Division of the subject into changes of the organic and inorganic
world--Inorganic causes of change divided into aqueous and
igneous--Aqueous causes first considered--Fall of rain--Recent
rain-prints in mud--Destroying and transporting power of running
water--Newly formed valleys in Georgia--Sinuosities of rivers--Two
streams when united do not occupy a bed of double
surface--Inundations in Scotland--Floods caused by landslips in the
White Mountains--Bursting of a lake in Switzerland--Devastations
caused by the Anio at Tivoli--Excavations in the lavas of Etna by
Sicilian rivers--Gorge of the Simeto--Gradual recession of the
cataract of Niagara.
_Division of the subject._--Geology was defined to be the science which
investigates the former changes that have taken place in the organic as
well as in the inorganic kingdoms of nature. As vicissitudes in the
inorganic world are most apparent, and as on them all fluctuations in
the animate creation must in a great measure depend, they may claim our
first consideration. The great agents of change in the inorganic world
may be divided into two principal classes, the aqueous and the igneous.
To the aqueous belong Rain, Rivers, Torrents, Springs, Currents, and
Tides; to the igneous, Volcanoes, and Earthquakes. Both these classes
are instruments of decay as well as of reproduction; but they may also
be regarded as antagonist forces. For the aqueous agents are incessantly
laboring to reduce the inequalities of the earth's surface to a level;
while the igneous are equally active in restoring the unevenness of the
external crust, partly by heaping up new matter in certain localities,
and partly by depressing one portion, and forcing out another, of the
earth's envelope.
It is difficult, in a scientific arrangement, to give an accurate view
of the combined effects of so many forces in simultaneous operation;
because, when we consider them separately, we cannot easily estimate
either the extent of their efficacy, or the kind of results which they
produce. We are in danger, therefore, when we attempt to examine the
influence exerted singly by each, of overlooking the modifications which
they produce on one another; and these are so complicated, that
sometimes the igneous and aqueous forces co-operate to produce a joint
effect, to which neither of them unaided by the other could give
rise,--as when repeated earthquakes unite with running water to widen a
valley; or when a thermal spring rises up from a great depth, and
conveys the mineral ingredients with which it is impregnated from the
interior of the earth to the surface. Sometimes the organic combine with
the inorganic causes; as when a reef, composed of shells and corals,
protects one line of coast from the destroying power of tides or
currents, and turns them against some other point; or when drift timber,
floated into a lake, fills a hollow to which the stream would not have
had sufficient velocity to convey earthy sediment.
It is necessary, however, to divide our observations on these various
causes, and to classify them systematically, endeavoring as much as
possible to keep in view that the effects in nature are mixed and not
simple, as they may appear in an artificial arrangement.
In treating, in the first place, of the aqueous causes, we may consider
them under two divisions; first, those which are connected with the
circulation of water from the land to the sea, under which are included
all the phenomena of rain, rivers, glaciers, and springs; secondly,
those which arise from the movements of water in lakes, seas, and the
ocean, wherein are comprised the phenomena of waves, tides, and
currents. In turning our attention to the former division, we find that
the effects of rivers may be subdivided into, first, those of a
destroying and transporting, and, secondly, those of a renovating
nature; in the former are included the erosion of rocks and the
transportation of matter to lower levels; in the renovating class, the
formation of deltas by the influx of sediment, and the shallowing of
seas; but these processes are so intimately related to each other, that
it will not always be possible to consider them under their separate
heads.
_Fall of Rain._--It is well known that the capacity of the atmosphere to
absorb aqueous vapor, and hold it in suspension, increases with every
increment of temperature. This capacity is also found to augment in a
higher ratio than the augmentation of the heat. Hence, as was first
suggested by the geologist, Dr. Hutton, when two volumes of air, of
different temperatures, both saturated with moisture, mingle together,
clouds and rain are produced, for a mean degree of heat having resulted
from the union of the two moist airs, the excess of vapor previously
held in suspension by the warmer of the two is given out, and if it be
in sufficient abundance is precipitated in the form of rain.
As the temperature of the atmosphere diminishes gradually from the
equator towards the pole, the evaporation of water and the quantity of
rain diminish also. According to Humboldt's computation, the average
annual depth of rain at the equator is 96 inches, while in lat. 45
degrees it is only 29 inches, and in lat. 60 degrees not more than 17
inches. But there are so many disturbing causes, that the actual
discharge, in any given locality, may deviate very widely from this
rule. In England, for example, where the average fall at London is
24-1/2 inches, as ascertained at the Greenwich Observatory, there is
such irregularity in some districts, that while at Whitehaven, in
Cumberland, there fell in 1849, 32 inches, the quantity of rain in
Borrowdale, near Keswick (only 15 miles to the westward), was no less
than 142 inches![262] In like manner, in India, Colonel Sykes found by
observations made in 1847 and 1848, that at places situated between 17
degrees and 18 degrees north lat., on a line drawn across the Western
Ghauts in the Deccan, the fall of rain varied from 21 to 219
inches.[263] The annual average in Bengal is probably below 80 inches,
yet Dr. G. Hooker witnessed at Churrapoonjee, in the year 1850, a fall
of 30 inches in 24 hours, and in the same place during a residence of
six months (from June to November) 530 inches! This occurred on the
south face of the Khasia (or Garrow) mountains in Eastern Bengal (see
map, Chap. XVIII.), where the depth during the whole of the same year
probably exceeded 600 inches. So extraordinary a discharge of water,
which, as we shall presently see, is very local, may be thus accounted
for. Warm, southerly winds, blowing over the Bay of Bengal, and becoming
laden with vapor during their passage, reach the low level delta of the
Ganges and Brahmapootra, where the ordinary heat exceeds that of the
sea, and where evaporation is constantly going on from countless marshes
and the arms of the great rivers. A mingling of two masses of damp air
of different temperatures probably causes the fall of 70 or 80 inches of
rain, which takes place on the plains. The monsoon having crossed the
delta, impinges on the Khasia mountains, which rise abruptly from the
plain to a mean elevation of between 4000 and 5000 feet. Here the wind
not only encounters the cold air of the mountains, but, what is far more
effective as a refrigerating cause, the aerial current is made to flow
upwards, and to ascend to a height of several thousand feet above the
sea. Both the air and the vapor contained in it, being thus relieved of
much atmospheric pressure, expand suddenly, and are cooled by
rarefaction. The vapor is condensed, and about 500 inches of rain are
thrown down annually, nearly twenty times as much as falls in Great
Britain in a year, and almost all of it poured down in six months. The
channel of every torrent and river is swollen at this season, and much
sandstone horizontally stratified, and other rocks are reduced to sand
and gravel by the flooded streams. So great is the superficial waste (or
_denudation_), that what would otherwise be a rich and luxuriantly
wooded region, is converted into a wild and barren moorland.
