On the Revolutions of the Earth's Surface
By Sir JAMES HALL, Bart. Pr. R. S. ED. & F.R.S. LONDON.
PART I
PART II
PART I
Transactions of the Royal Society of Edinburgh, vol v, 1812. P.
139-167.
[Read March 16. 1812]
WE are never more disposed to give credit to a philosophical system,
than when we meet with a case of its successful application, unknown to
the author, or containing circumstances which he had not taken into
account when he formed that system.
The facts brought forward in the following paper, which, according to
my view of the subject, clearly indicate the operation of immense
torrents, can be accounted for, I think, in a very satisfactory manner,
by the Huttonian Theory, and consequently afford some very powerful
arguments in support of it.
But such was by no means the view taken of this subject by Dr
HUTTON himself, or by Mr PLAYFAIR, who, since his death, has so
ably illustrated and maintained his geological opinions; These
gentlemen have expressed themselves, on all occasions, in a manner
peculiarly hostile to the employment of such torrents as geological
agents, believing that all the phenomena may be traced to the influence
of diurnal causes only.
I have no hesitation in declaring my hearty concurrence in what I
consider as the essence of the Huttonian Theory; I mean as to all that
relates to the influence of internal heat in the formation of our rocks
and mountains: But I could never help differing from Dr HUTTON, as to
the particular mode in which he conceived our continents to have risen
from the bottom of the sea, by a motion so gentle, as to leave no trace
of the event, and so as to have had no share in producing the present
state of the Earth's surface.
At an early period of life, while I imbibed from the delightful
conversation of my worthy friend Dr HUTTON, the spirit of his
geological views, I retained my attachment to opinions suggested by M.
DE SAUSSURE's observations, which, at a still earlier period, I had
acquired in the Alps, and which had. been rivetted in my mind by the
sight of the phenomena from which they have been inferred. The facts
also observed in the Russian empire, and brought forward by Professor
PALLAS, relative to the productions of the tropics, which are found
upon the banks of the Frozen Sea, appeared to me of sufficient. force
to justify his belief, that, at some remote period, a torrent of water
had swept across the continent of Asia. I have, therefore, been always
disposed to combine the doctrines of HUTTON with those professed by the
gentlemen just named, relative to marine inundations; and a number of
facts which I have observed in this country, will contribute, I hope,
to throw some additional light upon this difficult subject.
Before mentioning my own observations, I beg leave to lay before the
Society the state in which I found the question. The central ridge of
the Alps, of which Mont Blanc is the principal mountain, consists, as
is well known, of Granite. Nearly parallel to it, and at a distance of
fifty-four miles to the north-west (by Sir GEORGE SHUCKBURGH's
measurement) lies the mountainous ridge of Jura, fifty or sixty miles
in length, composed entirely of calcareous matter. Between these two
ridges are interposed the valley and lake of Geneva and other valleys
and inferior hills. The native place of the granite is confined
to the central ridge; but over the surface just described, innumerable
detached granitic blocks are very irregularly scattered, which have
originated, as we must presume, in that central ridge, and have been
transported to their present position by some mechanical power.
Upon the southern side of the Baltic, a multitude of similar blocks are
found, also scattered irregularly over a sandy district, of which a
particular and detailed account is given in M. DE Luc's Geological
Travels, vol. i.
The origin and history of such blocks, which occur in various other
places, have given rise to considerable discussion, and the question is
inseparably connected with other points of magnitude in geology, such
as the formation of valleys and lakes, and the distribution and
arrangement of various beds of clay and gravel, and of all the loose
and alluvial assemblages which occur under various forms in all parts
of the globe, and constitute its most valuable districts in point of
fertility.
M. DE SAUSSURE ascribes the transportation of the granitic blocks on
the Alps to the action of an immense torrent of water, or Debacle, as
he terms it, which at some remote period, flowing over the summit of
the Alps, had carried these blocks along with it.
Dr HUTTON and Mr. PLAYFAIR, as I have already said, deny the necessity
of introducing such an agent, since the circumstances, as they
conceive, might have been produced by the usual action of rivers.
But this simple view seems to be excluded, when we consider both the
magnitude and the positions of these blocks. Their size, in some
cases, amounts, as. in the valley of Monetier upon Saleve, to 1200
cubic feet, and in the case of those on the Gotean de Boisy, to 2250,
and even to 10,296 cubic feet, which is the measure of the block called
Pierre a' Martin.
To move a mass of granite of even fifty or sixty cubic feet, and to
carry it a few yards, would require the utmost efforts of the Rhone or
the Arve, as they flow near Geneva, in their highest floods, but that
such blocks could be conveyed by one of them along its whole course, is
contrary, I conceive, to all experience, and still more when we
consider that these rivers are divided at their source from beneath the
Glaciers into forty or fifty small streams. Yet from the Glaciers,
these blocks must have come; and when we take into account the
magnitude of some of the granitic masses, it is clear that the task is
beyond the power of any river that flows on the surface of the earth;
nay, it seems more than water, under any predicament, could accomplish,
and more than could be expected from the Debacle itself, however
extravagant its magnitude may appear: but we shall again return to the
subject, and shew in what manner this difficulty may be explained.
These stones do not lie merely in the beds of rivers, but occur all
over the country, and on the summits of mountains, where rivers could
least be conceived to have flowed; nor are they confined to that side
of the country, or to the side of the lake of Geneva which lies next
the Alps; for we find them in particular on the face of Jura, which
fronts the central ridge and the lake, at an elevation of 2000 feet
above the latter. A set of low hills also intervene, which
occasionally hide that central ridge from the view; and it is
principally where the snowy summits are visible from the face of Jura,
by means of some depression in these intervening hills, that we find
those travelled masses; as I remember well to have witnessed, at some
of the places which SAUSSURE has pointed out, where, in high
situations, on the face of Jura, I rode through great assemblages of
granitic blocks, three or four feet in diameter.
The force of this fact is admitted, but an attempt is made, even
under that admission, to refer the whole to diurnal actions, by
supposing, that from the spot where these blocks lie, up to the summit
of Mont Blanc, one continued solid plane has ascended, along which, on
a declivity computed at one in thirty, these blocks may have been
hurried by a stream of water; and that subsequently, in the course of
ages, all the intervening mass had been washed away, so as to reduce
the country to its present situation. But this hypothesis removes
the difficulty of the intervening valleys only; for the transportation
in this case would be scarcely less difficult than it would be to
Geneva, as matters stand at this day; -and a circumstance occurs,
founded upon the observations of Sir GEORGE SHUCKBURGH, which seems
entirely to preclude this hypothesis.
