CHAPTER IX.

Mechanical forces at work in the disintegration of rocks—Rains Landslips—Effects of frosts—Glaciers and icebergs—Abrading power of rivers—Suggested volume on the geology of rivers—Some of its probable contents—Scene in a woody ravine—First idea of the origin of the ravine one of primeval cataclysms—Proved to be incorrect—Love of the marvellous long the bane of geology—More careful examination shows the operations of Nature to be singularly uniform and gradual—The doctrine of slow and gradual change not less poetic than that of sudden paroxysms—The origin of the ravine may be sought among some of the quieter processes of Nature—Features of the ravine—Lessons of the waterfall—Course of the stream through level ground—True history of the ravine—Waves and currents—What becomes of the waste of the land—The Rhone and the Leman Lake—Deltas on the sea-margin—Reproductive effects of currents and waves—Usual belief in the stability of the land and the mutability of the ocean—The reverse true—Continual interchange of land and sea part of the economy of Nature—The continuance of such a condition of things in future ages rendered probable by its continuance during the past.

Theforms of decomposition noticed in the last chapter were chiefly of a chemical kind. Their effects were observable alike on the surface of the earth and below ground; in the latter case we saw them excavating caverns and long irregular chasms, in the former we noted the production of debris which if undisturbed went to the formation of soils. It must be borne in mind however, that in these operations other forces than simply those of a chemical kind come into play. The percolation of water and the removal of insoluble particles on the exposed parts of rocks rank as mechanical processes. So also do those by which new surfaces of mineral masses are brought within the sphere of the chemical agencies, such as the action of frosts, rains, rivers, and waves. In short, as already noticed, any subdivision of the forces at work in effecting the decomposition of rocks must ever be more or less arbitrary; but it remainsnevertheless useful, if we bear in mind that the exactly defined boundary lines are of our making, not Nature's. With this caution we may proceed to examine what are termed the mechanical agencies in the disintegration of mineral masses, and in so doing, we shall find that the chemical forces are not less helpful to the mechanical than the latter to the former.

First, we may notice the effect of rains in washing away the disintegrated particles to lower levels or into river-courses whereby fresh portions of rock become exposed to the decomposing forces. Rains also act powerfully in altering the form of cliff-lines and steep declivities, especially where these consist more or less of friable earthy matter. After a long continuance of wet weather, I have seen the abrupt sides of a river-channel that were formed of a stiff blue clay completely cut up by rents of various dimensions, whereby large masses had subsided many feet, while others had rolled down altogether and lay in the bed of the stream where they were undergoing a rapid abrasion. The cause of this alteration was obvious. The rains pouring down from the sloping grounds on either side of the river had excavated deep channels on the abrupt face of the cliffs, while a considerable quantity of water finding its way through the soil, had permeated through joints and crevices in the clay some feet from the edge of the bank. By the combined operation of these causes, masses of clay several yards in extent lost their cohesion and either settled down a few feet, or found their way to the bottom. Such landslips are of frequent occurrence where large masses of rock of a hard compact nature rest upon loose shales and clays more or less inclined. Whole hills have been known to be hurled in this way into the valleys below.

But these results become perhaps still more marked where to the ordinary operations of water there are added those of intense frost. The effects of a severe winter (such, for instance, as a Canadian one), in loosening the particles of rocks and facilitating the breaking-up of large masses, must be ranked among the mostpowerful agencies of nature. In such a season, the percolating water with which nearly every surface-rock is charged becomes frozen, and in the act of congelation expands. The result of this dilatation is to exert great pressure on the particles of the rock, and thereby loosen their cohesion. When thaw comes the frozen liquid contracts again, but the loosened particles have no such elastic power, and so, having lost hold of each other, crumble down. If the season be a changeable one, frost and thaw quickly alternating, the amount of waste produced becomes very great. Not only is the outer surface of the stone decomposed, but the water filtering through the joints of the rock freezes there, and thus on the arrival of milder weather vast masses become detached from the cliffs, and roll down, to be worn by the grinding action either of waves or of rivers, as the case may be. Spring at last sets in with its warmth and its showers; the snow rapidly melts away; the whole country streams with water; every valley and hollow has its red turbid rivulet, that bears a burden of muddy sediment into the nearest river; and thus the loosened portions of the rocks get washed away down to sea, leaving a new surface for the action of next winter. We can easily understand, therefore, that in certain regions the combined effects of frost and thaw may work in the course of ages changes of almost inconceivable extent, and that the agency of ice must be not less varied and important on the land than, in the case of the boulder clay, we found it to be in the ocean.

