the destructive action of inland waters, or we may truly say, the
destructive action of _rain_, is manifested,—viz., in landslips.
These, of course, are also caused not unfrequently by vulcanian action,
but equally of course landslips so caused do not belong to our present
subject. Landslips caused directly or indirectly by rain, are often
quite as extensive as those occasioned by vulcanian energy, and they
are a great deal more common. We may cite as a remarkable instance
a landslip of nearly half a mile in breadth, now in progress, in a
district of the city of Bath called Hedgmead, which forms a portion
of the slope of Beacon Hill. It is attributed to the action of a
subterranean stream on a bed of gravel, the continued washing away of
which causes the shifting; but the heavy rains of 1876–77 caused the
landslip to become much more marked.
Besides slow landslips, however, rain not unfrequently causes great
masses of earth to be precipitated suddenly, and where such masses fall
into the bed of a river, local deluges of great extent and of the most
destructive character often follow. The following instances, cited in
an abridged form from the pages of Lyell’s “Principles of Geology,”
attest the terrible nature of catastrophes such as these.
Two dry seasons in the White Mountains of New Hampshire were followed
by heavy rains on August 28, 1826. From the steep and lofty slopes
of the River Saco great masses of rock and stone were detached, and
descending carried along with them “in one promiscuous and frightful
ruin, forests, shrubs, and the earth which sustained them.” “Although
there are numerous indications on the steep sides of these hills of
former slides of the same kind, yet no tradition had been handed down
of any similar catastrophe within the memory of man, and the growth of
the forest on the very spots now devastated clearly showed that for a
long interval nothing similar had occurred. One of these moving masses
was afterwards found to have slid three miles, with an average breadth
of a quarter of a mile.” At the base of the vast chasms formed by these
natural excavations, a confused mass of ruins was seen, consisting of
transported earth, gravel, rocks, and trees. Forests were prostrated
with as much ease as if they had been mere fields of grain; if they
resisted for a while, “the torrent of mud and rock accumulated behind
till it gathered sufficient force to burst the temporary barrier.”
“The valleys of the Amonoosuck and Saco presented, for many miles, an
uninterrupted scene of desolation, all the bridges being carried away,
as well as those over the tributary streams. In some places the road
was excavated to the depth of from fifteen to twenty feet; in others
it was covered with earth, rocks, and trees to as great a height.
The water flowed for many weeks after the flood, as densely charged
with earth as it could be without being changed into mud, and marks
were seen in various localities of its having risen on either side of
the valley to more than twenty-five feet above the ordinary level.”
But perhaps the most remarkable evidence of the tremendous nature
of this cataclysm is to be found in Lyell’s statements respecting
the condition of the region nineteen years later. “I found the signs
of devastation still very striking,” he says; “I also particularly
remarked that the surface of the bare granite rocks had been smoothed
by the passage over them of so much mud and stone.” Professor Hubbard
mentions in _Silliman’s Journal_ that “in 1838 the deep channels worn
by the avalanches of mud and stone, and the immense heaps of boulders
and blocks of granite in the river channel, still formed a picturesque
feature in the scenery.”
It will readily be understood that when destruction such as this
follows from landslips along the borders of insignificant rivers,
those occurring on the banks of the mighty rivers which drain whole
continents are still more terrible. The following account from the pen
of Mr. Bates the naturalist, indicates the nature of the landslips
which occur on the banks of the Amazon. “I was awoke before sunrise,
one morning,” he says, “by an unusual sound resembling the roar of
artillery; the noise came from a considerable distance, one crash
succeeding another. I supposed it to be an earthquake, for, although
the night was breathlessly calm, the broad river was much agitated,
and the vessel rolled heavily. Soon afterwards another loud explosion
took place, followed by others which lasted for an hour till the day
dawned, and we then saw the work of destruction going forward on the
other side of the river, about three miles off. Large masses of forest,
including trees of colossal size, probably 200 feet in height, were
rocking to and fro, and falling headlong one after another into the
water. After each avalanche the wave which it caused returned on the
crumbly bank with tremendous force, and caused the fall of other masses
by undermining. The line of coast over which the landslip extended was
a mile or two in length; the end of it, however, was hid from our view
by an intervening island. It was a grand sight; each downfall created
a cloud of spray; the concussion in one place causing other masses
to give way a long distance from it, and thus the crashes continued,
swaying to and fro, with little prospect of termination. When we glided
out of sight two hours after sunrise the destruction was still going
on.”