After the current of warm air has been thus drained of a large portion
of its moisture, it still continues its northerly course to the opposite
flank of the Khasia range, only 20 miles farther north, and here the
fall of rain is reduced to 70 inches in the year. The same wind then
blows northwards across the valley of the Brahmapootra, and at length
arrives so dry and exhausted at the Bhootan Himalaya (lat. 28 degrees
N.), that those mountains, up to the height of 5000 feet, are naked and
sterile, and all their outer valleys arid and dusty. The aerial current
still continuing its northerly course and ascending to a higher region,
becomes further cooled, condensation again ensues, and Bhootan, above
5000 feet, is densely clothed with vegetation.[264]
In another part of India, immediately to the westward, similar phenomena
are repeated. The same warm and humid winds, copiously charged with
aqueous vapor from the Bay of Bengal, hold their course due north for
300 miles across the flat and hot plains of the Ganges, till they
encounter the lofty Sikkim mountains. (See map, Chap. XVIII.) On the
southern flank of these they discharge such a deluge of rain that the
rivers in the rainy season rise twelve feet in as many hours. Numerous
landslips, some of them extending three or four thousand feet along the
face of the mountains, composed of granite, gneiss, and slate, descend
into the beds of streams, and dam them up for a time, causing temporary
lakes, which soon burst their barriers. "Day and night," says Dr.
Hooker, "we heard the crashing of falling trees, and the sound of
boulders thrown violently against each other in the beds of torrents. By
such wear and tear rocky fragments swept down from the hills are in part
converted into sand and fine mud; and the turbid Ganges, during its
annual inundation, derives more of its sediment from this source than
from the waste of the fine clay of the alluvial plains below.[265]
On the verge of the tropics a greater quantity of rain falls annually
than at the equator. Yet parts even of the tropical latitudes are
entirely destitute of rain: Peru, for example, which owes its vegetation
solely to rivers and nightly dews. In that country easterly winds
prevail, blowing from the Pacific, and these being intercepted by the
Andes, and cooled as they rise, are made to part with all their moisture
before reaching the low region to the leeward. The desert zone of North
Africa, between lat. 15 degrees and 30 degrees N., is another instance
of a rainless region. Five or six consecutive years may pass in Upper
Egypt, Nubia, and Dongola, or in the Desert of Sahara, without rain.
From the facts above mentioned, the reader will infer that in the course
of successive geological periods there will be great variations in the
quantity of rain falling in one and the same region. At one time there
may be none whatever during the whole year; at another a fall of 100 or
500 inches; and these two last averages may occur on the two opposite
flanks of a mountain-chain, not more than 20 miles wide. While,
therefore, the valleys in one district are widened and deepened
annually, they may remain stationary in another, the superficial soil
being protected from waste by a dense covering of vegetation. This
diversity depends on many geographical circumstances, but principally on
the height of the land above the sea, the direction of the prevailing
winds, and the relative position, at the time being, of the plains,
hills, and the ocean, conditions all of which are liable in the course
of ages to undergo a complete revolution.
_Recent rain-prints._--When examining, in 1842, the extensive mud-flats
of Nova Scotia, which are exposed at low tide on the borders of the Bay
of Fundy, I observed not only the foot-prints of birds which had
recently passed over the mud, but also very distinct impressions of
rain-drops. A peculiar combination of circumstances renders these
mud-flats admirably fitted to receive and retain any markings which may
happen to be made on their surface. The sediment with which the waters
are charged is extremely fine, being derived from the destruction of
cliffs of red sandstone and shale, and as the tides rise fifty feet and
upwards, large areas are laid dry for nearly a fortnight between the
spring and neap tides. In this interval the mud is baked in summer by a
hot sun, so that it solidifies and becomes traversed by cracks, caused
by shrinkage. Portions of the hardened mud between these cracks may then
be taken up and removed without injury. On examining the edges of each
slab, we observe numerous layers, formed by successive tides, each layer
being usually very thin, sometimes only one-tenth of an inch thick. When
a shower of rain falls, the highest portion of the mud-covered flat is
usually too hard to receive any impressions; while that recently
uncovered by the tide near the water's edge is too soft. Between these
areas a zone occurs, almost as smooth and even as a looking-glass, on
which every drop forms a cavity of circular or oval form, and, if the
shower be transient, these pits retain their shape permanently, being
dried by the sun, and being then too firm to be effaced by the action of
the succeeding tide, which deposits upon them a new layer of mud. Hence
we often find, in splitting open a slab an inch or more thick, on the
upper surface of which the marks of recent rain occur, that an inferior
layer, deposited during some previous rise of the tide, exhibits on its
under side perfect casts of rain-prints, which stand out in relief, the
moulds of the same being seen on the layer below. But in some cases,
especially in the more sandy layers, the markings have been somewhat
blunted by the tide, and by several rain-prints having been joined into
one by a repetition of drops falling on the same spot; in which case the
casts present a very irregular and blistered appearance.
