Man and Nature Part 11

"Such is their aspect in dry weather. But no tongue can give an adequate description of their devastations in one of those sudden floods which resemble, in almost none of their phenomena, the action of ordinary river water. They are now no longer overflowing brooks, but real seas, tumbling down in cataracts, and rolling before them blocks of stone, which are hurled forward by the shock of the waves like b.a.l.l.s shot out by the explosion of gunpowder. Sometimes ridges of pebbles are driven down when the transporting torrent does not rise high enough to show itself, and then the movement is accompanied with a roar louder than the crash of thunder. A furious wind precedes the rus.h.i.+ng water and announces its approach. Then comes a violent eruption, followed by a flow of muddy waves, and after a few hours all returns to the dreary silence which at periods of rest marks these abodes of desolation.

"This is but an imperfect sketch of this scourge of the Alps. Its devastations are increasing with the progress of clearing, and are every day turning a portion of our frontier departments into barren wastes.

"The unfortunate pa.s.sion for clearing manifested itself at the beginning of the French Revolution, and has much increased under the pressure of immediate want. It has now reached an extreme point, and must be speedily checked, or the last inhabitant will be compelled to retreat when the last tree falls.

"The elements of destruction are increasing in violence. Rivers might be mentioned whose beds have been raised ten feet in a single year. The devastation advances in geometrical progression as the higher slopes are bared of their wood, and 'the ruin from above,' to use the words of a peasant, 'helps to hasten the desolation below.'

"The Alps of Provence present a terrible aspect. In the more equable climate of Northern France, one can form no conception of those parched mountain gorges where not even a bush can be found to shelter a bird, where, at most, the wanderer sees in summer here and there a withered lavender, where all the springs are dried up, and where a dead silence, hardly broken by even the hum of an insect, prevails. But if a storm bursts forth, ma.s.ses of water suddenly shoot from the mountain heights into the shattered gulfs, waste without irrigating, deluge without refres.h.i.+ng the soil they overflow in their swift descent, and leave it even more seared than it was from want of moisture. Man at last retires from the fearful desert, and I have, the present season, found not a living soul in districts where I remember to have enjoyed hospitality thirty years ago."

In 1853, ten years after the date of Blanqui's memoir, M. de Bonville, prefect of the Lower Alps, addressed to the Government a report in which the following pa.s.sages occur:

"It is certain that the productive mould of the Alps, swept off by the increasing violence of that curse of the mountains, the torrents, is daily diminis.h.i.+ng with fearful rapidity. All our Alps are wholly, or in large proportion, bared of wood. Their soil, scorched by the sun of Provence, cut up by the hoofs of the sheep, which, not finding on the surface the gra.s.s they require for their sustenance, scratch the ground in search of roots to satisfy their hunger, is periodically washed and carried off by melting snows and summer storms.

"I will not dwell on the effects of the torrents. For sixty years they have been too often depicted to require to be further discussed, but it is important to show that their ravages are daily extending the range of devastation. The bed of the Durance, which now in some places exceeds 2,000 metres [about 6,600 feet, or a mile and a quarter] in width, and, at ordinary times, has a current of water less than 10 metres [about 33 feet] wide, shows something of the extent of the damage.[220] Where, ten years ago, there were still woods and cultivated grounds to be seen, there is now but a vast torrent: there is not one of our mountains which has not at least one torrent, and new ones are daily forming.

"An indirect proof of the diminution of the soil is to be found in the depopulation of the country. In 1852, I reported to the General Council that, according to the census of that year, the population of the department of the Lower Alps had fallen off no less than 5,000 souls in the five years between 1846 and 1851.

"Unless prompt and energetic measures are taken, it is easy to fix the epoch when the French Alps will be but a desert. The interval between 1851 and 1856 will show a further decrease of population. In 1862, the ministry will announce a continued and progressive reduction in the number of acres devoted to agriculture; every year will aggravate the evil, and, in a half century, France will count more ruins, and a department the less."