According to his measurement and scale, as given in the
Philosophical Transactions, vol. lxvii, the height of Mont Blanc is
14,432 feet above the lake, and the distance in a straight line from
Jura to Mont Blanc, I find, by his scale, to be nearly fifty-four
miles, and 2,000 feet is the height at which these granite blocks occur
on Jura. The slope, therefore, along which these blocks must have
descended, would be nearly that of one in twenty-three. But
without inquiring whether or not this declivity would be sufficient for
the purpose, it is of consequence to attend to another circumstance
pointed out in these measurements. The line above which snow lies
perpetually, during all summer, is there noted, and lies by the scale
at 7,500 feet perpendicular below the summit of Mont Blanc. It is
easy, then, to calculate to what horizontal distance from the centre of
the ridge, this limit of perpetual snow would extend, and we thus find
it to be 82 miles. But SAUSSURE has found the junction of the
granite with the surrounding strata at the Buet, and by the same scale,
I find the distance of this mountain from Mont Blanc is about 10
miles. It is obvious, then, that in the supposed situation of the
Alps, in which the granitic mass could not be conceived to extend
farther than it now does from the middle, that the whole granitic
surface must not only have been buried in everlasting snow, but that
the extent of this snow must have been three times that of the
granitic. Now there is every reason to suppose, since the
temperature diminishes as the height increases, that within this upper
third, the temperature has never, in the hottest day in summer,
reached so high as 32°; that is to say, that water has never there
existed in a liquid form. We cannot, then, conceive any block,
however small, to have been conveyed from thence by means of water,
acting by the usual diurnal laws.
M. DE LUC admits the reality of M. DE SAUSSURE'S debacle, and accounts
for it in the same manner; but he does not ascribe to it, either in
Germany or in the Alps, the transportation of the granitic rocks, the
presence of which he accounts for very differently. He supposes that
these blocks have nowhere migrated along the surface, but have been
ejected from below, at the places where they now lie. This
ejection he produces by a very extraordinary hypothesis, founded
entirely on gratuitous suppositions, and which affords no plausible
solution of the difficulty, by which we might be tempted to adopt it.
M. DE Luc mentions a theory formed by Mr. WREDE, a Professor of Berlin,
to account for the same blocks. He embraces a belief which seems to
have been in considerable favour, that the level of the Baltic Sea has
been sinking for ages past, and he extends his belief of the same
change taking place in the whole of the North Sea. His idea is, that a
change has been gradually going forward in the position of the centre
of gravity of the earth, which has been moving to the southward, and
that an equivalent rise is experienced in the southern hemisphere. That
when the North Sea stood at its highest level, the granitic blocks had
been transported across the Baltic, by means of the winds, on floats of
ice, and settling in their present places, had been left by the
retiring waters.
M. DE Luc, in a very satisfactory manner, in my opinion, refutes one
essential part of this system, by shewing, from the form of the new
lands, and from other circumstances, that no change in level whatever
has taken place on the surface of that sea for many ages back, and,
indeed, ever since the surrounding country has possessed its present
form. As to the other part of his system, which relates to the
transportation of these blocks by means of ice, we shall soon have
occasion to resume its consideration.
Besides the consequences resulting from these particular phenomena,
there are other material branches of Dr HUTTON'S theory connected with
the same views, which in my opinion require to be carefully revised.
The theory which he has advanced respecting the formation of valleys,
by mere diurnal action, appears to me liable to great objections; and I
cannot help agreeing with M. DE Luc, in much that he has urged against
Mr. PLAYFAIR on this point. I also concur with my friend Sir
GEORGE MacKENZIE, in some general views, suggested to him by the aspect
of certain rocks in Iceland, and in rejecting the slow operations of
the atmosphere (Travels in Iceland, p. 39.)
The difficulty is peculiarly great where a lake occupies the bottom of
the valley, and is very conspicuous in the case of the lake of Geneva
lately mentioned. For, granting all that has been advanced in
support of the diurnal formation of the valley in which this lake lies,
granting that it has been excavated by the diurnal actions, since these
granitic blocks were deposited upon Mount Jura; it still remains to be
explained, how the lake of Geneva itself was formed, the depth of which
amounts in some places (SAUSSURE, art. 44.) to 950 feet. Now, as has
very fairly been stated, this lake is constantly receiving all the
spoils of the district which lies above it, called the Vallais, yet
delivers nothing at Geneva, situated below it, but pure water. It is
evident, then, that every known diurnal action tends to fill up this
lake, and none to excavate it. This is therefore admitted by Mr.
PLAYFAIR, to be a case "in which hypothetical reasonings are warranted
by the strictest rules of philosophical investigation, and where we
must therefore have recourse to an agent that is invisible.”
Illustrations of the Huttonian Theory, p. 366.
This acknowledged difficulty seems calculated to entitle the following
speculations to a patient hearing from this gentleman; and he has
already, in another part of the same work, expressed himse1f with
regard to them, and other kindred opinions, in a manner highly
encouraging to discussion, and which affords one of the most striking
examples of candour that has ever occurred.
“These arguments," Mr. PLAYFAIR says, (referring to those he had just
been stating) art. 367. page 412. “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, perhaps I am deceived,
neither PALLAS nor SAUSSURE, nor DOLOMIEU, nor any 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 seems 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 find his strong holds.”
The charge of obscurity here brought forward, is very applicable to the
diluvian system of SAUSSURE and DE Luc, in what regards the origin of
the torrent, or the disposal of the water after it had overwhelmed the
land The same charge seems not, however, to apply to the
suggestion of Professor PALLAS, who, in an early work, entitled,
Observations sur la Formation des Montagnes, first published in 1777,
has very explicitly ascribed the inundation by which he conceives the
continent of Asia to have been overwhelmed to the action of volcanoes
rising in the Indian Sea, and forming the Moluccas, the Philippines,
and other islands, known or supposed to be volcanic.
It must be admitted, that such an event may occasionally have happened.
But the occurrence of similar catastrophes may be inferred in a manner
still more general and unequivocal, from those Plutonic revolutions
the reality of which has been established by DR HUTTON'S observations.
According to his system, all our strata once lay at the bottom of the
sea, and have been raised into their present situation by the
subterranean and submarine exertions of heat, similar to that which
appears externally in the volcanoes. And the angular fractures
exhibited by these beds on many occasions, prove that this elevation
was performed in such cases when the mass was in a hard state. It is
obvious, then, that the same principle which I lately attempted to
apply (page 87. of this volume) to the volcanic phenomena, as exhibited
in the Atrio del Cavallo, will apply to those Plutonic revolutions; and
we are authorised on these principles to expect, that the liquid
substances of basalt and of granite, in their progress through the
rents of our strata, have been exposed to congelation, like the lava in
the rents of Mount Vesuvius; that their progress has been arrested, and
the protruding energy accumulated in a similar manner, till it acquired
sufficient power to break through that obstacle; or through some other
opposed to it; and it is manifest, however gradual and uniform the
propelling force may have been previous to its accumulation, that the
ultimate laceration must have been performed, by a sudden and violent
motion, producing an earthquake at the surface, and thus affording a
more extensive and more satisfactory solution of that tremendous
Phenomenon that is furnished by the steam of Mr. MITCHELL; though I am
ready to admit, that in the volcanic actions, the production and
condensation of steam, on many occasions, has produced very powerful
effects. In the Plutonic regions, as the restraining mass was
beyond all comparison stronger, and thicker and heavier, than in the
most violent volcanic action, it must have exerted a power of
resistance greater in the same proportion; consequently, the time of
that constraint must have been of incomparably longer duration, and the
violence of the shock, when the fracture did take place, though no less
sudden, must have been incomparably more powerful.
Thus, as Vesuvius, in the course of the middle ages, was once at rest
during several centuries, we have reason to presume that the Plutonic
action, after being suspended for several thousand years, should rush
forward with a degree of violence proportioned to the time of its
previous constraint, and capable of fulfilling all the conditions of
SAUSSURE's debacle, or the wave of PALLAS; and the existence of these
tremendous events is thus indicated a priori by the Huttonian
principles.