The finest examples which I have seen of these rain-prints were sent to
me by Dr. Webster, from Kentville, on the borders of the Bay of Mines,
in Nova Scotia. They were made by a heavy shower which fell on the 21st
of July, 1849, when the rise and fall of the tides were at their
maximum. The impressions (see fig. 13) consist of cup-shaped or
hemispherical cavities, the average size of which is from one-eighth to
one-tenth of an inch across, but the largest are fully half an inch in
diameter, and one-tenth of an inch deep. The depth is chiefly below the
general surface or plane of stratification, but the walls of the cavity
consist partly of a prominent rim of sandy mud, formed of the matter
which has been forcibly expelled from the pit. All the cavities having
an oval form are deeper at one end, where they have also a higher rim,
and all the deep ends have the same direction, showing towards which
quarter the wind was blowing. Two or more drops are sometimes seen to
have interfered with each other; in which case it is usually possible to
determine which drop fell last, its rim being unbroken.
[Illustration: Fig. 13.
Recent rain-prints, formed July 21, 1849, at Kentville, Bay of Fundy,
Nova Scotia. The arrow represents the direction of the shower.]
On some of the specimens the winding tubular tracks of worms are seen,
which have been bored just beneath the surface (see fig. 13, _left
side_). They occasionally pass under the middle of a rain-mark, having
been formed subsequently. Sometimes the worms have dived beneath the
surface, and then reappeared. All these appearances, both of rain-prints
and worm-tracks, are of great geological interest, as their exact
counterparts are seen in rocks of various ages, even in formations of
very high antiquity.[266] Small cavities, often corresponding in size to
those produced by rain, are also caused by air-bubbles rising up through
sand or mud; but these differ in character from rain-prints, being
usually deeper than they are wide, and having their sides steeper.
These, indeed, are occasionally vertical, or overarching, the opening at
the top being narrower than the pit below. In their mode, also, of
mutual interference they are unlike rain-prints.[267]
In consequence of the effects of mountains in cooling currents of moist
air, and causing the condensation of aqueous vapor in the manner above
described, it follows that in every country, as a general rule, the more
elevated regions become perpetual reservoirs of water, which descends
and irrigates the lower valleys and plains. The largest quantity of
water is first carried to the highest region, and then made to descend
by steep declivities towards the sea; so that it acquires superior
velocity, and removes more soil, than it would do if the rain had been
distributed over the plains and mountains equally in proportion to their
relative areas. The water is also made by these means to pass over the
greatest distances before it can regain the sea.
It has already been observed that in higher latitudes, where the
atmosphere being colder is capable of holding less water in suspension,
a diminished fall of rain takes place. Thus at St. Petersburg, the
amount is only 16 inches, and at Uleaborg in the Gulf of Bothnia (N.
lat. 65 degrees), only 13-1/2 inches, or less than half the average of
England, and even this small quantity descends more slowly in the
temperate zone, and is spread more equally over the year than in
tropical climates. But in reference to geological changes, frost in the
colder latitude acts as a compensating power in the disintegration of
rocks, and the transportation of stones to lower levels.
Water when converted into ice augments in bulk more than one-twentieth
of its volume, and owing to this property it widens the minute crevices
(or _joints_) of rocks into which it penetrates. Ice also in various
ways, as will be shown in the next chapter, gives buoyancy to mud and
sand, even to huge blocks of stone, enabling rivers of moderate size and
velocity to carry them to a great distance.
The mechanical force exerted by running water in undermining cliffs, and
rounding off the angles of hard rock, is mainly due to the intermixture
of foreign ingredients. Sand and pebbles, when hurried along by the
violence of the stream, are thrown against every obstacle lying in their
way, and thus a power of attrition is acquired, capable of wearing
through the hardest siliceous stones, on which water alone could make no
impression.
_Newly formed valleys._--When travelling in Georgia and Alabama, in
1846, I saw in both those States the commencement of hundreds of valleys
in places where the native forest had recently been removed. One of
these newly formed gulleys or ravines is represented in the annexed
woodcut (fig. 14), from a drawing which I made on the spot. It occurs
three miles and a half due west of Milledgeville, the capital of
Georgia, and is situated on the farm of Pomona, on the direct road to
Macon.[268]
Twenty years ago, before the land was cleared, it had no existence; but
when the trees of the forest were cut down, cracks three feet deep were
caused by the sun's heat in the clay; and, during the rains, a sudden
rush of water through the principal crack deepened it at its lower
extremity, from whence the excavating power worked backwards, till, in
the course of twenty years, a chasm, measuring no less than 55 feet in
depth, 300 yards in length, and varying in width from 20 to 180 feet,
was the result. The high road has been several times turned to avoid
this cavity, the enlargement of which is still proceeding, and the old
line of road may be seen to have held its course directly over what is
now the wildest part of the ravine. In the perpendicular walls of this
great chasm appear beds of clay and sand, red, white, yellow, and
green, produced by the decomposition in situ of hornblendic gneiss,
with layers and veins of quartz, which remain entire, to prove that the
whole mass was once solid and crystalline.
[Illustration: Fig. 14.
Ravine on the farm of Pomona, near Milledgeville, Georgia, as it
appeared January, 1846.
Excavated in twenty years, 55 feet deep, and 180 feet broad.]