Time has verified the predictions of De Bonville. The later census returns show a progressive diminution in the population of the departments of the Lower Alps, the Isere, the Drome, Ariege, the Upper and the Lower Pyrenees, the Lozere, the Ardennes, the Doubs, the Vosges, and, in short, in all the provinces formerly remarkable for their forests. This diminution is not to be ascribed to a pa.s.sion for foreign emigration, as in Ireland, and in parts of Germany and of Italy; it is simply a transfer of population from one part of the empire to another, from soils which human folly has rendered uninhabitable, by ruthlessly depriving them of their natural advantages and securities, to provinces where the face of the earth was so formed by nature as to need no such safeguards, and where, consequently, she preserves her outlines in spite of the wasteful improvidence of man.[221]

Highly colored as these pictures seem, they are not exaggerated, although the hasty tourist through Southern France and Northern Italy, finding little in his high road experiences to justify them, might suppose them so. The lines of communication by locomotive train and diligence lead generally over safer ground, and it is only when they ascend the Alpine pa.s.ses and traverse the mountain chains, that scenes somewhat resembling those just described fall under the eye of the ordinary traveller. But the extension of the sphere of devastation, by the degradation of the mountains and the transportation of their debris, is producing a.n.a.logous effects upon the lower ridges of the Alps and the plains which skirt them; and even now one needs but an hour's departure from some great thoroughfares to reach sites where the genius of destruction revels as wildly as in the most frightful of the abysses which Blanqui has painted.[222]

There is one effect of the action of torrents which few travellers on the Continent are heedless enough to pa.s.s without notice. I refer to the elevation of the beds of mountain streams in consequence of the deposit of the debris with which they are charged. To prevent the spread of sand and gravel over the fields and the deluging overflow of the raging waters, the streams are confined by walls and embankments, which are gradually built higher and higher as the bed of the torrent is raised, so that, to reach a river, you ascend from the fields beside it; and sometimes the ordinary level of the stream is above the streets and even the roofs of the towns through which it pa.s.ses.[223]

The traveller who visits the depths of an Alpine ravine, observes the length and width of the gorge and the great height and apparent solidity of the precipitous walls which bound it, and calculates the ma.s.s of rock required to fill the vacancy, can hardly believe that the humble brooklet which purls at his feet has been the princ.i.p.al agent in accomplis.h.i.+ng this tremendous erosion. Closer observation will often teach him, that the seemingly unbroken rock which overhangs the valley is full of cracks and fissures, and really in such a state of disintegration that every frost must bring down tons of it. If he compute the area of the basin which finds here its only discharge, he will perceive that a sudden thaw of the winter's deposit of snow, or one of those terrible discharges of rain so common in the Alps, must send forth a deluge mighty enough to sweep down the largest ma.s.ses of gravel and of rock.[224] The simple measurement of the cubical contents of the semi-circular hillock which he climbed before he entered the gorge, the structure and composition of which conclusively show that it must have been washed out of this latter by torrential action, will often account satisfactorily for the disposal of most of the matter which once filled the ravine.

It must further be remembered, that every inch of the violent movement of the rocks is accompanied with crus.h.i.+ng concussion, or, at least, with great abrasion, and, as you follow the deposit along the course of the waters which transport it, you find the stones gradually rounding off in form, and diminis.h.i.+ng in size until they pa.s.s successively into gravel, sand, impalpable slime.

I do not mean to a.s.sert that all the rocky valleys of the Alps have been produced by the action of torrents resulting from the destruction of the forests. All the greater, and many of the smaller channels, by which that chain is drained, owe their origin to higher causes. They are primitive fissures, ascribable to disruption in upheaval or other geological convulsion, widened and scarped, and often even polished, so to speak, by the action of glaciers during the ice period, and but little changed in form by running water in later eras.[225]

In these valleys of ancient formation, which extend into the very heart of the mountains, the streams, though rapid, have lost the true torrential character, if, indeed, they ever possessed it. Their beds have become approximately constant, and their walls no longer crumble and fall into the waters that wash their bases. The torrent-worn ravines, of which I have spoken, are of later date, and belong more properly to what may be called the crust of the Alps, consisting of loose rocks, of gravel, and of earth, strewed along the surface of the great declivities of the central ridge, and acc.u.mulated thickly between their solid b.u.t.tresses. But it is on this crust that the mountaineer dwells. Here are his forests, here his pastures, and the ravages of the torrent both destroy his world, and convert it into a source of overwhelming desolation to the plains below.