I shall now consider, whether such traces have not been actually left
by these waves, as to place the reality of their existence beyond all
doubt. We have already alluded to some of these, and shall have
occasion to mention some others; but in a case of this sort, no
historical, nor even traditional authority need be expected; for though
no limit could be assigned to the magnitude which such a wave might
actually reach, there is a decided limit to the magnitude of one that
could be recorded; since, by exterminating all witnesses, every wave
beyond a certain size would infallibly be the cause of its own
oblivion. Those of a moderate extent are most likely to have been
recorded by man, having sufficient power to produce the most awful
impressions, and yet sparing a sufficient number of witnesses to
transmit the event to future generations.
In our attempts to elucidate this subject, we shall refer, in the first
place, to specimens of this tremendous phenomenon in still lower stages
of its power, since they alone have been recorded in an authentic and
intelligible form. I mean, by referring to those agitations of the sea
which have accompanied most of the great Earthquakes in our own times,
as we learn from the following regular statements.
In the Philosophical Transactions, vol. XLIX. p. 424. we have an
account of the earthquake at Cadiz, by Mr. B. BEWICKE, merchant there,
1st November 1755, in which it is said; “an hour after" (the first
shock) “looking out to sea, we saw a wave coming, at eight miles
distance, which was at least sixty feet higher than common. Every body
began to tremble; the centinels left their posts, as well they did; it
came against the west part of the town, which is very rocky; the rocks
abated a great deal of its force; at last it came upon the walls, and
beat in the breast-work, and carried pieces of eight or ten tons
weight, forty or fifty yards from the walls," &c.
We have another account of the same by Don ANTONIO DE ULLOA, Phil.
Trans. vol. XLIX. p. 427. who describes the wave as having returned
five times after the first. And similar facts are stated of
other scenes of this sort.
One circumstance in addition to those mentioned, which has accompanied
all these great events, and which seems at first sight to contradict
our explanation of them is, that in all the agitations of the water,
the first event has been a retreat of the sea.
Mr. MITCHELL says, Philosophical Transactions, vol. LI. p. 566-611,
speaking of the earthquake at Lisbon in 1755, “the bar (at the mouth of
the Tagus) was seen dry from shore to shore, then suddenly the sea,
like a mountain, came rolling in. This phenomenon accompanied the
same earthquake at the island of Madeira, where we are told, that at
the city of Funchal, the sea, which was quite calm, was observed to
retire suddenly some paces, then rising with a great swell, without the
least noise, and as suddenly advancing, entered the city." He further
states, in the same page, that "in the northern part of the island the
inundation was more violent, the sea retiring there above one hundred
paces at first, and suddenly returning, overflowed the shore, forcing
open doors," &c. Again, in page 466 he says, "The great earthquake
that destroyed Lima and Callao in 1746, seems also to have come from
the sea; for several of the ports on the coast were overwhelmed by a
great wave, which did not arrive till four or five minutes after the
earthquake began, and which was preceded by a retreat of the waters, as
well as that at Lisbon.”
Looking into the original account of these calamities in DON ANTONIO DE
ULLOA's Travels into South America, French edition, vol. i. p.467, I
find that he mentions two events of this sort, which took place at
Callao; one in 1687, in which the sea first retreated, and then
returned with such force as to overwhelm Callao and other places. And
again in 1746, when, in the course of twenty-four hours, two hundred
shocks were felt; on this occasion, the sea retiring as it had done
formerly, on similar occasions, returned furiously, and overwhelmed
Callao so completely, that nothing remained of the whole town but a
remnant of Fort Santa Cruz. Out of twenty-three vessels at anchor
in the harbour, nineteen were sunk, and the four others, one of which
was a frigate, were carried to a great distance on shore, and left on
dry land. At Callao, out of four thousand inhabitants, only two
hundred were saved.
Mr. MITCHELL endeavours to account for this, by his favourite doctrine
of steam, according to which he conceives, that the strata at the
bottom of the sea have been raised as a roof; he then continues,
(p.613.) “now while the roof is raising, the waters of the ocean, over
it, retreat, and flow from thence every way; this, however, being
brought about slow1y, they will have time to retreat so gently, as to
occason no great disturbance; but as soon as some part of the roof
falls in, the cold water contained in its fissures mixing with the
steam, will immediately produce a vacuum, in the same manner as the
water injected into the cylinder of a steam-engine, and the earth
subsiding, and leaving a hollow place above, the waters will flow every
way towards it, and cause a retreat of the sea on all the shores round
about." But the gradual elevation of the roof, keeping tight all the
while, which is here assumed as the steam was collecting, can by no
means be admitted; since, as we have shewn, a mass such as this, when
forced upwards, would have yielded suddenly.
This phenomenon, however, along with all the rest mentioned, may, I
conceive, be traced as consequences of the simple, but rapid, elevation
of the bottom, which we have ascribed to the forcible introduction from
below of stone in liquid fusion. Water being almost incompressible, or
elastic in an extreme degree, it is obvious, that an elevation of that
portion of the fluid which lay immediately upon the part of the bottom
raised, would take place almost simultaneously. This elevation
would of course be the greatest in the middle, the quantity of matter
in the vertical line being the least; but there is no doubt that an
action of the same kind, diminishing in a high ratio, as the distance
from the centre increased, would be extended all round; the surface of
the fluid being thrown into the form of some species of conchoid,
produced by the revolution round the vertical axis c b of the line d c
e, (Plate. VI. fig. 1.) the highest point of which is at c, with
a contrary flexure at f and h, and going off beyond these
points, in such a manner as to have the original level surface of the
water d b e for an asymptote. This conchoidal elevation, produced in
the first moment, being just equal in bulk to the quantity of water
displaced by the solid mass (a a) elevated at the bottom.
If this mass stopped suddenly, as would naturally happen, when, in
consequence of the rupture the protruding liquid stone has found a
vent; it is evident, that every part of the water put in motion during
the first moment, would tend, according to the first law of motion,
still to continue moving during the next and following moments, but
this tendency could not be everywhere effectual because, if all the
water that rose in the first moment were to rise also in the second, a
vacuum would be the consequence. In the second moment, then, a struggle
must take place between the different portions of the fluid; the middle
portion, which had acquired the greatest velocity, would continue to
flow upwards, and its place not being supplied from below, as, during
the first moment, suction would be exerted downwards, upon all the
surrounding fluid. The consequence must be, that the rise which each
particle would have performed by its separate momentum, during this
second moment, would be counteracted by the pressure of the atmosphere,
and entirely done away at a certain distance from the centre, as at f
and h, fig. 2.; beyond that limit the water would sink below the medium
level, as at k and i. An annular depression would thus be
produced, surrounding the central elevation, vertical sections of which
are exhibited by d k f and h i e; that depression below the medium
level being exactly equal in bulk to the additional elevation at the
centre, occasioned by the momentum, the vertical section of which is f
g h c. The middle elevation, and the annular depression, would
continue to increase during a period, and to an amount regulated by
that class of laws, which determines the magnitude and duration of
waves, as relative to the wind which produces them; and the water at
the end of that period would be thrown into the position denoted by the
dotted line in fig. 2, and by the single continued line in fig3,
indicated in both figures by the letters d k f g h i e.