I infer, from the rapidity of the denudation which only began here after
the removal of the native wood, that this spot, elevated about 600 feet
above the sea, has been always covered with a dense forest, from the
remote time when it first emerged from the sea. The termination of the
cavity on the right hand in the foreground is the head or upper end of
the ravine, and in almost every case, such gulleys are lengthened by the
streams cutting their way backwards. The depth at the upper end is
often, as in this case, considerable, and there is usually at this
point, during floods, a small cascade.
_Sinuosities of rivers._--In proportion as such valleys are widened,
sinuosities are caused by the deflection of the stream first to one
side and then to the other. The unequal hardness of the materials
through which the channel is eroded tends partly to give new directions
to the lateral force of excavation. When by these, or by accidental
shiftings of the alluvial matter in the channel, the current is made to
cross its general line of descent, it eats out a curve in the opposite
bank, or in the side of the hills bounding the valley, from which curve
it is turned back again at an equal angle, so that it recrosses the line
of descent, and gradually hollows out another curve lower down in the
opposite bank, till the whole sides of the valley, or river bed, present
a succession of salient and retiring angles. Among the causes of
deviation from a straight course, by which torrents and rivers tend in
mountainous regions to widen the valleys through which they flow, may be
mentioned the confluence of lateral torrents, swollen irregularly at
different seasons by partial storms, and discharging at different times
unequal quantities of sand, mud, and pebbles, into the main channel.
[Illustration: Fig. 15.]
When the tortuous flexures of a river are extremely great, as often
happens in alluvial plains, the aberration from the direct line of
descent may be restored by the river cutting through the isthmus which
separates two neighboring curves. Thus in the annexed diagram, the
extreme sinuosity of the river has caused it to return for a brief space
in a contrary direction to its main course, so that a peninsula is
formed, and the isthmus (at _a_) is consumed on both sides by currents
flowing in opposite directions. In this case an island is soon
formed,--on either side of which a portion of the stream usually
remains.
_Transporting power of water._--In regard to the transporting power of
water, we may often be surprised at the facility with which streams of a
small size, and descending a slight declivity, bear along coarse sand
and gravel; for we usually estimate the weight of rocks in air, and do
not reflect on their comparative buoyancy when submerged in a denser
fluid. The specific gravity of many rocks is not more than twice that of
water, and very rarely more than thrice, so that almost all the
fragments propelled by a stream have lost a third, and many of them a
half, of what we usually term their weight.
It has been proved by experiment, in contradiction to the theories of
the earlier writers on hydrostatics, to be a universal law, regulating
the motion of running water, that the velocity at the bottom of the
stream is everywhere less than in any part above it, and is greatest at
the surface. Also that the superficial particles in the middle of the
stream move swifter than those at the sides. This retardation of the
lowest and lateral currents is produced by friction; and when the
velocity is sufficiently great, the soil composing the sides and bottom
gives way. A velocity of three inches per second at the bottom is
ascertained to be sufficient to tear up fine clay,--six inches per
second, fine sand,--twelve inches per second, fine gravel,--and three
feet per second, stones of the size of an egg.[269]
When this mechanical power of running water is considered, we are
prepared for the transportation before alluded to of large quantities of
gravel, sand, and mud, by torrents which descend from mountainous
regions. But a question naturally arises, How the more tranquil rivers
of the valleys and plains, flowing on comparatively level ground, can
remove the prodigious burden which is discharged into them by their
numerous tributaries, and by what means they are enabled to convey the
whole mass to the sea? If they had not this removing power, their
channels would be annually choked up, and the valleys of the lower
country, and plains at the base of mountain-chains, would be continually
strewed over with fragments of rock and sterile sand. But this evil is
prevented by a general law regulating the conduct of running
water,--that two equal streams do not, when united, occupy a bed of
double surface. Nay, the width of the principal river, after the
junction of a tributary, sometimes remains the same as before, or is
even lessened. The cause of this apparent paradox was long ago explained
by the Italian writers, who had studied the confluence of the Po and its
feeders in the plains of Lombardy.
The addition of a smaller river augments the velocity of the main
stream, often in the same proportion as it does the quantity of water.
Thus the Venetian branch of the Po swallowed up the Ferranese branch and
that of Panaro without any enlargement of its own dimensions. The cause
of the greater velocity is, first, that after the union of two rivers
the water, in place of the friction of four shores, has only that of two
to surmount; 2dly, because the main body of the stream being farther
distant from the banks, flows on with less interruption; and lastly,
because a greater quantity of water moving more swiftly, digs deeper
into the river's bed. By this beautiful adjustment, the water which
drains the interior country is made continually to occupy less room as
it approaches the sea; and thus the most valuable part of our
continents, the rich deltas and great alluvial plains, are prevented
from being constantly under water.
_River floods in Scotland_, 1829.--Many remarkable illustrations of the
power of running water in moving stones and heavy materials were
afforded by the storm and floods which occurred on the 3d and 4th of
August, 1829, in Aberdeenshire and other counties in Scotland. The
elements during this storm assumed all the characters which mark the
tropical hurricanes; the wind blowing in sudden gusts and whirlwinds,
the lightning and thunder being such as is rarely witnessed in our
climate, and heavy rain falling without intermission. The floods
extended almost simultaneously, and with equal violence over that part
of the northeast of Scotland which would be cut off by two lines drawn
from the head of Lochrannoch, one towards Inverness and the other to
Stonehaven. The united line of the different rivers which were flooded,
could not be less than from five to six hundred miles in length; and the
whole of their courses were marked by the destruction of bridges, roads,
crops, and buildings. Sir T. D. Lauder has recorded the destruction of
thirty-eight bridges, and the entire obliteration of a great number of
farms and hamlets. On the Nairn, a fragment of sandstone, fourteen feet
long by three feet wide and one foot thick, was carried above 200 yards
down the river. Some new ravines were formed on the sides of mountains
where no streams had previously flowed, and ancient river-channels,
which had never been filled from time immemorial, gave passage to a
copious flood.[270]
The bridge over the Dee at Ballater consisted of five arches, having
upon the whole a water-way of 260 feet. The bed of the river, on which
the piers rested, was composed of rolled pieces of granite and gneiss.