Besides this action in winter, which goes on more or less in every country wherever the temperature sinks sufficiently low to permit of the freezing of water, ice effects many changes on the surfaces of rocks when it takes the form of glaciers and icebergs. We have already noted the operation of a glacier during its slow progress in crushing down large fragments of stone, scratching and abrading the rocks over which it passes, and eventually producing a vast quantity of mud, which is carrieddown by streams to form new accumulations either in lakes or seas. We have also marked the effects of the drifting iceberg in materially modifying the contour of submarine hills, and depositing over the ocean-bottom mud, gravel, and boulders. Nothing further, therefore, need be done here than simply to keep these agencies in view, as playing an important part in the disintegration of rocks.

Another highly interesting aqueous action is that of streams and rivers, in scooping out for themselves channels through sometimes the hardest and most solid rock. Such effects may be seen all over the globe, in the old world and in the new, in the bed of the tiniest rivulet, as well as in the course of the mightiest river. And accordingly, in all the long list of geological agents, we find none so well known and so often described alike by poets, historians, and scientific writers, as well in ancient as in modern times. What a delightful volume might be written about the geology of rivers! It would, perhaps, begin with that "great river," the Euphrates, along whose green banks lay the birthplace of the human race, tracing out the features of its progress from the ravines and cataracts of Armenia, with all their surrounding relics of ancient art, down into the plains of Assyria, amid date-palms and Arab villages, onwards to the mounds of Nineveh and Babylon, and thence to the waters of the Persian Gulf. Well-nigh as remote, and perhaps still more interesting in its human history, would be the story of the Nile. We should have to follow that river from the mystic region of its birth,[53]marking the character of the rocks through which winds its earlier channel, and the effects upon them of the floods of untold centuries; it would be needful, too, to note the influence of the waters on the lower grounds, from where the stream flows over the cataracts of Syene, down through the alluvial plains of Egypt; and lastly, the concluding and perhapsmost onerous part of our labour would be the investigation of the delta, marking its origin and progress, its features in ancient times, as made known to us in the graphic chapters of Herodotus, and the changes which the lapse of more than twenty centuries has since wrought in its configuration. The rivers of Europe would detain us long, not less perhaps by their historic interest than by the variety and attractiveness of their physical phenomena. One could scarce help lingering over the Rhine, with its source among Alpine glaciers, its lakes and gorges, its castles and antique towns; and when once the narrative entered the classic ground of Italy, it would perhaps become more antiquarian than geological. The ravine of Tivoli, for instance, would certainly lay claim to a whole chapter for itself, with its long-continued river action, its ancient travertin, its beautiful calcareous incrustations, and above all its exquisite scenery.

[53]"Fontium qui celat origines Nilus" a description not less true now than in the days of the Sabine bard.

"Domus Albuneæ resonantis,Et præceps Anio, ac Tiburni lucus, et udaMobilibus pomaria rivis."[54]

"Domus Albuneæ resonantis,

Et præceps Anio, ac Tiburni lucus, et uda

Mobilibus pomaria rivis."[54]

[54]"Albuna's grey re-echoing home,And Anio, headlong in his foam,And grove of Tivoli,And orchards with their golden gleam,Whose boughs are dipping in the streamThat hurries to the sea."Hor.Carm.L vii. 12.

[54]

"Albuna's grey re-echoing home,And Anio, headlong in his foam,And grove of Tivoli,And orchards with their golden gleam,Whose boughs are dipping in the streamThat hurries to the sea."