We might consider here the action of glaciers in gradually grinding
down the mountain slopes, the destructive action of avalanches, and a
number of other forms in which snow and ice break down by slow degrees
the upraised portions of the earth. For in reality all these forms of
destructive action take their origin in the same process whence running
waters and heavy rainfalls derive their power. All these destructive
agencies are derived from the vapour of water in the air. But it seems
better to limit the reader’s attention in this place to the action of
water in the liquid form; and therefore we proceed to consider the
other ways in which rain wears down the land.
Hitherto we have considered effects which are produced chiefly along
the courses of rivers, or in their neighbourhood. But heavy rainfall
acts, and perhaps in the long run as effectively (when we remember the
far wider region affected) over wide tracts of nearly level ground, as
along the banks of torrents and rivers.
The rain which falls on plains or gently undulating surfaces, although
after a while it dries up, yet to some degree aids in levelling the
land, partly by washing down particles of earth, however slowly, to
lower levels, partly by soaking the earth and preparing a thin stratum
of its upper surface to be converted into dust, and blown away by the
wind. But it is when very heavy storms occur that the levelling action
of rain over widely extending regions can be most readily recognized.
Of this fact observant travellers cannot fail to have had occasional
evidence. Sir Charles Lyell mentions one instance observed by him,
which is specially interesting. “During a tour in Spain,” he says,
“I was surprised to see a district of gently undulating ground in
Catalonia, consisting of red and grey sandstone, and in some parts
of red marl, almost entirely denuded of herbage, while the roots of
the pines, holm oaks, and some other trees, were half exposed, as if
the soil had been washed away by a flood. Such is the state of the
forests, for example, between Oristo and Vich, and near San Lorenzo.
But being overtaken by a violent thunderstorm, in the month of August,
I saw the whole surface, even the highest levels of some flat-topped
hills, streaming with mud, while on every declivity the devastation
of torrents was terrific. The peculiarities in the physiognomy of the
district were at once explained, and I was taught that, in speculating
on the greater effects which the direct action of rain may once have
produced on the surface of certain parts of England, we need not revert
to periods when the heat of the climate was tropical.” He might have
cited instances of such storms occurring in England. For example,
White, in his delightful “Natural History of Selborne,” describes
thus the effects of a storm which occurred on June 5, 1784: “At about
a quarter after two the storm began in the parish of Harpley, moving
slowly from north to south, and from thence it came over Norton Farm
and so to Grange Farm, both in this parish. Had it been as extensive
as it was violent (for it was very short) it must have ravaged all the
neighbourhood. The extent of the storm was about two miles in length
and one in breadth. There fell prodigious torrents of rain on the farms
above mentioned, which occasioned a flood as violent as it was sudden,
doing great damage to the meadows and fallows by deluging the one and
washing away the soil of the other. The hollow lane towards Alton was
so torn and disordered as not to be passable till mended, rocks being
removed which weighed two hundredweight.”
We have mentioned the formation of dust, and the action of wind upon
it, as a cause tending to level the surface of the land. It may appear
to many that this cause is too insignificant to be noticed among those
which modify the earth’s surface. In reality, however, owing to its
continuous action, and to its always acting (in the main) in one
direction, this cause is much more important than might be supposed.