During a third period, the water flowing from without the annular
depression, to fill it up, a wave of depression would be propagated
outwards in all directions, and would visibly reach to places where the
elevation produced by the direct impulse in the first moment, had not
been at all perceptible, in consequence of their distance from the
centre. It is owing to this cause, I am convinced, that the
catastrophes at Lisbon, at Cadiz, at Madeira, and at Lima, in so far as
the position of the surface of the sea was concerned, were all first
announced by a retreat of the sea, as shewn at m, in fig. 4.
During another successive period, the undulatory motion continuing, the
wave of elevation would advance towards the shore at m, and being met
by the retiring water, would produce the curling form called the
breaking of a wave, as shewn at m in figure 5. At that instant
the calamitous effects described in all the great earthquakes, as
resulting from the sea, have been produced.
I have made a few experiments lately with explosions of some pounds of
gunpowder under water, in order to try the effect of sudden impulse. In
every case, a very manifest heave of the surface was produced at the
instant of the explosion, and at that same instant, a very smart
percussion was felt. This was always followed in two or three seconds
by a distinct and separate agitation of the water, occasioned by the
rising of the permanently elastic gases produced in the explosion. The
form given to the wave in fig. 1 has been sketched from the
recollection of that first momentary impulse.
We have thus a theory of those waves which seem almost invariably to
have accompanied the great earthquakes; supposing these to have been
occasioned by submarine elevations, and to form an intermediate step
between the Volcanic eruptions, and the truly Plutonic operations by
which our continents have been elevated.
But, to return to the alpine scenes with which we set out: If the
phenomena on the banks of the lake of Geneva, to which we have alluded,
were really occasioned by a torrent of water, its magnitude must have
been such as to leave few vestiges of the human race, in Europe at
least, to record the circumstance; and we can only expect to find
proofs of its reality in geological facts. The events of Lisbon and of
Callao, though on a scale comparatively diminutive, help to lead our
imagination to the conception of this colossal disaster. It may still,
however, be alleged, as I have already hinted, that it would be
impossible for water of any depth whatever, or moving with any
velocity, to carry blocks of such magnitude to such situations; and the
consideration is of so great importance, that I am induced, in
attempting to unite the ideas of SAUSSURE with those of HUTTON, to
retain part of the system proposed by M. WREDE, in so far as to
consider the granitic blocks as having been made to float, by means of
a mass of ice; attached to each.
A circumstance happened lately in this country which tends to encourage
that idea. Upon the coast of the Murray Frith, a large block of stone,
four or five feet in diameter, lying within high-water mark; and well
known as having served to denote the boundary between two estates, was,
during a stormy night in winter, transported to the distance of ninety
yards, and the persons upon the spot were convinced, that this
migration was performed by means of a large cake of ice, formed round
the stone, and attached to it; and that the whole had. been lifted and
carried forward by the rising tide. The course of this stone was marked
upon the sand below, by a deep and broad furrow, which remained visible
for a long time afterwards, as I have been informed by several members
of this Society, who saw it after an interval of more than a year.
By supposing that an agent of this sort acted in concert with our
diluvian wave, the difficulty with regard to the transportation of the
granitic blocks; seems to be in a great measure removed; and this
explanation happens to be peculiarly applicable to the case in point;
the native place of these blocks being covered profusely with ice, in
the state best calculated for fulfilling the office here assigned to
it. M. DE SAUSSURE has given the following clear account of the
formation of the Glaciers. In any region whose temperature is such that
the heat of summer is not sufficiently powerful to melt all the snow of
winter, the remaining snow, which, by partial thawing, and. subsequent
freezing, is turned into ice, must accumulate year after year to an
unbounded degree, so that the mass must become at last so lofty, as to
be unable to bear its own weight, and must produce a motion downwards,
which will be greatly assisted by fractures, and by the sliding of the
ice upon the ground. In this manner the glaciers are produced, which
consist of an assemblage of great blocks of ice, each as big as a
house, which, sliding and rolling upon each other during summer, make
their way from the high frozen valleys where they were formed, through
narrow glens, into the rich and warm vales below, in which they remain
a considerable time, presenting a curious spectacle among the meadows
and woods, some of which they overtop. These masses, however, melt at
last, in this new situation, and leave vast assemblages of stones,
which had been attached to them, forming a ridge called the Marene of
the Glacier.
It is obvious, then, that a wave washing over these high alpine valleys
in summer, would float and carry off all the ice in the glaciers, and
accumulated in the higher valleys, and, along with the ice, all the
blocks of stone imbedded in it, or attached to it in any way. The
stream, with this load, would find its way through every opening, and
would in a particular manner flow through those depressions, which at
this day, as we have said, afford a view of the snowy summit of Mount
Blanc, from certain places on the face of Jura where these blocks
abound.
The enormous masses already mentioned, which are found near Geneva, and
at the Coteau de Boisy, may now be accounted for; and the same system
will apply also to the blocks upon the Baltic, which may have been
brought to their present place, not by a permanent and steady position
of the ocean, varying by slow degrees, as has been alleged by M. WREDE;
but by a sudden diluvian wave washing over some district, situated
either at a sufficiently high level, or near enough to the pole to be
the seat of glaciers. I am not at present acquainted with any facts by
which the native place of these blocks can be traced; but I trust we
shall not long remain in that state of uncertainty, since there are
means by which that point will be found of very easy decision, as I
shall endeavour to shew in a subsequent part of this paper by examples
in theneighbourhood of Edinburgh.
It is well known, that granite is found native in Sweden; so these
blocks may have been carried across the Baltic, as those of Mont Blanc
have crossed the valley of Geneva. It is possible also, that they may
have been transported by the help of ice from the Alps across Germany,
by the very same torrent we have been considering, and which had left a
portion of its load behind it on Mount Jura.
This view would afford a natural account of the production of Holland,
and of a great part of that quarter of Europe, which consists entirely
of sand, and whose magnitude appears to me very far to surpass any
deposition that could reasonably be ascribed to the present
rivers. All this sand may be conceived to have been hurried along
by that mighty stream, and deposited when the torrent began to spread,
and lose its force by diffusion. It is likely, too, that an
immense quantity of this sand would be carried far into the ocean, and
its deposition being there modified in various ways, by local
tides and currents, might assume the character of horizontal strata, so
as to lay the foundation for future productions of freestone or of
killas. We might thus, by the help of this diluvian agent,
complete the great circle of events, so elegantly pointed out by Dr
HUTTON, but which the diurnal agents seem quite insufficient to fulfil.
As the sand was depositing upon the low countries, the blocks of
granite, with their accompanying ice, from whatever quarter they
originated, would still keep floating, and thus account for a striking
fact stated by M. DE Luc. He observes, that the granitic blocks
lying in the district between Berlin and the Baltic Sea, occur
frequently, and almost constantly, in very numerous assemblages, upon
the summits of the sandy hills with which that country is interspersed,
whilst none are to be met with in the intervening valleys. That they
also abound on the islands of the Baltic; and these blocks shew
themselves upon the beach only in those places where the sea reaches
the base of some of the hills on which they lie.
The present theory affords an easy solution of this fact. In the
descending stream, these hills, constituting shallow sand-banks, would
afford the first resting place for the floating blocks of ice, which,
grounding upon them, would accumulate to very numerous
assemblages, and there deposit their granitic charge, while all the
other similar blocks flowed onwards, in the deep water
between.