_Transporting Power of Rivers._

An instance that fell under my own observation in 1857, will serve to show something of the eroding and transporting power of streams which, in these respects, fall incalculably below the torrents of the Alps. In a flood of the Ottaquechee, a small river which flows through Woodstock, Vermont, a milldam on that stream burst, and the sediment with which the pond was filled, estimated after careful measurement at 13,000 cubic yards, was carried down by the current. Between this dam and the slack water of another, four miles below, the bed of the stream, which is composed of pebbles interspersed in a few places with larger stones, is about sixty-five feet wide, though, at low water, the breadth of the current is considerably less. The sand and fine gravel were smoothly and evenly distributed over the bed to a width of fifty-five or sixty feet, and for a distance of about two miles, except at two or three intervening rapids, filled up all the interstices between the stones, covering them to the depth of nine or ten inches, so as to present a regularly formed concave channel, lined with sand, and reducing the depth of water, in some places, from five or six feet to fifteen or eighteen inches. Observing this deposit after the river had subsided and become so clear that the bottom could be seen, I supposed that the next flood would produce an extraordinary erosion of the banks and some permanent changes in the channel of the stream, in consequence of the elevation of the bed and the filling up of the s.p.a.ces between the stones through which formerly much water had flowed; but no such result followed. The spring freshet of the next year entirely washed out the sand its predecessor had deposited, carried it to ponds and still-water reaches below, and left the bed of the river almost precisely in its former condition, though, of course, with the slight displacement of the pebbles which every flood produces in the channels of such streams. The pond, though often previously discharged by the breakage of the dam, had then been undisturbed for about twenty-five years, and its contents consisted almost entirely of sand, the rapidity of the current in floods being such that it would let fall little lighter sediment, even above an obstruction like a dam. The quant.i.ty I have mentioned evidently bears a very inconsiderable proportion to the total erosion of the stream during that period, because the wash of the banks consists chiefly of fine earth rather than of sand, and after the pond was once filled, or nearly so, even this material could no longer be deposited in it. The fact of the complete removal of the deposit I have described between the two dams in a single freshet, shows that, in spite of considerable obstruction from roughness of bed, large quant.i.ties of sand may be taken up and carried off by streams of no great rapidity of inclination; for the whole descent of the bed of the river between the two dams--a distance of four miles--is but sixty feet, or fifteen feet to the mile.

_The Po and its Deposits._

The current of the river Po, for a considerable distance after its volume of water is otherwise sufficient for continuous navigation, is too rapid for that purpose until near Piacenza, where its velocity becomes too much reduced to transport great quant.i.ties of mineral matter, except in a state of minute division. Its southern affluents bring down from the Apennines a large quant.i.ty of fine earth from various geological formations, while its Alpine tributaries west of the Ticino are charged chiefly with rock ground down to sand or gravel.[226] The bed of the river has been somewhat elevated by the deposits in its channel, though not by any means above the level of the adjacent plains as has been so often represented. The dikes, which confine the current at high water, at the same time augment its velocity and compel it to carry most of its sediment to the Adriatic. It has, therefore, raised neither its own channel nor its alluvial sh.o.r.es, as it would have done if it had remained unconfined. But, as the surface of the water in floods is from six to fifteen feet above the general level of its banks, the Po can, at that period, receive no contributions of earth from the was.h.i.+ng of the fields of Lombardy, and there is no doubt that a large proportion of the sediment it now deposits at its mouth descended from the Alps in the form of rock, though reduced by the grinding action of the waters, in its pa.s.sage seaward, to the condition of fine sand, and often of silt.[227]