The bridge was built of granite, and had stood uninjured for twenty
years; but the different parts were swept away in succession by the
flood, and the whole mass of masonry disappeared in the bed of the
river. "The river Don," observes Mr. Farquharson, in his account of the
inundations, "has upon my own premises forced a mass of four or five
hundred tons of stones, many of them two or three hundred pounds'
weight, up an inclined plane, rising six feet in eight or ten yards, and
left them in a rectangular heap, about three feet deep on a flat
ground:--the heap ends abruptly at its lower extremity."[271]
The power even of a small rivulet, when swollen by rain, in removing
heavy bodies, was exemplified in August, 1827, in the College, a small
stream which flows at a slight declivity from the eastern watershed of
the Cheviot Hills. Several thousand tons' weight of gravel and sand were
transported to the plain of the Till, and a bridge, then in progress of
building, was carried away, some of the arch-stones of which, weighing
from half to three quarters of a ton each, were propelled two miles down
the rivulet. On the same occasion, the current tore away from the
abutment of a mill-dam a large block of greenstone-porphyry, weighing
nearly two tons, and transported it to the distance of a quarter of a
mile. Instances are related as occurring repeatedly, in which from one
to three thousand tons of gravel are, in like manner, removed by this
streamlet to still greater distances in one day.[272]
_Floods caused by landslips_, 1826.--The power which running water may
exert in the lapse of ages, in widening and deepening a valley, does not
so much depend on the volume and velocity of the stream usually flowing
in it, as on the number and magnitude of the obstructions which have, at
different periods, opposed its free passage. If a torrent, however
small, be effectually dammed up, the size of the valley above the
barrier, and its declivity below, and not the dimensions of the torrent,
will determine the violence of the dabacle. The most universal source of
local deluges, are landslips, slides, or avalanches, as they are
sometimes called, when great masses of rock and soil, or sometimes ice
and snow, are precipitated into the bed of a river, the boundary cliffs
of which have been thrown down by the shock of an earthquake, or
undermined by springs or other causes. Volumes might be filled with the
enumeration of instances on record of these terrific catastrophes; I
shall therefore select a few examples of recent occurrence, the facts of
which are well authenticated.
Two dry seasons in the White Mountains, in New Hampshire (United
States), were followed by heavy rains on the 28th August, 1826, when
from the steep and lofty declivities which rise abruptly on both sides
of the river Saco, innumerable rocks and stones, many of sufficient size
to fill a common apartment, were detached, and in their descent swept
down before 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. The natural excavations commenced generally in a trench a few
yards in depth and a few rods in width, and descended the mountains,
widening and deepening till they became vast chasms. At the base of
these hollow ravines was seen a confused mass of ruins, consisting of
transported earth, gravel, rocks, and trees. Forests of spruce-fir and
hemlock, a kind of fir somewhat resembling our yew in foliage, were
prostrated with as much ease as if they had been fields of grain; for,
where they disputed the ground, 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 their 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 its ordinary level. Many sheep and
cattle were swept away, and the Willey family, nine in number, who in
alarm had deserted their house, were destroyed on the banks of the Saco;
seven of their mangled bodies were afterwards found near the river,
buried beneath drift-wood and mountain ruins.[273] Eleven years after
the event, 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, says Professor Hubbard, a picturesque feature in
the scenery.[274]
When I visited the country in 1845, eight years after Professor Hubbard,
I found the signs of devastation still very striking; I also
particularly remarked that although the surface of the bare granitic
rocks had been smoothed by the passage over them of so much mud and
stone, there were no continuous parallel and rectilinear furrows, nor
any of the fine scratches or striae which characterize _glacial_ action.
The absence of these is nowhere more clearly exemplified than in the
bare rocks over which passed the great "Willey slide" of 1826.[275]
But the catastrophes in the White Mountains are insignificant, when
compared to those which are occasioned by earthquakes, when the boundary
hills, for miles in length, are thrown down into the hollow of a valley.
I shall have opportunities of alluding to inundations of this kind, when
treating expressly of earthquakes, and shall content myself at present
with selecting an example of a flood due to a different cause.