We are thus enabled to give a tolerable account of the granite blocks;
but the formation of valleys, and the excavation of lakes, in
particular of the lake of Geneva, remain to be explained. I
trust, however, that the same principles will extricate us from this
difficulty also.
If my view is correct, as to the violent manner in which our continents
have risen from the bottom of the sea, to their utmost elevation in the
atmosphere, it is quite obvious, that the cold and hard external crust,
while it communicated such shocks to the ocean, must itself have
undergone the greatest agitation, and must have been rent and broken in
every conceivable mode. The stone, in liquid fusion, introduced
into the rents, would assist the elevation, in so far as it tended to
facilitate the shifts, by enabling one mass to slide on the other; but
forcing its way upwards, this liquid would at last reach a temperature,
in which, as we have said, it would congeal; its further progress in
that direction, with respect to the neighbouring substance, being thus
effectually stopt, and the propelling force from below, continuing to
act, the local elevation would be converted into a more general one,
either where the strata were in a flexible state, by means of an
horizontal thrust, producing an elevation along with the convolution of
the strata, (in a manner lately pointed out in this Society), or, where
the neighbouring substances were inflexible, by a penetration of the
liquid through the mass, giving rise also to a vertical heave of the
whole. In either case, a number of rents would be formed, in the
hard outer crust, which would widen upwards as the heave advanced,
thus forming the rudiments of valleysof every degree of wideness, from
the narrowest ravine to the most open vale.
All this progress, or at least great part of it, being performed by
successive starts, each of which, while under water, producing a
tremendous wave; the mass, in rising, would be repeatedly exposed to
these diluvian torrents, some formed by itself, and some by
neighbouring elevations, so as at last, after long exposure to agents,
partly diluvian and partly diurnal, it would arrive at its present
situation and condition.
In consequence of the diversity of these elevations in place and in
magnitude, acting upon substances of every sort, an endless diversity
of effect would be the result. In some cases a rent, in
consequence of the locality of the heave, would be rendered both large
and deep in the middle, while it remained nearly closed at both
ends. Water, therefore, could not flow through it without
stagnating by the way, and thus a lake would be formed, the depth of
which might be very great in the middle, though its extremities were
shallow. This applies to the formation of the lake of Geneva,
either before or since the passage of the granitic blocks. It is
applicable also to all lakes which occur in alpine or rocky
districts. It will appear in what follows, that an account no
less satisfactory may be given on other principles of those which
belong to alluvial countries.
An example of the relative changes among rocks, produced by motions of
this sort, occurs in the mass on the coast of Berwickshire, with which
we have been so much occupied in a former meeting, as affording a
display of the convolutions of the strata. Upon this coast, the
killas and sandstone meet on the East near Eyemouth, and on the West in
the parish of Cockburnspath, at the Siccar Point, where the junction is
beautifully displayed. In the neighbourhood of both junctions, the
sandstone strata, laid open by the shore, present to view a striking
picture of former revolutions, as I have endeavoured to represent by
two sketches taken upon the spot. One is at the Hallahole near
the Press, between Berwick and Eyemouth (Plate VII.) on the east; and
the other at the Coveshore (Plate VIII) in the parish of
Cockburnspath on the west.
In both, the sandstone, remote from the junction, is seen in an
honzontal position which seems to be its general character, and which
upon the west it maintains to a great extent; but in the neighbourhood
of the junction, the beds become absolute ly perpendicular, in a rock
which, in both cases, rises to the height of thirty or forty
feet. This change of position takes place differently in the two
cases. That upon the East is sudden and immediate, the whole
change being performed in a single cliff, at the bottom of which the
beds are horizontal, and at its summit vertical, nay, leaning over upon
themselves; on the other it is gradual, the strata being seen in
succession to pass through every intermediate declivity, the change
occupying more than a mile of coast.
This local elevation seems unequivocally to denote a great
perpendicular shift. Either the sandstone mass has sunk and left
the killas behind it, or the killas has risen from below, leaving the
sandstone, and dragging its contiguous beds partly up along with
it. As we know that the whole once lay deep below the sea, and
that it is now raised, it seems more simple to ascribe these changes to
one action, and to believe that the last supposition is the true one.
A fact which I witnessed in Calabria, not long after it occurred,
belongs to the same class, and may bear a similar interpretation. What
is called the Plain of Calabria is an alluvial district, situated along
the western coast, behind which there rises a ridge of primitive
mountains. When the terrible earth-quake of 1783 took place, a very
extraordinary circumstance happened, the remains of which I went to see
in 1785, and found it corresponding to DOLOMIEU’S description. A naked
scar of red earth, facing the plain, made its appearance upon the
primitive mass, all along the line of junction between the two; this
scar being ten or fifteen feet high, and appearing almost universally
in a tract of many miles. DOLOMIEU has given an ingenious theory
of this phenomenon, which appeared to me satisfactory at the time. He
supposes, that in consequence of the agitation of the alluvial mass by
the earthquake, it had undergone a subsidence, or tasserment (as
it is expressed in French), similar to what happens when a bushel of
wheat is struck by the hand; and that in consequence of the subsidence
of the loose country, the firm country had been left behind. I am now,
however, inclined to suggest a different explanation of the same fact,
and to connect it, by analogy, with some of those lately mentioned, by
conceiving that the middle, or rocky mass, may have been raised,
leaving the alluvial mass behind it, and that its motion has been
the proximate cause of the earthquake. It may further be observed,
in favour of this view, that if the alluvial mass did subside in
the manner pointed out by DOLOMIEU, the districts in the neighbourhood
of the sea must have been submerged. It would have been easy, if
such a conjecture had occurred on the spot, to bring it to the test of
observation; for the opposite side of the coast of the peninsula must
have furnished some facts either in confirmation or refutation of the
idea. I mention it here, as a hint to future travellers, in
tracing the result of earthquakes.
I cannot too strongly recommend to geological travellers, the
examination of those scenes where earthquakes, and their attendant
waves, have exerted their influence, and which have probably left
behind them very striking monuments of their power. I conceive
that in this way, the doctrines already advanced in this paper, and
which I am now about more fully to illustrate, will be submitted to the
test of direct observation.
The fact which I have met with most strictly in point, occurs in
HUMBOLDT’S Account of Mexico, Eng. Trans. London, 1811, vol. ii p. 212.
At San Pedro de Jerullo, “in the month of June 1759, a
subterraneous noise was heard. Hollow noises of a most alarming
nature (brarnidos) were accompanied by frequent earthquakes, which
succeeded one another from fifty to sixty days, to the great
consternation of the in habitants of the Hacienda. From the beginning
of September, every thing seemed to announce the complete
re-establishment of tranquillity, when, in the night between the 28th
and 29th, the horrible subterraneous noise recommenced. The
affrighted inhabitants fled to the mountains of Aquasarco. A
tract of ground, from three to four square miles in extent, which goes
by the name of the Malpays, rose up in the shape of a bladder.
The bounds of this convulsion are still distinguishable in the
fractured strata. The Malpays near its edges is only twelve
metres (thirty-nine feet) above the old level of the plain, called
Playas de Jorullo; but the convexity of the ground thus thrown up,
increases progressively towards the centre, to an elevation of one
hundred and sixty metres (five hundred and twenty-four feet)."
