now, had very frequent recourse to this method, and that
they still use it occasionally, is very true. But from this, the only
inference that can be fairly deduced is, that the changes at the bottom
of the sea are very slow, and the variation but little; not merely
from one year to another, but even from one century to another. The
rules by which the mariner judged of his position from the quality of
the earth which the lead brought up, and which were deduced no doubt
from observations made at no very great distance of time, might be
sufficient for his purpose, though a slow change had been all the while
going forward. Such observations could at best have little accuracy,
and could not be affected by small variations. It is the slowness of
the change, that makes the experience of one age applicable, in this,
as in innumerable other instances, to the observations of the next. If
a long interval is taken, we will look in vain for the same uniformity
of results. A pilot, who would at present judge of his position in the
German Ocean, by comparing his soundings with those taken by PYTHEAS,
(supposing them known) in his navigation of that sea, more than 2000
years ago, could hardly be expected to determine his latitude and
longitude with great exactness; and I know not if the most zealous
advocate for the immutability of the earth's surface, would be willing
to trust his safety in a ship that was guided by such antiquated rules.
NOTE XX. § 118.
_Inequalities in the Planetary Motions._
385. The assertion that, in the planetary motions, we discover no
mark, either of the commencement or termination of the present order,
refers to the late discoveries of LA GRANGE and LA PLACE, which have
contributed so much to the perfection of physical astronomy. From
the principle of universal gravitation, these mathematicians have
demonstrated, that all the variations in our system are periodical;
that they are confined within certain limits; and consist of alternate
diminution and increase. The orbits of the planets change not only
their position, but even their magnitude and their form: the longer
axis of each has a slow angular motion; and, though its length remains
fixed, the shorter axis increases and diminishes, so that the form of
the orbit approaches to that of a circle, and recedes from it by turns.
In the same manner, the obliquity of the ecliptic, and the inclination
of the planetary orbits, are subject to change; but the changes are
small, and, being first in one direction, and then in the opposite,
they can never accumulate so as to produce a permanent or a progressive
alteration. Thus, in the celestial motions, no room is left for the
introduction of disorder; no irregularity or disturbance, arising from
the mutual action of the planets, is permitted to increase beyond
certain limits, but each of them, in time, affords a correction for
itself. The general order is constant, in the midst of the variation of
the parts; and, in the language of La Place, there is a certain mean
condition, about which our system perpetually _oscillates_ performing
small vibrations on each side of it, and never receding from it
far.[199] The system is thus endowed with a stability, which can refill
the lapse of unlimited duration; it can only perish by an external
cause, and by the introduction of laws, of which at present no vestige
is to be traced.
[Footnote 199: Exposition du Systéme du Monde, par La Place, Livre iv.
chap. 6. p. 199. 2d edit.]
386. The same _calculus_ to which we are indebted for these sublime
conclusions, informs us of two circumstances, which mark the law
here treated of as an effect of wise design, to the entire exclusion
both of necessity and chance. One of these circumstances consists in
the planetary motions being all in the same direction, or all _in
consequentia_ as it is called by the astronomers. This is essential
to the compensation and stability above mentioned:[200] had one planet
circulated round the sun in a direction from east to west, and another
in a direction from west to east, the disturbances they would have
produced on one another's motion would not necessarily have been
periodical; their irregularities might have continually increased, and
they might have deviated in the course of ages from their original
condition, beyond any limits that can be assigned.
[Footnote 200: La Place, _ibid._]
The other circumstance, on which the stability of our system depends,
is the small eccentricity of the planetary orbits, or their near
approach to circles. Were their orbits very eccentric, an opening
would be given to progressive change, that might so far increase, as
to prove the destruction of the whole. But neither the movement of all
the planets in the same direction, nor the small eccentricity of their
orbits, can be ascribed to accident, since that either of these should
happen by chance, in as many instances as there are planets, both
primary and secondary, is almost infinitely improbable. Again, that any
necessity in the nature of things should have either determined the
_direction_ of the planetary motions, or proportioned the _quantity_
of them to the intensity of the central force, cannot be admitted, as
these are things unavoidably conceived to be quite independent of one
another. It remains, therefore, that we consider the laws, which make
the disturbances in our system correct themselves, and by that means
give firmness and permanence to it, as a proof of the consummate wisdom
with which the whole is constructed.
387. The geological system of Dr Hutton, resembles, in many respects,
that which appears to preside over the heavenly motions. In both, we
perceive continual vicissitude and change, but confined within certain
limits, and never from a certain, mean condition, which is such, that,
in the lapse of time, the deviations from it on the one side, must
become just equal to the deviations from it on the other. In both, a
provision is made for duration of unlimited extent, and the lapse of
time has no effect to wear out or destroy a machine, constructed with
so much wisdom. Where the movements are all so perfect, their beginning
and end must be alike invisible.
NOTE XXI. § 122.
_Changes in the apparent
Level of the Sea._
388. In speaking of the natural epochs marked out by the phenomena
of the mineral kingdom, we have supposed a greater simplicity, and
separation of effects from one another, than probably takes place in
nature. We have, for instance, abstracted, in speaking of the waste
and degradation of the land, from that elevation which may have been
carried on at the same time. This appeared necessary to be done, in
order to simplify as much as possible the view that was to be given of
the whole; but there can be no doubt, that, while the land has been
gradually worn down by the operations on its surface, it has been
raised up by the expansive forces acting from below. There is even
reason to think, that the elevation has not been uniform, but has been
subject to a kind of oscillation, insomuch, that the continents have
both ascended and descended, or have had their level alternately raised
and depressed, independently of all action at the surface, and this
within a period comparatively of no great extent.
It will be easily understood, that the facts we are going to state,
each taken singly, prove nothing more than a change of the line in
which the surface of the sea intersects the surface of the land,
leaving it uncertain to which of the two the change ought really to be
ascribed. Taken in combination, however, these facts may determine what
each of them separately cannot ascertain. I shall first, therefore,
mention some of the principal observations relative to the change above
mentioned, and shall then compare them, in order to discover whether it
is most probable that this change has been produced by the motion of
the land or of the sea.
389. If we begin with examining the coasts of our own island, we shall
find clear evidence every where, that the sea once reached higher up
upon the land than it does at present. The marks of an ancient sea
beach are to be seen beyond the present limits of the tide, and beds
of sea shells, not mineralized, are found in the loose earth or soil,
sometimes as high as thirty feet above the present level of the sea.
Some of these on the shores of the Frith of Forth are very well known,
and have been often mentioned. Indeed, on the shores of that frith,
many monuments appear, which would seem to carry the difference
between the present and the ancient level of the sea, to more than
forty feet. The ground on which the Botanic Garden of Edinburgh is
situated, after a thin covering of soil is removed, consists entirely
of sea sand, very regularly stratified, with layers of a black
carbonaceous matter, in thin lamellæ, interposed between them. Shells I
believe are but rarely found in it, but it has every other appearance
of a sea beach. The height of this ground above the present level of
the sea is certainly not less than forty feet.