_Flood in the valley of Bagnes_, 1818.--The valley of Bagnes is one of
the largest of the lateral embranchments of the main valley of the
Rhone, above the Lake of Geneva. Its upper portion was, in 1818,
converted into a lake by the damming up of a narrow pass, by avalanches
of snow and ice, precipitated from an elevated glacier into the bed of
the river Dranse. In the winter season, during continued frost, scarcely
any water flows in the bed of this river to preserve an open channel, so
that the ice barrier remained entire until the melting of the snows in
spring, when a lake was formed above, about half a league in length,
which finally attained in some parts a depth of about two hundred feet,
and a width of about seven hundred feet. To prevent or lessen the
mischief apprehended from the sudden bursting of the barrier, an
artificial gallery, seven hundred feet in length, was cut through the
ice, before the waters had risen to a great height. When at length they
accumulated and flowed through this tunnel, they dissolved the ice, and
thus deepened their channel, until nearly half of the whole contents of
the lake were slowly drained off. But at length, on the approach of the
hot season, the central portion of the remaining mass of ice gave way
with a tremendous crash, and the residue of the lake was emptied in half
an hour. In the course of its descent, the waters encountered several
narrow gorges, and at each of these they rose to a great height, and
then burst with new violence into the next basin, sweeping along rocks,
forests, houses, bridges, and cultivated land. For the greater part of
its course the flood resembled a moving mass of rock and mud, rather
than of water. Some fragments of granitic rocks, of enormous magnitude,
and which from their dimensions, might be compared without exaggeration
to houses, were torn out of a more ancient alluvion, and borne down for
a quarter of a mile. One of the fragments moved was sixty paces in
circumference.[276] The velocity of the water, in the first part of its
course, was thirty-three feet per second, which diminished to six feet
before it reached the Lake of Geneva, where it arrived in six hours and
a half, the distance being forty-five miles.[277]
This flood left behind it, on the plains of Martigny, thousands of trees
torn up by the roots, together with the ruins of buildings. Some of the
houses in that town were filled with mud up to the second story. After
expanding in the plain of Martigny, it entered the Rhone, and did no
farther damage; but some bodies of men, who had been drowned above
Martigny, were afterwards found, at the distance of about thirty miles,
floating on the farther side of the Lake of Geneva, near Vevay.
The waters, on escaping from the temporary lake, intermixed with mud and
rock, swept along, for the first four miles, at the rate of above twenty
miles an hour; and M. Escher, the engineer, calculated that the flood
furnished 300,000 cubic feet of water every second--an efflux which is
five times greater than that of the Rhine below Basle. Now, if part of
the lake had not been gradually drained off, the flood would have been
nearly double, approaching in volume to some of the largest rivers in
Europe. It is evident, therefore, that when we are speculating on the
excavating force which a river may have exerted in any particular
valley, the most important question is, not the volume of the existing
stream, nor the present levels of its channel, nor even the nature of
the rocks, but the probability of a succession of floods at some period
since the time when the valley may have been first elevated above the
sea.
For several months after the dabacle of 1818, the Dranse, having no
settled channel, shifted its position continually from one side to the
other of the valley, carrying away newly-erected bridges, undermining
houses, and continuing to be charged with as large a quantity of earthy
matter as the fluid could hold in suspension. I visited this valley four
months after the flood, and was witness to the sweeping away of a
bridge, and the undermining of part of a house. The greater part of the
ice-barrier was then standing, presenting vertical cliffs 150 feet high,
like ravines in the lava-currents of Etna or Auvergne, where they are
intersected by rivers.
Inundations, precisely similar, are recorded to have occurred at former
periods in this district, and from the same cause. In 1595, for example,
a lake burst, and the waters, descending with irresistible fury,
destroyed the town of Martigny, where from sixty to eighty persons
perished. In a similar flood, fifty years before, 140 persons were
drowned.
_Flood at Tivoli_, 1826.--I shall conclude with one more example derived
from a land of classic recollections, the ancient Tibur, and which,
like all the other inundations above alluded to, occurred within the
present century. The younger Pliny, it will be remembered, describes a
flood on the Anio, which destroyed woods, rocks, and houses, with the
most sumptuous villas and works of arts.[278] For four or five centuries
consecutively, this "headlong stream," as Horace truly called it, has
often remained within its bounds, and then, after so long an interval of
rest, has at different periods inundated its banks again, and widened
its channel. The last of these catastrophes happened 15th Nov. 1826,
after heavy rains, such as produced the floods before alluded to in
Scotland. The waters appear also to have been impeded by an artificial
dike, by which they were separated into two parts, a short distance
above Tivoli. They broke through this dike; and leaving the left trench
dry, precipitated themselves, with their whole weight, on the right
side. Here they undermined, in the course of a few hours, a high cliff,
and widened the river's channel about fifteen paces. On this height
stood the church of St. Lucia, and about thirty-six houses of the town
of Tivoli, which were all carried away, presenting as they sank into the
roaring flood, a terrific scene of destruction to the spectators on the
opposite bank. As the foundations were gradually removed, each building,
some of them edifices of considerable height, was first traversed with
numerous rents, which soon widened into large fissures, until at length
the roofs fell in with a crash, and then the walls sunk into the river,
and were hurled down the cataract below.[279]
The destroying agency of the flood came within two hundred yards of the
precipice on which the beautiful temple of Vesta stands; but fortunately
this precious relic of antiquity was spared, while the wreck of modern
structures was hurled down the abyss. Vesta, it will be remembered, in
the heathen mythology, personified the stability of the earth; and when
the Samian astronomer, Aristarchus, first taught that the earth revolved
on its axis, and round the sun, he was publicly accused of impiety, "for
removing the everlasting Vesta from her place." Playfair observed, that
when Hutton ascribed instability to the earth's surface, and represented
the continents which we inhabit as the theatre of incessant change and
movement, his antagonists, who regarded them as unalterable, assailed
him in a similar manner with accusations founded on religious
prejudices.[280] We might appeal to the excavating power of the Anio as
corroborative of one of the most controverted parts of the Huttonian
theory; and if the days of omens had not gone by, the geologists who now
worship Vesta might regard the late catastrophe as portentous. We may,
at least, recommend the modern votaries of the goddess to lose no time
in making a pilgrimage to her shrine, for the next flood may not respect
the temple.
_Excavation of rocks by running water._--The rapidity with which even
the smallest streams hollow out deep channels in soft and destructible
soils is remarkably exemplified in volcanic countries, where the sand
and half-consolidated tuffs opposed but a slight resistance to the
torrents which descend the mountain-side. After the heavy rains which
followed the eruption of Vesuvius in 1824, the water flowing from the
Atrio del Cavallo cut, in three days, a new chasm through strata of tuff
and ejected volcanic matter, to the depth of twenty-five feet. I found
the old mule-road, in 1828, intersected by this new ravine.