In this most striking and interesting scene, we have an actual specimen
of those violent and sudden operations by which our continents have
been raised to their present position, according to my view of HUTTON’S
Theory; and had this event come to his knowledge, which happened in his
day, it might have induced him to admit the probability of those sudden
elevations, indicated by so many facts.
The process of elevation, whether performed gently or rapidly, is free
from a difficulty to which the Systems of both SAUSSURE and WERNER are
exposed: both of these geologists conceive that our continents and
rocky districts were once covered with water, which has since: flowed
away, these rocks maintaining their original position; now, to lay a
rock bare in this manner, we must dispose not only of the water which
covered the immediate mass of rock, but also of that body of the same
fluid which occupied an equal level all over the globe. This difficulty
was strongly felt by Professor PALLAS, who says, (Observations sur la
Formation des Montagnes, p.79.), that were the summits of the mountains
supposed to have been covered, the mass of water required to equal and
surmount them round all the globe could not be disposed of within the
earth, even were its inside made up of caverns. On that account, he
denies that the summit of the hills has been covered. He burdens
himself, however, with a very considerable share of the same
difficulty, by supposing that the sea had stood at such a level as to
submerge hills of 100 fathoms high.
According to our theory, there is no such embarrassment. We suppose
these low hills, as well as the high ones, to have been raised from the
bottom of the sea, which need not be considered as ever having stood
above its present level. And I think myself authorised by the
facts stated in the course of this paper, in deviating so far from the
Huttonian hypothesis, as to believe that the elevation of the land was
formed by successive starts, similar to volcanic eruptions, though far
more rare and more powerful; and that the percussions impressed by
these starts upon the waters of the ocean, were such as to form waves,
sometimes of a moderate force, as those at Lisbon or Callao; sometimes
of overwhelming magnitude, and capable of producing the effects
described in the Alps, in Germany, and in Russia.
As the inferences derived from these distant facts are called in
question by some gentlemen of the highest authority in this Society, I
am happy to have it in my power to produce a set of observations made
in this immediate neighbourhood, which seem in a manner no less
satisfactory, to lead to similar conclusions. These are the subject of
the second part of this communication.
On the Revolutions of the Earth's Surface.
By Sir JAMES HALL, Bart. Pr. R. S. ED. & F.R.S. LOND.
PART II
BEING AN ACCOUNT OF THE DILUVIAN FACTS IN THE NEIGHBOURHOOD OF
EDINBURGH..
(Read June 8. l812.)
Transactions of the Royal Society of Edinburgh, 1812, vol VII, 169-211.
IF such tremendous agents did in reality exert their influence in the
Alps, it is not conceivable that other countries, situated lower, and
composed of materials much more frail, could have been spared. We
hear in fact of similar traces, more or less distinct, in all quarters
of the globe. From the bones of animals, natives of the hottest
climates, which are found over a frozen region of vast extent in the
Russian empire, PALLAS concludes, that an enormous torrent had
transported their carcases across the ridge of Tartary.
On my return from the Alps, looking for traces of the same agents in
this country, I found them in abundance, particularly in the
neighbourhood of Edinburgh; and the circumstances of this sort which I
have met with, both on a large scale and in detail, seem to afford more
precise information as to these events than has hitherto been furnished
by the alpine phenomena.
In order to acquire principles upon which these observations may be
made to advantage, and by which the truth or falsehood of the systems
to which they lead may be brought to a certain test, it is of
importance to examine the results of similar actions, in such analogous
cases as lie in all respects within reach of observation. If a torrent,
like that which is supposed to have inundated the Alps, had flowed over
this country, it must have left behind it traces of various sorts,
resembling in some degree those that occur in the course of any common
river which has recently overflowed its banks. Thus in both cases, sand
and mud, and loose stones, must have been transported and deposited,
and fixed objects must have been overwhelmed and abraded, by the action
of the water and of these moving bodies: The relation of the stream to
these objects, either met or transported, is very different, it is
true, in the two cases, but a sufficient agreement exists between them
to guide us on the present occasion.
It is an undoubted truth that where an obstacle occurs in the course of
a fluid, which is in the act of transporting and depositing heavy
substances, the deposition in the neighbourhood of that obstacle
undergoes considerable modification. The theory of these modifications
would be difficult to determine; but their effects may easily be
traced, by observation in the bed of any stream after a flood, or in
the drifting of snow, fallen during a high wind, or which is in the act
of falling.
Where the obstacle is high and narrow; where its height, for instance,
is at least equal to its breadth, we perceive that a stagnation of the
fluid takes place in the side towards which the stream is flowing, and
a deposition is formed of the transported substances, constituting a
tail or prolongation, which extends in the direction of the stream, by
a gradual descent to the distance frequently of eight or ten times the
height of the obstacle. At the same time, an acceleration
is occasioned at certain places in the neighbourhood, by which the
general deposition, which was going on at the time, and which would
have been universal had no interruption occurred, is there prevented.
This acceleration takes place along that face of the obstacle which
fronts the stream, along both its sides, and along those of the tail,
forming the stagnation just mentioned: the consequence is
the production of a hollow or depression in all those places, below the
leve1of the general deposition in the neighbourhood. The case is
different where the obstacle is of great breadth compared with its
height. When its breadth, for example, is five or six times greater
than its height, the effect is often reversed, and an excavation is
occasioned on the side toward which the stream is flowing.
The action of a current of water upon sand, or of wind upon snow,
previously deposited, produces effects similar to those just pointed
out. Thus, a firm body, occurring in the midst of such an assemblage,
mitigates or prevents the action behind the obstacle, that is, on the
side towards which the current is flowing, and, at the same time,
augments the corrosive energy in front of it, and along the two sides.
Effects are thus produced similar to those in the case last mentioned;
and it may be difficult in many cases to determine whether a particular
assemblage has been produced, by a modification of the first
deposition, or by a second action upon an assemblage previously formed.
The action of a current, with the assistance of the solid masses of
every size transported by it, upon the substance of an solid rock
opposed to it, is subject to certain laws, the principles of which
must, in many respects, be common to it in the two cases just
mentioned, as we shall have occasion to point out more fully in the
course of this paper.
In attempting to apply these principles to the great scale of geology,
and to vindicate my opinions on this curious subject, I shall appeal to
a series of facts which are very accessible to this assembly, the
greatest part of them lying within two or three miles of this city. In
that view, I have given, along with this paper, a plan, on actual
survey, taken on this account, of a small district in the neighbourhood
of Edinburgh, comprehending the Corstorphine Hill and its immediate
neighbourhood. I thus hope to indicate the place of each specimen
alluded to, in such a manner, that, provided the rock remains in
existence, it may be in the power of an observer to discover it at any
future period; however much it may have been concealed by those
accidents to which such specimens are perpetually exposed.
The country in the neighbourhood of Edinburgh is what all are agreed to
ca1l Secondary, consisting of beds of sandstone, and occasionally of
limestone; and coal, interstratified with thick assemblages of shale,
in loose and frail strata: This mass is traversed with the utmost
irregularity, by dikes or veins of whinstone, which occur also in vast
interjected masses, sometimes lying in great amorphous blocks, and
sometimes in thick beds, parallel to the strata. The strata,
themselves, as might be expected, are thrown, by means of its
intrusion, into much irregularity, and though nearly parallel to each
other in any particular spot, exhibit the utmost variety, when
different places are compared together, as to their dip and
direction. This contrast is conspicuous in Salisbury Craig on the
east and in Corstorphine Hill on the west of Edinburgh. Each consists
of a thick mass of whinstone, parallel to the strata beneath it, which,
in the first mentioned hill, dip rapidly to the east, and in the second
to the west.