We know little of the history of the Po, or of the geography of the coast near the point where it enters the Adriatic, at any period more than twenty centuries before our own. Still less can we say how much of the plains of Lombardy had been formed by its action, combined with other causes, before man accelerated its levelling operations by felling the first woods on the mountains whence its waters are derived. But we know that since the Roman conquest of Northern Italy, its deposits have amounted to a quant.i.ty which, if recemented into rock, recombined into gravel, common earth, and vegetable mould, and restored to the situations where eruption or upheaval originally placed, or vegetation deposited it, would fill up hundreds of deep ravines in the Alps and Apennines, change the plan and profile of their chains, and give their southern and northern faces respectively a geographical aspect very different from that they now present. Ravenna, forty miles south of the princ.i.p.al mouth of the Po, was built like Venice, in a lagoon, and the Adriatic still washed its walls at the commencement of the Christian era. The mud of the Po has filled up the lagoon, and Ravenna is now four miles from the sea. The town of Adria, which lies between the Po and the Adige, at the distance of some four or five miles from each, was once a harbor famous enough to have given its name to the Adriatic sea, and it was still a seaport in the time of Augustus. The combined action of the two rivers has so advanced the coast line that Adria is now about fourteen miles inland, and, in other places, the deposits made within the same period by these and other neighboring streams have a width of twenty miles.

What proportion of the earth with which they are charged these rivers have borne out into deep water, during the last two thousand years, we do not know, but as they still transport enormous quant.i.ties, as the North Adriatic appears to have shoaled rapidly, and as long islands, composed in great part of fluviatile deposits, have formed opposite their mouths, it must evidently have been very great. The floods of the Po occur but once, or sometimes twice in a year.[228] At other times, its waters are comparatively limpid and seem to hold no great amount of mud or fine sand in mechanical suspension; but at high water it contains a large proportion of solid matter, and according to Lombardini, it annually transports to the sh.o.r.es of the Adriatic not less than 42,760,000 cubic metres, or very nearly 55,000,000 cubic yards, which carries the coast line out into the sea at the rate of more than 200 feet in a year.[229] The depth of the annual deposit is stated at eighteen centimetres, or rather more than seven inches, and it would cover an area of not much less than ninety square miles with a layer of that thickness. The Adige, also, brings every year to the Adriatic many million cubic yards of Alpine detritus, and the contributions of the Brenta from the same source are far from inconsiderable. The Adriatic, however, receives but a small proportion of the soil and rock washed away from the Italian slope of the Alps and the northern declivity of the Apennines by torrents. Nearly the whole of the debris thus removed from the southern face of the Alps between Monte Rosa and the sources of the Adda--a length of watershed not less than one hundred and fifty miles--is arrested by the still waters of the Lakes Maggiore and Como, and some smaller lacustrine reservoirs, and never reaches the sea. The Po is not continuously embanked except for the lower half of its course.

Above Piacenza, therefore, it spreads and deposits sediment over a wide surface, and the water withdrawn from it for irrigation at lower points, as well as its inundations in the occasional ruptures of its banks, carry over the adjacent soil a large amount of slime.

If we add to the estimated annual deposits of the Po at its mouth, the earth and sand transported to the sea by the Adige, the Brenta, and other less important streams, the prodigious ma.s.s of detritus swept into Lago Maggiore by the Tosa, the Maggia, and the Ticino, into the lake of Como by the Maira and the Adda, into the lake of Garda by its affluents, and the yet vaster heaps of pebbles, gravel, and earth permanently deposited by the torrents near their points of eruption from mountain gorges, or spread over the wide plains at lower levels, we may safely a.s.sume that we have an aggregate of not less than four times the quant.i.ty carried to the Adriatic by the Po, or 220,000,000 cubic yards of solid matter, abstracted every year from the Italian Alps and the Apennines, and removed out of their domain by the force of running water.[230]