"Albuna's grey re-echoing home,

And Anio, headlong in his foam,

And grove of Tivoli,

And orchards with their golden gleam,

Whose boughs are dipping in the stream

That hurries to the sea."

Hor.Carm.L vii. 12.

And when could we exhaust all, that might be said about the rivers of our own land?

"Of utmost Tweed, or Ouse, or gulfy Dun,Or Trent, who, like some Earth-born giant, spreadsHis thirty arms along the indented meadsOr sullen Mole, that runneth underneath;Or Severn swift, guilty of maidens' death;Or rocky Avon, or of sedgy Lee;Or coaly Tyne, or ancient hallow'd Dee;Or Humber loud, that keeps the Scythian's name;Or Medway smooth, or royal-towered Thame."

"Of utmost Tweed, or Ouse, or gulfy Dun,

Or Trent, who, like some Earth-born giant, spreads

His thirty arms along the indented meads

Or sullen Mole, that runneth underneath;

Or Severn swift, guilty of maidens' death;

Or rocky Avon, or of sedgy Lee;

Or coaly Tyne, or ancient hallow'd Dee;

Or Humber loud, that keeps the Scythian's name;

Or Medway smooth, or royal-towered Thame."

Passing to the new world, a vast field would spread out beforeus: the Mississippi, the Atchafalaya, the Ohio, the St. Lawrence, the Amazon, and many other rivers that in some cases rise high among the regions of perpetual snow, and after traversing large areas of country in the temperate zone, fall into the waters of tropical seas. By studying such examples of river-action and delta-formation as are presented by these gigantic streams, we should arrive at some conception of the conditions anciently at work in producing our present coal-fields. Nor would our researches assume aught like completion until after a scrutiny of all the larger and more important rivers of the globe. Such a work could be undertaken, perhaps, only by another Humboldt. Its successful accomplishment would certainly insure the highest renown to its author, and incalculable benefits to science.

From what we have seen of the wide waste and decay everywhere in progress on the solid lands of our planet, it becomes no difficult matter to perceive what a number of agencies must be at work in the formation of a river channel. Let the reader take his stand in some wooded ravine, where the shelving rocks on either side are hung all over with verdure, and a tiny streamlet murmurs on beneath with a flow so quiet and gentle as scarcely to shake the long pendant willow branches that dip into its surface, while the polished pebbles that strew its bed lie unmoved by the rippling current that glides over them. If in the midst of such a scene the question were to arise in his mind, How came this deep, narrow ravine into existence? what answer would in all likelihood be the first to suggest itself? His eye would scan the precipitous walls of the dell, with their rocks cleft through to a depth of perchance fifty feet. It would require no great scrutiny to assure him that the beds on the one side formed the onward prolongations of those on the other, and that consequently there must have been a time ere yet the ravine existed, when these beds stretched along unbroken. Satisfied with these results, his first impulse might beto bethink him of some primeval earthquake, when the solid land rocked to and fro like a tempested sea, and broke up into great rents and yawning chasms. Into one of these clefts he might suppose the little streamlet had eventually found its way, moistening the bare and barren rocks, until at length their surface put on a livery of moss, or lichen, or liver-wort, and the birch, the alder, and the willow, found a nestling-place in their crevices. Such a view of the origin of the woody dell would be certainly a very natural one, and in some instances might be sufficiently correct, but in the present case it will not explain the phenomena. If the reader will kindly permit me to visit the locality in his company, perhaps we may be able to light upon the true explanation, and see a few appearances worthy our attention.