The surface of this district, together with the alluvial part of its
mass, bears every mark of the effects which a wave of sufficient
magnitude to overwhelm it, might be expected to occasion upon so
multifarious an assemblage.
Raised from below by the violent and abrupt means already alluded to,
in my last communication to the Society, this district would present to
a stream overwhelming it at any subsequent period, numberless points of
attack. Many of the rocks being rent in various ways, the hardest parts
being in a shivered state, would easily be carried forward. The
soft beds of shale or of slate-clay being laid open to the attacks of
the current, would be deeply abraded by its action, and thus masses,
both stratified and unstratified, that were originally unbroken, would
be undermined, and, yielding to their own weight, would add to the
quantity of moving matter, and extend the field of attack upon the
weaker parts. The water would thus be loaded with a multitude of blocks
of every size, shape, and quality, and with a quantity of clay, which
being soon reduced to mud, through which these stones were irregularly
and confusedly scattered, would flow at the bottom of the water, and
along with it, and would be deposited, according to the laws already
pointed out, when the stream approached to a state of rest. Such seems
to have been the origin of that body of compact blue clay which forms a
material part of our low districts, bearing every indication of having
flowed as a mass into its present situation; for it is totally devoid
of stratification, though frequently of great thickness.
This mass shews itself in several places, in the bed of the water of
Leith, where the banks have been laid open by natural or artificial
means. It was well displayed formerly, and may still be seen
above the Bells' Mills Quarry, and is now exposed to view upon the
right hand, after crossing the Dean Bridge, on the old road to
Queensferry. It here presents to view a face of about twenty or thirty
feet deep, though it often extends to forty or fifty feet. We find it
also upon the shore to the west of Leith, as laid open by the sea; and
I am informed by the person who conducted the work, that at the Fort in
that neighbourhood, in a search for water, it has been penetrated to
the depth of eighty feet from the surface, being fifty below high-water
mark. It is obvious, that the power of such mud, when flowing as a
stream, in transporting heavy bodies, and in abrading assemblages
previously formed, must bear some relation to the resistance which it
would oppose to any object forced through it, and of course, that
The existence of assemblages of this sort, affords, by its simple
testimony, a powerful argument in favour of a stream having overflowed
this country, superior in magnitude to any known river; and the facts
seem to meet the challenge held out by Mr PLAYFAIR in the following
passage, Illustrations, art. 366. "Lastly," he says, "if there were
anywhere 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."
Now it appears to me, that these vast assemblages, in which blocks of
every size, and shape, and quality, some sharp, some round, are
confusedly scattered through clay, are inexplicable by any diurnal
cause, and do call for some particular solution.
Such parts of the torrent as encountered less of the strata of shale
and clay, would hurry along with them the comminuted sandstone, and
deposite it in the form of sand and of gravel. Vast accumulations have
thus been formed in all our lower districts, the external figure of
which, and of the clay, has acquired, as we shall soon have occasion to
point out, a character peculiar to itself, and having externally a
smoothness and regularity, which forms a striking contrast with
the abrupt and most irregular dislocation which very commonly
occurs in the solid mass within.
In the midst of this general wreck of all the frail parts, the
strongest masses, principally those which, like Arthur's Seat, have
been powerfully pervaded with whinstone, would resist and defy all the
impetuosity of the stream. The principles which we have endeavoured to
lay down, as to the influence of firm obstacles on depositions and
abrasions, would thus be brought into action.
The rock upon which the Castle of Edinburgh stands, together with the
site of the Old Town, exhibit the most perfect example that could be
conceived of the application of these principles. The rock itself,
about two hundred feet in height, above its base, and bare on three
fourths of its circumference, consists of one of the most complete and
uniform masses of whin stone that is known in this country. Its form is
rudely cylindrical, and from it the ridge upon which the Old Town
stands, composed partly of deposites, and partly of protected strata,
extends, gently sloping, for about a mile to the eastward, from the
Castle to the Abbey of Holyroodhouse, where the tail terminates. Round
the western, southern, and northern sides, a hollow valley occurs,
which, towards the north, is still a marsh, and was once a lake, being
known by the name of the North Loch.
Corstorphine Hill, which, as seen from Edinburgh, occupies the horizon
to the north-west, affords, in one respect, an example of the other
case just mentioned. It consists of a ridge of about a mile and a half
in extent, rising in the middle, declining gently at both ends, and
pointing from north to south, with a declination of about 20° to
the east. It presents a smooth face of whinstone to the west,
towards which the mass dips in parallelism with the strata beneath it.
Upon its eastern side a hollow valley occurs, in which the old castle
of Craig-crook stands, and from its southern extremity a tail extends
to the eastward, lying between Ravelstone and Murrayfield, upon the
southern face of which Murrayfield stands.
Thus, the Castle of Edinburgh, gives an example of the effect of a
narrow obstacle; and this hill, of a broad one, in so far as it has a
valley on the side towards which the stream was flowing.
We have endeavoured, in the last communication to this Society, to
account for the formation of such lakes as occur in alpine and rocky
districts. The circumstances just pointed out, explain the formation of
those which belong to districts formed of frail and moveable
substances. At Lochend a striking example occurs, of a lake
produced upon the upper side of an obstacle, in consequence of local
acceleration.
Immediately on the east of Corstorphine Hill, a set of firm rocks, or
little hills of sandstone occur, rising up from this hollow, or
standing upon its eastern side. Of these, Ravelstone, Craigleith, and
Blackcraig, are the principal, well known as excellent quarries. From
each of them a tail or prolongation extends to the eastward, formed
chiefly of the blue clay already mentioned, together with beds of sand
and gravel. These decline very gently, and maintain, to a
considerable distance, the individual character given to each by the
firm mass producing it. These ridges, however, are occasionally
interrupted by the interference of the same principles; as we see well
illustrated near the rock of Craigleith Quarry, by which the tail
extending from the Maiden Craig, (another sandstone mass to the
westward of it,) is abruptly cut off.
From Corstorphine Hill to the eastward, the country embracing all the
space between Edinburgh and the sea presents one continued series of
ridges, upon one of which the New Town of Edinburgh stands. It is an
important circumstance, that these ridges maintain a very correct
parallelism with each other, with the tail of the Castle rock, and of
the Calton Hill, and with the alluvial prolongations that extend to the
eastward from all the eminences of this neighbourhood. And a series of
parallel ridges occur also on the south side of Edinburgh, extending
from all the rocky eminences, as may be well seen on the road leading
to Dalkeith, which passes over several of these; one of the most
remarkable of which is, that on which the village of Libberton stands.
Such an arrangement cannot have been the work of the diurnal waters
produced by our common rains; for the course of such waters, flowing by
the action of gravity, and guided by the general slope of the country,
which declines towards the Frith of Forth, ought to have produced
depositions nearly at right angles to those under consideration. It is
in vain that a vast duration is ascribed to the influence of an agent,
unless it can be shewn, that its action has a tendency to produce the
alleged result. If it has a tendency to produce a different result,
that difference would be augmented in proportion to the duration of the
action. Now, the diurnal operations are everywhere found in the
act of corroding and altering the forms here alluded to; but they are
nowhere seen to produce them. This class of facts, on the other hand,
all conspire in giving probability to the hypothesis of a diluvian
wave, which affords an easy explanation of all the large features
of this country.