The present rate of deposit at the mouth of the Po has continued since the year 1600, the previous advance of the coast, after the year 1200, having been only one third as rapid. The great increase of erosion and transport is ascribed by Lombardini chiefly to the destruction of the forests in the basin of that river and the valleys, of its tributaries, since the beginning of the seventeenth century.[231] We have no data to show the rate of deposit in any given century before the year 1200, and it doubtless varied according to the progress of population and the consequent extension of clearing and cultivation. The transporting power of torrents is greatest soon after their formation, because at that time their points of delivery are lower, and, of course, their general slope and velocity more rapid, than after years of erosion above, and deposit below, have depressed the beds of their mountain valleys, and elevated the channels of their lower course. Their eroding action also is most powerful at the same period, both because their mechanical force is then greatest, and because the loose earth and stones of freshly cleared forest ground are most easily removed. Many of the Alpine valleys west of the Ticino--that of the Dora Baltea for instance--were nearly stripped of their forests in the days of the Roman empire, others in the Middle Ages, and, of course, there must have been, at different periods before the year 1200, epochs when the erosion and transportation of solid matter from the Alps and the Apennines were as great as since the year 1600.

Upon the whole, we shall not greatly err if we a.s.sume that, for a period of not less than two thousand years, the walls of the basin of the Po--the Italian slope of the Alps, and the northern and northeastern declivities of the Apennines--have annually sent down into the Adriatic, the lakes, and the plains, not less than 150,000,000 cubic yards of earth and disintegrated rock. We have, then, an aggregate of 300,000,000,000 cubic yards of such material, which, allowing to the mountain surface in question an area of 50,000,000,000 square yards, would cover the whole to the depth of six yards.[232] There are very large portions of this area, where, as we know from ancient remains--roads, bridges, and the like--from other direct testimony, and from geological considerations, very little degradation has taken place within twenty centuries, and hence the quant.i.ty to be a.s.signed to localities where the destructive causes have been most active is increased in proportion.

If this vast ma.s.s of pulverized rock and earth were restored to the localities from which it was derived, it certainly would not obliterate valleys and gorges hollowed out by great geological causes, but it would reduce the length and diminish the depth of ravines of later formation, modify the inclination of their walls, reclothe with earth many bare mountain ridges, essentially change the line of junction between plain and mountain, and carry back a long reach of the Adriatic coast many miles to the west.[233]

It is, indeed, not to be supposed that all the degradation of the mountains is due to the destruction of the forests--that the flanks of every Alpine valley in Central Europe below the snow line were once covered with earth and green with woods, but there are not many particular cases, in which we can, with certainty, or even with strong probability, affirm the contrary.

We cannot measure the share which human action has had in augmenting the intensity of causes of mountain degradation, but we know that the clearing of the woods has, in some cases, produced within two or three generations, effects as blasting as those generally ascribed to geological convulsions, and has laid waste the face of the earth more hopelessly than if it had been buried by a current of lava or a shower of volcanic sand. Now torrents are forming every year in the Alps.

Tradition, written records, and a.n.a.logy concur to establish the belief that the ruin of most of the now desolate valleys in those mountains is to be ascribed to the same cause, and authentic descriptions of the irresistible force of the torrent show that, aided by frost and heat, it is adequate to level Mont Blanc and Monte Rosa themselves, unless new upheavals shall maintain their elevation.

It has been contended that all rivers which take their rise in mountains originated in torrents. These, it is said, have lowered the summits by gradual erosion, and, with the material thus derived, have formed shoals in the sea which once beat against the cliffs; then, by successive deposits, gradually raised them above the surface, and finally expanded them into broad plains traversed by gently flowing streams. If we could go back to earlier geological periods, we should find this theory often verified, and we cannot fail to see that the torrents go on at the present hour, depressing still lower the ridges of the Alps and the Apennines, raising still higher the plains of Lombardy and Provence, extending the coast still farther into the Adriatic and the Mediterranean, reducing the inclination of their own beds and the rapidity of their flow, and thus tending to become river-like in character.