The gradual erosion of deep chasms through some of the hardest rocks, by
the constant passage of running water, charged with foreign matter, is
another phenomenon of which striking examples may be adduced.
Illustrations of this excavating power are presented by many valleys in
central France where the channels of rivers have been barred up by solid
currents of lava, through which the streams have re-excavated a passage,
to the depth of from twenty to seventy feet and upwards, and often of
great width. In these cases there are decisive proofs that neither the
sea, nor any denuding wave or extraordinary body of water, has passed
over the spot since the melted lava was consolidated. Every hypothesis
of the intervention of sudden and violent agency is entirely excluded,
because the cones of _loose_ scoriae, out of which the lavas flowed, are
oftentimes at no great elevation above the rivers, and have remained
undisturbed during the whole period which has been sufficient for the
hollowing out of such enormous ravines.
_Recent excavation by the Simeto._--But I shall at present confine
myself to examples derived from events which have happened since the
time of history.
[Illustration: Fig. 16.
Recent excavation of lava at the foot of Etna by the river Simeto.]
At the western base of Etna, a current of lava (A A, fig. 16),
descending from near the summit of the great volcano, has flowed to the
distance of five or six miles, and then reached the alluvial plain of
the Simeto, the largest of the Sicilian rivers, which skirts the base of
Etna, and falls into the sea a few miles south of Catania. The lava
entered the river about three miles above the town of Aderno, and not
only occupied its channel for some distance, but, crossing to the
opposite side of the valley, accumulated there in a rocky mass.
Gemmellaro gives the year 1603 as the date of the eruption.[281] The
appearance of the current clearly proves, that it is one of the most
modern of those of Etna; for it has not been covered or crossed by
subsequent streams or ejections, and the olives which had been planted
on its surface were all of small size, when I examined the spot in 1828,
yet they were older than the natural wood on the same lava. In the
course, therefore, of about two centuries, the Simeto has eroded a
passage from fifty to several hundred feet wide, and in some parts from
forty to fifty feet deep.
The portion of lava cut through is in no part porous or scoriaceous, but
consists of a compact homogeneous mass of hard blue rock, somewhat
inferior in weight to ordinary basalt, and containing crystals of
olivine and glassy felspar. The general declivity of this part of the
bed of the Simeto is not considerable; but, in consequence of the
unequal waste of the lava, two water-falls occur at Passo Manzanelli,
each about six feet in height. Here the chasm (B, fig. 16) is about
forty feet deep, and only fifty broad.
The sand and pebbles in the river-bed consist chiefly of a brown
quartzose sandstone, derived from the upper country; but the materials
of the volcanic rock itself must have greatly assisted the attrition.
This river, like the Caltabiano on the eastern side of Etna, has not yet
cut down to the ancient bed of which it was dispossessed, and of which
the probable position is indicated in the annexed diagram (C, fig. 16).
On entering the narrow ravine where the water foams down the two
cataracts, we are entirely shut out from all view of the surrounding
country; and a geologist who is accustomed to associate the
characteristic features of the landscape with the relative age of
certain rocks, can scarcely dissuade himself from the belief that he is
contemplating a scene in some rocky gorge of a primary district. The
external forms of the hard blue lava are as massive as any of the most
ancient trap-rocks of Scotland. The solid surface is in some parts
smoothed and almost polished by attrition, and covered in others with a
white lichen, which imparts to it an air of extreme antiquity, so as
greatly to heighten the delusion. But the moment we reascend the cliff
the spell is broken; for we scarcely recede a few paces, before the
ravine and river disappear, and we stand on the black and rugged surface
of a vast current of lava, which seems unbroken, and which we can trace
up nearly to the distant summit of that majestic cone which Pindar
called "the pillar of heaven," and which still continues to send forth a
fleecy wreath of vapor, reminding us that its fires are not extinct, and
that it may again give out a rocky stream, wherein other scenes like
that now described may present themselves to future observers.
[Illustration: Fig. 17. Lake Erie. The Falls.
Limestone Shale.
Lewiston. Niagara River. Queenstown.]
_Falls of Niagara._--The falls of Niagara afford a magnificent example
of the progressive excavation of a deep valley in solid rock. That river
flows over a flat table-land, in a depression of which Lake Erie is
situated. Where it issues from the lake, it is nearly a mile in width,
and 330 feet above Lake Ontario, which is about 30 miles distant. For
the first fifteen miles below Lake Erie the surrounding country,
comprising Upper Canada on the west, and the state of New York on the
east, is almost on a level with its banks, and nowhere more than thirty
or forty feet above them.[282] (See fig. 17.) The river being
occasionally interspersed with low wooded islands, and having sometimes
a width of three miles, glides along at first with a clear, smooth, and
tranquil current, falling only fifteen feet in as many miles, and in
this part of its course resembling an arm of Lake Erie. But its
character is afterwards entirely changed, on approaching the Rapids,
where it begins to rush and foam over a rocky and uneven limestone
bottom, for the space of nearly a mile, till at length it is thrown down
perpendicularly 165 feet at the Falls. Here the river is divided into
two sheets of water by an island, the largest cataract being more than a
third of a mile broad, the smaller one having a breadth of six hundred
feet. When the water has precipitated itself into an unfathomable pool,
it rushes with great velocity down the sloping bottom of a narrow chasm,
for a distance of seven miles. This ravine varies from 200 to 400 yards
in width from cliff to cliff; contrasting, therefore, strongly in its
breadth with that of the river above. Its depth is from 200 to 300 feet,
and it intersects for about seven miles the table-land before described,
which terminates suddenly at Queenstown in an escarpment or long line of
inland cliff facing northwards, towards Lake Ontario. The Niagara, on
reaching the escarpment and issuing from the gorge, enters the flat
country, which is so nearly on a level with Lake Ontario, that there is
only a fall of about four feet in the seven additional miles which
intervene between Queenstown and the shores of that lake.