An important principle of the theory of running streams must here also
be considered, namely, that the shape assumed by such a stream flowing
through sand or other loose matter, bears a distinct relation to the
magnitude of the stream; the radius of curvature of its bendings being
in proportion to that magnitude. Thus, all the water collected from
this neighbourhood, is capable of producing no more than a paltry
brook, as appears from the Water of Leith, which we see meandering
between two of the ridges just mentioned. This meandering course, suits
the diminutive size of the brook; whereas these ridges being straight,
or, mathematically speaking, having a curvature whose radius is of
infinite length, we are led, by a very obvious analogy, and in
concurrence with what has been observed in other parts of the globe, to
believe that a cause very different from any now in activity, and far
more powerful, has exerted its influence upon this spot; that a stream
has flowed over it, capable of overwhelming and disregarding objects by
which the Nile or the Ganges would have been turned out of their course.
But the testimonies in favour of this hypothesis are not derived from
these large features alone; and it is not conceivable, that such agents
could have been at work, without leaving behind them indications of
their influence still more unequivocal. These occur in the very places
indicated by the theory, and exhibit remarkable instances of abrasion.
In order to investigate them with success, we must have recourse once
more to the effects produced by one of our common streams.
Where a firm rock of any kind has been exposed to the action of a rapid
river, its surface acquires in consequence of that abrasion a peculiar
character, which every one recognizes at a glance, but which is
difficult to describe in words. The most obvious and universal
effect of such an action, is the rounding of all the original angles of
the rock not only the prominent, but also the entering angles.
For where an original hollow has occurred, coinciding at all with the
course of the stream, the water has undergone an acceleration along
that hollow, and has excavated for itself a waving groove more or less
longitudinal. The whole has thus acquired a peculiar character,
by an assemblage of flowing lines, which recalls the water-worn state
of the rock. Another set of forms also present themselves in all
such cases, which could not easily have been foreseen, and whose
existence we learn only from observation of the fact: we observe, that
the surface is in many parts excavated by shallow depressions of
various sizes, which I shall distinguish by the name of scoopings, as
resembling the effect which would be produced upon a soft body by the
oblique blow of a spoon or scoop. I conceive that they have in fact
been produced by the action of eddies of inferior force to the main
stream, but acting in company with it in different and sometimes
opposite directions. These various corrosions going on together, have
each produced its peculiar effect; and most of them being concave,
their meeting has given rise to the set of waving angular ridges which
constitute the most unequivocal feature of a water-worn rock. These
angular forms differ completely from those which occur in the broken
surface of a rock. These last are acute, rectilinear and abrupt; while
those others are continuous, flowing, and having their angles very
obtuse; so obtuse in some cases, as not to be visible, unless the light
strikes upon the rock in a peculiar direction.
In a small but rapid brook near my house in the country, these forms
occur on the surface of a smooth bed of sandstone over which it flows.
I observed lately, when the brook was low and clear, that, to a certain
distance below each of these obtuse angled ridges, the rock was covered
with green moss, while above the angle it was bare. The mode in which
these forms are produced and maintained, seems thus to be pointed out:
the main stream being possessed of just power enough to keep the rock
clear of moss, and the eddy being too weak for this purpose. In a
flood, I presume that the eddy acquires such power, that the whole rock
is cleared.
In whatever manner we account for the production of these forms, it is
certain, that they present themselves on the surface of water-worn
rocks. I have also observed them upon the surface of a mass of
snow which had been acted upon, and partly removed by a strong wind. As
the abrasion occasioned by a fluid in motion seems alone to possess
that power, we may reasonably conclude, then, that a fluid has acted
where such forms occur.
Now, it is a fact of which I shall presently lay various examples
before this assembly, that the very rocks over which, according to the
theory advanced in this paper, torrents of water have flowed, loaded
with sand, and gravel, and large stones, and accompanied with streams
of mud, are found to exhibit at their surface all the characters of
abrasion lately mentioned; -the rotundity and flowing character; -the
excavation of hollows into the form of waving grooves; -the concave
scoopings, and the obtuse-angled, and waving ridges.
Circumstances, then, seem to justify the conclusion, that, in fact,
mighty torrents have traversed these districts.
In addition to the facts mentioned, and in company with them, another
set also occur in these scenes, which powerfully corroborate the same
conclusion. As stones of considerable bulk are often carried down by
torrents, it is reasonable to expect, that upon rocks along which they
have been hurried, and on which they could not fail to act as grinders,
traces should shew themselves of that passage by scratches and
abrasions of various sorts; and I have little doubt, that such will be
found, when. the effects of great floods in rapid rivers are properly
examined; especially where a stream of mud has accompanied the
torrent. Not having had occasion, however, to visit any scene of
this sort, since the importance of the observation occurred to
me, I have as yet, only met in rivers with cases in which the surface
of the rock has been dressed to smoothness, and in which the abrasion
shews itself in the general rotundity of the grooves and scoopings, and
in, the obtuse angled and waving ridges just described.
But what I have hitherto looked for in vain in common rivers, occurs
universally in the diluvian scenes, where there is reason, from other
circumstances, to believe, that a powerful abrasion has taken place,
and where the surface has been protected from the injuries of the
weather. Where it has been exposed to that injury, we generally find,
that the large features of dressing, the grooves and scoopings, and
obtuse angled ridges, only remain. But where a mass of this kind,
either by accident or design, has been followed under ground to where
its surface has been protected by a covering of clay, an interesting
and striking scene presents itself; the surface is found to resemble
that of a wet road, along which a number of heavy and irregular bodies
have been recently dragged; indicating that every block that passed,
and every one of its corners, had left its trace behind it; and these
are rendered very distinctly visible, when the surface, is drenched
with water.
In many cases these furrows or scratches have been so deep as to resist
all the effects of the weather, and shew themselves in rocks that have
been always exposed, sometimes many yards in length. Occasionally,
single scratches, and parallel sets of them, deviate by five or six
degrees from the general direction; but the important circumstance is,
that such deviation is rare, the very great majority of both sets
agreeing in parallelism with each other, and with the general
direction, not only of the scoops and grooves of the rock upon which
they occur, but also of the ridges and large features of the district.
A rock covered with these furrows, has externally an appearance greatly
resembling what is called Slickenside, with this difference; that in
the slickenside, we can always discover some proof that one portion of
the main rock has performed a small slide upon the other; whereas, in
this case, every thing shews, that the rock under consideration has
stood firm, and has been abraded by a number of bodies in motion. The
circumstance just mentioned, of occasional deviations of parallelism,
seems also to distinguish this form from the slickenside; in which
last, I believe, the lines are invariably parallel.
The direction of the stream in the neighbourhood of Edinburgh, as
indicated by the medium result of a number of observations, appears to
have been from 10° S. of W. to 10° N. of E., by true bearings
taken with a needle, and allowing 27½° west of north as the
variation; and I have met with no case deviating more than 10° or
12° from that average on either side.
Text scanned and
edited by Douglas E. Cox