There are cases where torrents cease their ravages of themselves, in consequence of some change in the condition of the basin where they originate, or of the face of the mountain at a higher level, while the plain or the sea below remains in substantially the same state as before. If a torrent rises in a small valley containing no great amount of earth and of disintegrated or loose rock, it may, in the course of a certain period, wash out all the transportable material, and if the valley is then left with solid walls, it will cease to furnish debris to be carried down by floods. If, in this state of things, a new channel be formed at an elevation above the head of the valley, it may divert a part, or even the whole of the rain water and melted snow which would otherwise have flowed into it, and the once furious torrent now sinks to the rank of a humble and harmless brooklet. "In traversing this department," says Surell, "one often sees, at the outlet of a gorge, a flattened hillock, with a fan-shaped outline and regular slopes; it is the bed of dejection of an ancient torrent. It sometimes requires long and careful study to detect the primitive form, masked as it is by groves of trees, by cultivated fields, and often by houses, but, when examined closely, and from different points of view, its characteristic figure manifestly appears, and its true history cannot be mistaken.

Along the hillock flows a streamlet, issuing from the ravine, and quietly watering the fields. This was originally a torrent, and in the background may be discovered its mountain basin. Such _extinguished_ torrents, if I may use the expression, are numerous."[234]

But for the intervention of man and domestic animals, these latter beneficent revolutions would occur more frequently, proceed more rapidly. The new scarped mountains, the hillocks of debris, the plains elevated by sand and gravel spread over them, the sh.o.r.es freshly formed by fluviatile deposits, would clothe themselves with shrubs and trees, the intensity of the causes of degradation would be diminished, and nature would thus regain her ancient equilibrium. But these processes, under ordinary circ.u.mstances, demand, not years, generations, but centuries;[235] and man, who even now finds scarce breathing room on this vast globe, cannot retire from the Old World to some yet undiscovered continent, and wait for the slow action of such causes to replace, by a new creation, the Eden he has wasted.

_Mountain Slides._

I have said that the mountainous regions of the Atlantic States of the American Union are exposed to similar ravages, and I may add that there is, in some cases, reason to apprehend from the same cause even more appalling calamities than those which I have yet described. The slide in the Notch of the White Mountains, by which the Willey family lost their lives, is an instance of the sort I refer to, though I am not able to say that in this particular case, the slip of the earth and rock was produced by the denudation of the surface. It may have been occasioned by this cause, or by the construction of the road through the Notch, the excavations for which, perhaps, cut through the b.u.t.tresses that supported the sloping strata above.

Not to speak of the fall of earth when the roots which held it together, and the bed of leaves and mould which sheltered it both from disintegrating frost and from sudden drenching and dissolution by heavy showers, are gone, it is easy to see that, in a climate with severe winters, the removal of the forest, and, consequently, of the soil it had contributed to form, might cause the displacement and descent of great ma.s.ses of rock. The woods, the vegetable mould, and the soil beneath, protect the rocks they cover from the direct action of heat and cold, and from the expansion and contraction which accompany them. Most rocks, while covered with earth, contain a considerable quant.i.ty of water.[236] A fragment of rock pervaded with moisture cracks and splits, if thrown into a furnace, and sometimes with a loud detonation; and it is a familiar observation that the fire, in burning over newly cleared lands, breaks up and sometimes almost pulverizes the stones. This effect is due partly to the unequal expansion of the stone, partly to the action of heat on the water it contains in its pores. The sun, suddenly let in upon rock which had been covered with moist earth for centuries, produces more or less disintegration in the same way, and the stone is also exposed to chemical influences from which it was sheltered before.