It has long been the popular belief that the Niagara once flowed in a
shallow valley across the whole platform, from the present site of the
Falls to the escarpment (called the Queenstown heights), where it is
supposed that the cataract was first situated, and that the river has
been slowly eating its way backwards through the rocks for the distance
of seven miles. This hypothesis naturally suggests itself to every
observer, who sees the narrowness of the gorge at its termination, and
throughout its whole course, as far up as the Falls, above which point
the river expands as before stated. The boundary cliffs of the ravine
are usually perpendicular, and in many places undermined on one side by
the impetuous stream. The uppermost rock of the table-land at the Falls
consists of hard limestone (a member of the Silurian series), about
ninety feet thick, beneath which lie soft shales of equal thickness,
continually undermined by the action of the spray, which rises from the
pool into which so large a body of water is projected, and is driven
violently by gusts of wind against the base of the precipice. In
consequence of this action, and that of frost, the shale disintegrates
and crumbles away, and portions of the incumbent rock overhang 40 feet,
and often when unsupported tumble down, so that the Falls do not remain
absolutely stationary at the same spot, even for half a century.
Accounts have come down to us, from the earliest period of observation,
of the frequent destruction of these rocks, and the sudden descent of
huge fragments in 1818 and 1828, are said to have shaken the adjacent
country like an earthquake. The earliest travellers, Hennepin and Kalm,
who in 1678 and 1751 visited the Falls, and published views of them,
attest the fact, that the rocks have been suffering from dilapidation
for more than a century and a half, and that some slight changes, even
in the scenery of the cataract have been brought about within that time.
The idea, therefore, of perpetual and progressive waste is constantly
present to the mind of every beholder; and as that part of the chasm,
which has been the work of the last hundred and fifty years resembles
precisely, in depth, width, and character, the rest of the gorge which
extends seven miles below, it is most natural to infer, that the entire
ravine has been hollowed out in the same manner, by the recession of the
cataract.
It must at least be conceded, that the river supplies an adequate cause
for executing the whole task thus assigned to it, provided we grant
sufficient time for its completion. As this part of the country was a
wilderness till near the end of the last century, we can obtain no
accurate data for estimating the exact rate at which the cataract has
been receding. Mr. Bakewell, son of the eminent geologist of that name,
who visited the Niagara in 1829, made the first attempt to calculate
from the observations of one who had lived forty years at the Falls, and
who had been the first settler there, that the cataract had during that
period gone back about a yard annually. But after the most careful
inquiries which I was able to make, during my visit to the spot in
1841-2, I came to the conclusion that the average of one foot a year
would be a much more probable conjecture. In that case, it would have
required thirty-five thousand years for the retreat of the Falls, from
the escarpment of Queenstown to their present site. It seems by no means
improbable that such a result would be no exaggeration of the truth,
although we cannot assume that the retrograde movement has been uniform.
An examination of the geological structure of the district, as laid open
in the ravine, shows that at every step in the process of excavation,
the height of the precipice, the hardness of the materials at its base,
and the quantity of fallen matter to be removed, must have varied. At
some points it may have receded much faster than at present, but in
general its progress was probably slower, because the cataract, when it
began to recede, must have had nearly twice its present height.
From observations made by me in 1841, when I had the advantage of being
accompanied by Mr. Hall, state geologist of New York, and in 1842, when
I re-examined the Niagara district, I obtained geological evidence of
the former existence of an old river-bed, which, I have no doubt,
indicates the original channel through which the waters once flowed from
the Falls to Queenstown, at the height of nearly three hundred feet
above the bottom of the present gorge. The geological monuments alluded
to, consist of patches of sand and gravel, forty feet thick, containing
fluviatile shells of the genera Unio, Cyclas, Melania, &c., such as now
inhabit the waters of the Niagara above the Falls. The identity of the
fossil species with the recent is unquestionable, and these freshwater
deposits occur at the edge of the cliffs bounding the ravine, so that
they prove the former extension of an elevated shallow valley, four
miles below the falls, a distinct prolongation of that now occupied by
the Niagara, in the elevated region intervening between Lake Erie and
the Falls. Whatever theory be framed for the hollowing out of the ravine
further down, or for the three miles which intervene between the
whirlpool and Queenstown, it will always be necessary to suppose the
former existence of a barrier of _rock_, not of loose and destructible
materials, such as those composing the drift in this district, somewhere
immediately below the whirlpool. By that barrier the waters were held
back for ages, when the fluviatile deposit, 40 feet in thickness, and
250 feet above the present channel of the river, originated. If we are
led by this evidence to admit that the cataract has cut back its way for
four miles, we can have little hesitation in referring the excavation of
the remaining three miles below to a like agency, the shape of the chasm
being precisely similar.
There have been many speculations respecting the future recession of the
Falls, and the deluge that might be occasioned by the sudden escape of
the waters of Lake Erie, if the ravine should ever be prolonged 16 miles
backwards. But a more accurate knowledge of the geological succession of
the rocks, brought to light by the State Survey, has satisfied every
geologist that the Falls would diminish gradually in height before they
travelled back two miles, and in consequence of a gentle dip of the
strata to the south, the massive limestone now at the top would then be
at their base, and would retard, and perhaps put an effectual stop to,
the excavating process.