First, then, how can we make sure that no convulsion of nature has produced a rent in the rocks, and so helped the streamlet to a channel? a simple question that may be well-nigh as simply answered. We stand in the centre of the dell on a broad ledge of stone, round whose well-worn sides the rivulet is ever eddying onwards. The block consists of a pale sandstone lying in a bed about three feet thick, that dips gently down the stream and underlies a seam of dull, soft, blue shale, full of small shells. We trace the edge of this sandstone bed across to the left-hand side of the ravine, and away up into the precipitous cliff, till it is lost amid the ferns and brushwood. There can be no doubt, therefore, that the ledge on which we were but now standing is a continuous portion of the rocks that form the left side of the ravine. Returning again to the centre of the stream, we proceed to trace out the course of the other end of the same bed, and find that it, too, strikes across to the rocks on the right-hand side without a break or fissure, and passes up into the cliff, of which it forms a part. Clearly, then, the sandstone bed runs in an unbroken, unfissured line, from the one side to the other, and the rocks of either cliff form onecontinuous series. There occurs no break or dislocation, which, of course, there must have been had the ravine owed its origin to any subterranean agency. And so we come to conclude that no great cataclysm in primeval times, no yawning abyss, or gaping chasm, has had anything whatever to do with the formation of our deep sequestered dell. What then? "Whither shall we turn," you ask, "to find another agency equally grand and powerful in its operation and mighty in its results?"

Stay, gentle reader. That craving for the grand and the sublime, that hungering after cataclysms and convulsions, that insatiable appetite for upheavals, and Titanic earth-throes, and all the mightier machinery of Nature, has done no little mischief to geology. Men have reasoned that gigantic results in the physical structure of the earth must have had equally gigantic causes operating in sublime conflict and in periodic paroxysms, now heaving a mountain chain to the clouds of heaven, now swallowing up a continent in the depths of the sea. Happily such extreme notions are fast passing away, though the old tendency in a modified form still abounds. A closer scrutiny of Nature as she actually shows herself, not as theorists fancy she should be, has revealed to us that her operations are for the most part slow, gradual, and uniform, and that she oftentimes produces the mightiest results by combinations of forces that to us might seem the very emblems of feebleness and inactivity. In place of sudden paroxysms she demands only an unlimited duration of time, and with the aid of but a few of these simple, tardy agents, she will eventually effect results perchance yet more gigantic than could be accomplished even by the grandest catastrophe. Nor in thus seeking to explain the past by defining what seems the usual mode of Nature's operations in the present, do we, as is sometimes alleged, deprive them of their high poetic element. Assuredly there is something thrilling to even the calmest imagination in contemplating the results of vast and sudden upheavals, in picturing the solid crustof the earth heaving like a ground-swell upon the ocean, in tracing amid

"Crags, knolls, and mounds, confusedly hurl'd,The fragments of an earlier world;"

"Crags, knolls, and mounds, confusedly hurl'd,

The fragments of an earlier world;"

and in conjuring up visions of earthquakes, and frightful abysses from which there ever rose a lurid glare as hill after hill of molten rock came belching up from the fires below. But while far from denying that such appearances may have been sometimes seen during the long lapse of the geological ages, and that they give no little vividness and sublimity to a geological picture, we claim for the doctrine of the tranquil and uniform operation during past time of existing laws and forces, an element not less poetic. In the former case the pervading idea is that of unlimited expenditure of power, in the latter that of unlimited lapse of time. In the one case the action is Titanic but transient, in the other it is tranquil but immensely protracted. The two doctrines in this way counterbalance each other; yet I cannot but think that however impressive it may be to stand in some lone glen, and while gazing at its dark jagged precipitous cliffs, to dream about the paroxysmal convulsions of some hour far back in the distant past, the scene becomes yet more impressive when we look on its nakedness and sublimity not as the sudden and capricious creation of a day, but as the gradual result of a thousand centuries. These cliffs may once have been low-browed rocks rising but a little way out of a broad grassy plain, and serving as a noon-tide haunt for animals of long extinct races. Thousands of years pass away and we see these same rocks higher and steeper in their outline, brown with algæ and ever wet with surf, while around them stretches a shoreless sea. Ages again roll on, and we mark still the same rocks shooting up as bleak crags covered with ice and snow. Another interval of untold extent elapses, and rock, snow, and ice have all disappeared beneath a broad ocean cumbered with ice-floes and wandering bergs.Again the curtain drops upon the scene, and when once more it rises, the cliffs stand out in much the same abrupt precipitous aspect with that which they now present, save that their bald foreheads look less seamed and scarred than now, and their dark sides show no trace of bush or tree. The white cascades that to-day pour down from their summits and sides—seeming in the distance like the white hairs of age—are insensibly deepening the scars and furrows on these ancient hills, and thus slowly but yet surely carrying on the process of degradation and decay. Musing on all this long series of stages in the formation of one single cliff-line, is there not something more sublime, something yet more impressive than if we pictured but the chance random result of the gigantic paroxysm of an hour?