We overlook its action as actually going on around us, because in a
few years, or in a few generations, it produces no change that can be
readily noticed. But in long periods of time it changes very markedly
the level of lower lands, and that too even in cities, where means
exist for removing the accumulations of dust which are continually
collecting on the surface of the earth. We know that the remains of
old Roman roads, walls, houses, and so forth, in this country, are
found, not at the present level of the surface, but several feet—in
some cases many yards—below this level. The same holds elsewhere,
under the same conditions—that is, where we know quite certainly
that the substances thus found underground were originally on the
surface, and that there has been neither any disturbance causing them
to be engulfed, nor any deposition of scoriæ, volcanic dust, or other
products of subterranean disturbance. We cannot hesitate to regard this
burying of old buildings as due to the continual deposition of dust,
which eventually becomes compacted into solid earth. We know, moreover,
that the formation of dust is in the main due to rain converting the
surface layers of the earth into mud, which on drying requires but the
frictional action of heavy winds to rise in clouds of dust. In some
soils this process goes on more rapidly than in others, as every one
who has travelled much afoot is aware. There are parts of England, for
instance, where, even in the driest summer, the daily deposition of
dust on dry and breezy days is but slight, others where in such weather
a dust layer at least a quarter of an inch in thickness is deposited
in the course of a day. If we assumed, which would scarcely seem an
exaggerated estimate, that in the course of a single year a layer of
dust averaging an inch in thickness is deposited over the lower levels
of the surface of the land, we should find that the average depth of
the layer formed in the last thousand years would amount to no less
than eighty-three feet. Of course in inhabited places the deposition
of dust is checked, though not so much as most persons imagine. There
is not probably in this country a single building five hundred years
old, originally built at a moderately low level, the position of whose
foundation does not attest the constant gathering of matter upon the
surface. The actual amount by which the lower levels are raised and
the higher levels diminished in the course of a thousand years may be
very much less, but that it must amount to many feet can scarcely be
questioned.
And as in considering the action of rain falling over a wide range of
country, we have to distinguish between the slow but steady action
of ordinary rains and the occasional violent action of great storms
of rain, so in considering the effects of drought following after
rain which has well saturated the land we have to distinguish between
ordinarily dusty times and occasions when in a very short time, owing
to the intensity of the heat and the violence of the wind large
quantities of dust are spread over a wide area. Darwin thus describes
the effect of such exceptional drought, as experienced in the years
1827–1832 in Buenos Ayres:—“So little rain fell that the vegetation,
even to the thistles, failed; the brooks were dried up, and the
whole country assumed the appearance of a dusty high road. This was
especially the case in the northern part of the province of Buenos
Ayres, and the southern part of Santa Fé.” He describes the loss of
life caused by the want of water, and many remarkable circumstances of
the drought which do not here specially concern us. He then goes on to
speak of the dust which gathered over the open country. “Sir Woodbine
Parish,” he says, “informed me of a very curious source of dispute.
The ground being so long dry, such quantities of dust were blown about
that in this open country the landmarks became obliterated, and people
could not tell the limits of their estates.” The dust thus scattered
over the land, whether left or removed, necessarily formed part of the
solid material brought from higher to lower levels, indirectly (in
this case) through the action of rain; for a drought can only convert
into friable matter earth which has before been thoroughly soaked. But
the action of rain, which had originally led to the formation of these
enormous masses of dust, presently took part in carrying the dust in
the form of mud to yet lower levels. “Subsequently to the drought of
1827 to 1832,” proceeds Darwin, “a very rainy season followed, which
caused great floods. Hence it is almost certain that some thousands
of the skeletons” (of creatures whose deaths he had described before)
“were buried by the deposits of the very next year. What could be the
opinion of a geologist, viewing such an enormous collection of bones,
of all kinds of animals and of all ages, thus embedded in one thick
earthy mass? Would he not attribute it to a flood having swept over the
surface of the land, rather than to the common order of things?” In
fact, a single great drought, followed by a very rainy season, must in
this instance, which was however altogether exceptional, have produced
a layer or stratum such as geologists would ordinarily regard as the
work of a much longer time and much more potent disturbing causes.