But in the climate of the United States as well as of the Alps, frost is a still more powerful agent in breaking up mountain ma.s.ses. The soil that protects the lime and sand stone, the slate and the granite from the influence of the sun, also prevents the water which filters into their crevices and between their strata from freezing in the hardest winters, and the moisture descends, in a liquid form, until it escapes in springs, or pa.s.ses off by deep subterranean channels. But when the ridges are laid bare, the water of the autumnal rains fills the minutest pores and veins and fissures and lines of separation of the rocks, then suddenly freezes, and bursts asunder huge, and apparently solid blocks of adamantine stone.[237] Where the strata are inclined at a considerable angle, the freezing of a thin film of water over a large interstratal area might occasion a slide that should cover miles with its ruins; and similar results might be produced by the simple hydrostatic pressure of a column of water, admitted by the removal of the covering of earth to flow into a crevice faster than it could escape through orifices below.

Earth or rather mountain slides, compared to which the catastrophe that buried the Willey family in New Hamps.h.i.+re was but a pinch of dust, have often occurred in the Swiss Italian, and French Alps. The land slip, which overwhelmed and covered to the depth of seventy feet, the town of Plurs in the valley of the Maira, on the night of the 4th of September, 1618, sparing not a soul of a population of 2,430 inhabitants, is one of the most memorable of these catastrophes, and the fall of the Rossberg or Rufiberg, which destroyed the little town of Goldau in Switzerland, and 450 of its people, on the 2d of September, 1806, is almost equally celebrated. In 1771, according to Wessely, the mountain peak Piz, near Alleghe in the province of Belluno, slipped into the bed of the Cordevole, a tributary of the Piave, destroying in its fall three hamlets and sixty lives. The rubbish filled the valley for a distance of nearly two miles, and, by damming up the waters of the Cordevole, formed a lake about three miles long, and a hundred and fifty feet deep, which still subsists, though reduced to half its original length by the wearing down of its outlet.[238]

On the 14th of February, 1855, the hill of Belmonte, a little below the parish of San Stefano, in Tuscany, slid into the valley of the Tiber, which consequently flooded the village to the depth of fifty feet, and was finally drained off by a tunnel. The ma.s.s of debris is stated to have been about 3,500 feet long, 1,000 wide, and not less than 600 high.[239]

Such displacements of earth and rocky strata rise to the magnitude of geological convulsions, but they are of so rare occurrence in countries still covered by the primitive forest, so common where the mountains have been stripped of their native covering, and, in many cases, so easily explicable by the drenching of incohesive earth from rain, or the free admission of water between the strata of rocks--both of which a coating of vegetation would have prevented--that we are justified in ascribing them for the most part to the same cause as that to which the destructive effects of mountain torrents are chiefly due--the felling of the woods.

In nearly every case of this sort the circ.u.mstances of which are known, the immediate cause of the slip has been, either an earthquake, the imbibition of water in large quant.i.ties by bare earth, or its introduction between or beneath solid strata. If water insinuates itself between the strata, it creates a sliding surface, or it may, by its expansion in freezing, separate beds of rock, which had been nearly continuous before, widely enough to allow the gravitation of the superinc.u.mbent ma.s.s to overcome the resistance afforded by inequalities of face and by friction; if it finds its way beneath hard earth or rock reposing on clay or other bedding of similar properties, it converts the supporting layer into a semi-fluid mud, which opposes no obstacle to the sliding of the strata above.

The upper part of the mountain which buried Goldau was composed of a hard but brittle conglomerate, called _nagelflue_, resting on an unctuous clay, and inclining rapidly toward the village. Much earth remained upon the rock, in irregular ma.s.ses, but the woods had been felled, and the water had free access to the surface, and to the crevices which sun and frost had already produced in the rock, and of course, to the slimy stratum beneath. The whole summer of 1806 had been very wet, and an almost incessant deluge of rain had fallen the day preceding the catastrophe, as well as on that of its occurrence. All conditions then, were favorable to the sliding of the rock, and, in obedience to the laws of gravitation, it precipitated itself into the valley as soon as its adhesion to the earth beneath it was destroyed by the conversion of the latter into a viscous paste. The ma.s.s that fell measured between two and a half and three miles in length by one thousand feet in width, and its average thickness is thought to have been about a hundred feet. The highest portion of the mountain was more than three thousand feet above the village, and the momentum acquired by the rocks and earth in their descent carried huge blocks of stone far up the opposite slope of the Rigi.