Let us not be deterred then from seeking an explanation of the origin of the ravine among some of the quieter and more unobtrusive forces of Nature. Give them but an unlimited period to work in and they will abundantly satisfy all our demands.

We return again to the rocky ledge in mid-channel, and proceed to ascend the course of the stream, marking as we go the changes in the character and features of the stone that forms the cliff on either hand. We come to a bare part of the ravine where brushwood and herbage find but a scanty footing and where accordingly the rocks can be attentively studied. The face of the escarpment shows a number of beds of pale grey sandstone alternating with courses of a dark crumbly shale. The sandstones being harder and firmer in texture stand out in prominent relief while the shales between have been wasted away, covering the bottom of the slope with loose debris. We can mark too that, as this decay goes on, the harder beds continually lose their support, cracking across chiefly along the lines of joint, and rolling down in huge angular blocks into the stream. In truth we cannot doubt that every year adds to this decay and thus slowly widens the dell, for the broken fragments do not form in heapsover the solid rock below so as to protect it from the weather, but are evidently carried away by the stream and hurried down the ravine onwards to the sea. From what has been said above relative to the disintegration of rocks by percolating water, frosts, and other causes, the reader will easily see how this rotting away of the sides of the ravine must be carried on; and he will not fail to mark that here we have at work an agency not yet considered, that of running water. The effects of the weather are seen in the crumbling, ruinous cliffs overhead; the effects of the streamlet are observable in the continual removal of the rubbish whereby a fresh surface is ever exposed to the decomposing forces, while at some points we can mark the water actually undermining an overhanging part of the cliff from which there are ever and anon vast masses precipitated into the channel where eventually they get worn down and carried away out to sea. "Still," you may remark, "these forces are at work only in widening a channel already made. How was the ravine formed at first?"

We continue our ascent. A scrambling walk through briars and hazel-bushes, sometimes on rocky ledges high among the cliffs, sometimes among the prostrate blocks that dam up the stream, brings us at last full in front of a sparkling waterfall that dashes over a precipitous face of rock some twenty feet high. The appearances observable here deserve a careful attention. Our eyes have not been long employed noting the more picturesque features of the scene ere they discover that the dark-brown band of rock forming the summit of the ledge over which the water tumbles is continuous all round the sides of the dell. There is consequently no break or dislocation here. Approaching the cascade we note the rock behind it so hollowed out that its upper bars project several feet beyond the under ones. In this way the body of water is shot clear over the top of the cliff without touching rock till it comes splashing down among the blocks in the channel. And yet this hollowedsurface is never dry; the spray of the fall constantly striking on it keeps it always dank and dripping. In some parts the rock stands out bare and worn, while on the less exposed portions there gathers a thick green scum which is replaced on the drier ledges by the soft cellular leaves of the liver-wort. Now our examination of the influence of percolating water upon even the hardest rocks teaches us that this moist soaked surface is just the very best condition for favouring the decay of the rock. Nay more, the green vegetation that mantles over the stone serves to prevent the water from running off too rapidly, and keeps the rock in a still more moist state than would otherwise happen. So that the portion of sandstone behind the cascade comes to be in a still more favourable situation for speedy decay than the ledge over which the water is rapidly driven. We can see, therefore, how in the lapse of years the corrosion may go on until the upper projecting part of the cliff loses its support and falls with a crash into the rocky pool below, while the form of the waterfall becomes thus greatly altered, and new surfaces are exposed to the wear and tear of the stream.