It may be well to consider in this place the question whether in
reality the quantity of rain which falls now during our winter months
does not greatly exceed that which formerly fell in that part of
the year. The idea is very prevalent that our winters have changed
entirely in character in recent times, and the fear (or the hope?) is
entertained that the change may continue in the same direction until
wet and mild winters replace altogether the cold which prevailed in
former years. There is no sufficient reason, however, for supposing
that any such change is taking place. It is, indeed, not difficult to
find in the meteorological annals of the first half of the present
century, instances of the occurrence of several successive winters very
unlike the greater number of those which we have experienced during
the last ten or twelve years. But if we take any considerable series
of years in the last century we find the alternations of the weather
very similar to those we at present recognize. Consider, for instance,
Gilbert White’s brief summary of the weather from 1768 onwards:—
For the winter of 1768–69 we have October and the first part of
November rainy; thence to the end of 1768 alternate rains and frosts;
January and February frosty and rainy, with gleams of fine weather; to
the middle of March, wind and rain.
For the winter of 1769–70 we have October frosty, the next fortnight
rainy, the next dry and frosty. December windy, with rain and intervals
of frost (the first fortnight very foggy); the first half of January
frosty, thence to the end of February mild hazy weather. March frosty
and brighter.
For 1770–71, from the middle of October to the end of the year, almost
incessant rains; January severe frosts till the last week, the next
fortnight rain and snow, and spring weather to the end of February.
March frosty.
For 1771–72, October rainy, November frost with intervals of fog and
rain, December bright mild weather with hoar frosts; then six weeks of
frost and snow, followed by six of frost, sleet, hail, and snow.
For 1772–73, October, November, and to December 22, rain, with mild
weather; to the end of 1772, cold foggy weather; then a week of frost,
followed by three of dark rainy weather. First fortnight of February
frost; thence to the end of March misty showery weather.
Passing over the winter of 1773–74, which was half rainy, half frosty,
what could more closely resemble the winter weather we have had so much
of during the last few years, than that experienced in the winter of
1774–75? From August 24 to the third week of November, there was rain,
with frequent intervals of sunny weather; to the end of December, dark
dripping fogs; to the end of the first fortnight in March, rain almost
every day.
And so on, with no remarkable changes, until the year 1792, the last of
Gilbert White’s records.
If we limit our attention to any given month of winter, we find the
same mixture of cold and dry with wet and open weather as we are
familiar with at present. Take, for instance, the month usually the
most wintry of all, viz., January. Passing over the years already
considered, we have January, 1776, dark and frosty with much snow
till the 26th (at this time the Thames was frozen over), then foggy
with hoar frost; January, 1777, frosty till the 10th, then foggy and
showery; 1778, frosty till the 13th, then rainy to the 24th, then
hard frost; 1779, frost and showers throughout January; 1780, frost
throughout; 1781, frost and snow to the 25th, then rain and snow;
1782, open and mild; 1783, rainy with heavy winds; 1784, hard frost;
1785, a thaw on the 2nd, then rainy weather to the 28th, the rest of
the month frosty; 1786, frost and snow till January 7, then a week
mild with much rain, the next week heavy snow, and the rest mild with
frequent rain; 1787, first twenty-four days dark moist mild weather,
then four days frost, the rest mild and showery; 1788, thirteen days
mild and wet, five days of frost, and from January 18 to the end of the
month dry windy weather; 1789, thirteen days hard frost, the rest of
the month mild with showers; 1790, sixteen days of mild foggy weather
with occasional rain, to the 21st frost, to the 28th dark with driving
rains, and the rest mild dry weather; 1791, the whole of January mild
with heavy rains; and lastly 1792, “some hard frost in January, but
mostly wet and mild.”