The Piz, which fell into the Cordevole, rested on a steeply inclined stratum of limestone, with a thin layer of calcareous marl intervening, which, by long exposure to frost and the infiltration of water, had lost its original consistence, and become a loose and slippery ma.s.s instead of a cohesive and tenacious bed.

_Protection against fall of Rocks and Avalanches by Trees._

Forests often subserve a valuable purpose in preventing the fall of rocks, by mere mechanical resistance. Trees, as well as herbaceous vegetation, grow in the Alps upon declivities of surprising steepness of inclination, and the traveller sees both luxuriant gra.s.s and flouris.h.i.+ng woods on slopes at which the soil, in the dry air of lower regions, would crumble and fall by the weight of its own particles. When loose rocks lie scattered on the face of these declivities, they are held in place by the trunks of the trees, and it is very common to observe a stone that weighs hundreds of pounds, perhaps even tons, resting against a tree which has stopped its progress just as it was beginning to slide down to a lower level. When a forest in such a position is cut, these blocks lose their support, and a single wet season is enough not only to bare the face of a considerable extent of rock, but to cover with earth and stone many acres of fertile soil below.[240]

In Switzerland and other snowy and mountainous countries, forests render a most important service by preventing the formation and fall of destructive avalanches, and in many parts of the Alps exposed to this catastrophe, the woods are protected, though too often ineffectually, by law. No forest, indeed, could arrest a large avalanche once in motion, but the mechanical resistance afforded by the trees prevents their formation, both by obstructing the wind, which gives to the dry snow of the _Staub-Lawine_, or dust avalanche, its first impulse, and by checking the disposition of moist snow to gather itself into what is called the _Rutsch-Lawine_, or sliding avalanche. Marschand states that, the very first winter after the felling of the trees on the higher part of a declivity between Saanen and Gsteig where the snow had never been known to slide, an avalanche formed itself in the clearing, thundered down the mountain, and overthrew and carried with it a hitherto unviolated forest to the amount of nearly a million cubic feet of timber.[241] The path once opened down the flanks of the mountain, the evil is almost beyond remedy. The snow sometimes carries off the earth from the face of the rock, or, if the soil is left, fresh slides every winter destroy the young plantations, and the restoration of the wood becomes impossible. The track widens with every new avalanche. Dwellings and their occupants are buried in the snow, or swept away by the rus.h.i.+ng ma.s.s, or by the furious blasts it occasions through the displacement of the air; roads and bridges are destroyed; rivers blocked up, which swell till they overflow the valley above, and then, bursting their snowy barrier, flood the fields below with all the horrors of a winter inundation.[242]

_Princ.i.p.al Causes of the Destruction of the Forest._

The needs of agriculture are the most familiar cause of the destruction of the forest in new countries; for not only does an increasing population demand additional acres to grow the vegetables which feed it and its domestic animals, but the slovenly husbandry of the border settler soon exhausts the luxuriance of his first fields, and compels him to remove his household G.o.ds to a fresher soil. With growing numbers, too, come the many arts for which wood is the material. The demands of the near and the distant market for this product excite the cupidity of the hardy forester, and a few years of that wild industry of which Springer's "Forest Life and Forest Trees" so vividly depicts the dangers and the triumphs, suffice to rob the most inaccessible glens of their fairest ornaments. The value of timber increases with its dimensions in almost geometrical proportion, and the tallest, most vigorous, and most symmetrical trees fall the first sacrifice. This is a fortunate circ.u.mstance for the remainder of the wood; for the impatient lumberman contents himself with felling a few of the best trees, and then hurries on to take his t.i.the of still virgin groves.