But we have not yet exhausted all that the rocks at the cascade can teach us. By dint of some exertion we climb the cliff and gain the upper edge of the fall. The rocks that form the bed of the stream are now seen to be deeply grooved and worn, every exposed surface having a smoothed blunted aspect. We can mark how the stone has split up along the natural lines of joint, whereby great facility is given to the removing power of the current, and how large irregular angulated blocks become detached and are swept down the stream. In not a few parts, too, we may notice circular holes of greater or less depth, in the bottom of each of which lie perhaps a pebble or two, that with a constant gyratory movement, caused by the eddying water, have eaten their way downwards into the solid rock. When the stream is in flood and comes roaring down the rocky gorge bearing along with it a vast amount of mud, gravel, andstones, one can easily see how the friction of the transported material must wear down the hard bed and sides of the channel, and how this process repeated month after month and year after year, must aid the decomposing forces in scooping out a deep ravine. From the cascade the ascent of the stream becomes steeper and the run of water is consequently more rapid. Soon however we emerge from the woody copse, and find ourselves on a flat alluvial cultivated plain through which the rivulet winds in a tortuous meandering course, bending back upon itself into loops that almost meet and well-nigh form broad flat islets. Strolling along this winding route we can mark the effects of the stream in eating away the soft clay and sand at one part of the bend and piling them up at another. Such loose material can present but little resistance to a stream swollen with rains, and consequently a large quantity of the mud and gravel along with the interspersed boulders must be swept away down into the dell at every season of flood. The matter thus removed will of course be still further comminuted in its passage, and at the same time will help to grind down the hard rock surfaces over which it is driven.

Here then may be found the whole history of the ravine. Originally the streamlet wound its devious course through a flat alluvial country with a channel sunk but a foot or two below the level of the plain. Such continued its character till it reached a low bluff, down which the water flowed more rapidly to gain a second level undulating region. The part of this bluff crossed by the stream was ere long bared of its covering of soil and clay, and the rock below came to be washed by a group of little cascades. Once exposed to the decomposing and disintegrating forces, the stone soon began to decay and the cascades ere long merged into one. By slow degrees the rock gave way and the waterfall retreated from the bluff. For perchance thousands of years the same process has been going on, now with greater, now with less rapidity, according to the natureof the rocks encountered and other modifying causes, until the fall has eaten its way back for well-nigh three miles and scooped out a wild rocky gorge some fifty or sixty feet deep. This is but a solitary and insignificant instance of what may be seen all over the world, for the process remains the same whether we stand beside a tiny rivulet in some lone Highland glen or listen to the roar of the falls of Niagara.

There is but one other principal agency at work in the demolition of rock-masses, the waves and currents of the ocean. But we have already noted the effects thus produced, and need not now retrace our steps further than to recall the vast amount of devastation which can be shown to have been effected in our own country by marine causes, both in breaching the existing shores and in scooping out valleys and grinding down hills at former periods when the land was either rising above or sinking below the level of the sea.

Having now satisfied ourselves that there goes on all over the world an incessant waste of the solid lands, that the disintegrated debris is washed down by rains and transported seawards by rivers, and that the waves are ever eating their way into the iron-bound coast-line as well as into the low alluvial shore, we naturally come to ask the result and end of all this decay. What becomes of that vast amount of mineral matter annually removed from the land? To be able to answer this question clearly and distinctly, let us look for a little at what takes place in lakes, at river-mouths, and in open sea.