There is nothing certainly in this record to suggest that any material
change has taken place in our January weather during the last eight
years. And if we had given the record of the entire winter for each of
the years above dealt with the result would have been the same.
We have, in fact, very striking evidence in Gilbert White’s account
of the cold weather of December, 1784, which he specially describes
as “very extraordinary,” to show that neither our severe nor our
average winter weather can differ materially from that which people
experienced in the eighteenth century. “In the evening of December 9,”
he says, “the air began to be so very sharp that we thought it would be
curious to attend to the motions of a thermometer; we therefore hung
out two, one made by Martin and one by Dolland” (_sic_, presumably
Dollond), “which soon began to show us what we were to expect; for by
ten o’clock they fell to twenty-one, and at eleven to four, when we
went to bed. On the 10th, in the morning the quicksilver in Dolland’s
glass was down to half a degree below zero, and that of Martin’s, which
was absurdly graduated only to four degrees above zero, sank quite
into the brass guard of the ball, so that when the weather became most
interesting this was useless. On the 10th, at eleven at night, though
the air was perfectly still, Dolland’s glass went down to one degree
below zero!” The note of exclamation is White’s. He goes on to speak
of “this strange severity of the weather,” which was not exceeded
that winter, or at any time during the twenty-four years of White’s
observations. Within the last quarter of a century, the thermometer,
on more than one occasion, has shown two or three degrees below zero.
Certainly the winters cannot be supposed to have been ordinarily
severer than ours in the latter half of the last century, when we find
that thermometers, by well-known instrument makers, were so constructed
as to indicate no lower temperature than four degrees above zero.
Let us return, after this somewhat long digression, to the levelling
action of rain and rivers.
If we consider this action alone, we cannot but recognize in it a cause
sufficient to effect the removal of all the higher parts of the land
to low levels, and eventually of all the low-lying land to the sea, in
the course of such periods as geology makes us acquainted with. The
mud-banks at the mouths of rivers show only a part of what rain and
river action is doing, yet consider how enormous is the mass which is
thus carried into the sea. It has been calculated that in a single week
the Ganges alone carries away from the soil of India and delivers
into the sea twice as much solid substance as is contained in the
great pyramid of Egypt. “The Irrawaddy,” says Sir J. Herschel, “sweeps
off from Burmah 62 cubit feet of earth in every second of time on an
average, and there are 86,400 seconds in every day, and 365 days in
every year; and so on for other rivers. Nor is there any reason to fear
or hope that the rains will cease, and this destructive process come to
an end. For though the quantity of water on the surface of the earth
is probably undergoing a slow process of diminution, small portions of
it year by year taking their place as waters under the earth,[44] yet
these processes are far too slow to appreciably affect the supply of
water till a far longer period has elapsed than that during which (in
all probability) life can continue upon the earth.
When we consider the force really represented by the downfall of rain,
we need not greatly wonder that the levelling power of rain is so
effective. The sun’s heat is the true agent in thus levelling the
earth, and if we regard, as we justly may, the action of water, whether
in the form of rain or river, or of sea-wave raised by wind or tide, as
the chief levelling and therefore destructive force at work upon the
earth, and the action of the earth’s vulcanian energies as the chief
restorative agent, then we may fairly consider the contest as lying
between the sun’s heat and the earth’s internal heat. There can be
little question as to what would be the ultimate issue of the contest
if land and sea and air all endured or were only so far modified as
they were affected by these causes. Sun-heat would inevitably prevail
in the long run over earth-heat. But we see from the condition of
our moon how the withdrawal of water and air from the scene must
diminish the sun’s power of levelling the irregularities of the earth’s
surface. We say advisedly _diminish_, not _destroy_; for there can be
no question that the solar heat alternating with the cold of the long
lunar night is still at work levelling, however slowly, the moon’s
surface; and the same will be the case with our earth when her oceans
and atmosphere have disappeared by slow processes of absorption.