The river Rhone rises among the Bernese Alps, and after a course of about 100 miles through the Canton of Valais, it enters the upper end of the Lake of Geneva. Its waters, where they mingle with those of the lake, are muddy and discoloured, but where they pass out at the town of Geneva are limpid and clear. The mud, therefore, which they bring into the lake must be deposited there, and as the stream may have continued to flow for thousands of years, we may reasonably expect to find sometrace of the large amount of sediment necessarily deposited during the whole or part of that long period. Accordingly, careful examination of the Lake of Geneva has shown that such accumulations have really been formed, and that their progress and amount during part of the historic period can be approximately calculated. Where the turbid current of the Rhone enters the still water of the lake, the mud slowly sinks to the bottom. In the lapse of centuries layer after layer has been thrown down, rendering the lake at this part sensibly shallower, until a large area or delta has been filled up and converted into a flat alluvial plain. Thus, a town which in the time of the Romans formed a harbour on the water's edge, now stands more than a mile and a half inland. This new-formed land is entirely the work of the stream, and if we could obtain a complete section of it from the surface to the bottom, "we should see a great series of strata, probably from 600 to 900 feet thick (the supposed original depth of the head of the lake), and nearly two miles in length, inclined at a very slight angle." These strata, which are said to have taken about eight centuries to form, "probably consist of alternations of finer and coarser particles; for, during the hotter months, from April to August, when the snows melt, the volume and velocity of the river are greatest, and large quantities of sand, mud, vegetable matter, and drift-wood, are introduced; but, during the rest of the year, the influx is comparatively feeble, so much so that the whole lake, according to Saussure, stands six feet lower."[55]If the present conditions continue for a sufficient length of time, the lake may be eventually filled up with mud, sand, and gravel, deposits that would eventually harden by pressure into shale and sandstone. So that the day may yet arrive when the blue waters of the Leman lake shall have passed away, when the Rhone perchance may have ceased to flow or found its way by some other channel, when the peasant may guide the plough where now the boatman pliesthe oar, and when the geologist shall trace out in quarries and excavations the successive deposits of hardened sediment with their lacustrine shells and drift-wood, and, musing on the changes of which they are the silent yet impressive witnesses, may sit down to pen a record of the gradual extinction of the Leman lake on that classic ground where an immortal historian described the decline and fall of the empire of Rome.

[55]Lyell'sPrinciples of Geology. Ninth edition, p. 252.

The alluvial matter deposited by the Rhone at its entrance into the Lake of Geneva suffers perhaps no change when it once reaches the bottom. Layer after layer accumulates tranquilly, without disturbance from surface currents or other causes, so that the renovating effects of the stream have here every advantage. It is otherwise, however, where a delta gathers at the mouth of a river upon the sea-margin. There tides and currents are ever demolishing what the stream has piled up. Often, too, owing to the prevalence of high winds from seawards, the river is dammed up for leagues, and the waters of the ocean encroach far on the delta, mingling in this way marine remains with those that are fluviatile or terrestrial. But with these modifications the process of delta-formation remains essentially the same, both in lakes and at the sea. The vast quantities of sand and gravel transported by rivers during the flood-season sink to the bottom as soon as the motion of the water will permit. This takes place at the shore, where eventually wide tracts of low alluvial land encroach upon the sea, covered with marshes and overgrown with vegetation. A section of any of these deltas, obtained in boring for water, shows a succession of sands and clays, with occasionally a few calcareous beds and quantities of peaty matter formed of vegetation either drifted or that grew on the spot.[56]If, now, a sufficient amount of matter were piled over these loose incoherent strata, they wouldeventually become as hard and compact as any of our ordinary building stones. The sand would subside into a firm compact sandstone; the clay, in like manner, would consolidate into fissile shale; the peat would become chemically altered into coal; the calcareous seams would take the form of layers of limestone; while the leaves, twigs, branches, and trunks, dispersed through all the beds, would get black and carbonized, so as precisely to resemble the lepidodendra, calamites, stigmariæ, &c., of the carboniferous rocks. And thus might a mass of fossiliferous strata, thousands of feet deep and thousands of square miles in extent, be amassed by the prolonged operation of a single river.

[56]The structure of maritime deltas, especially their relation to the growth and entombment of forests, will be more fully alluded to in a subsequent chapter, when we come to inquire into the origin of a coal-field.

It often happens that a delta is prevented from extending further seawards owing to the prevalence of some marine current that comes sweeping along the coast-line and cuts away the accumulations thrown down by the river. The sediment thus removed is often carried to great distances, and eventually settles down as a fine mud along the floor of the sea, entombing any fucoids, infusoria, shells, corals, fish-bones, or other relics that may lie at the bottom.

He who has witnessed a storm along a rocky coast-line, has marked the breakers battering against the weather-bleached cliffs, and heard the thunder-like rattle of the shingle at the recoil of every wave, needs not to be told how vast an amount of sediment must in this way be formed. The pebbles of the beach are ground down still smaller, the sand produced by their friction finds its way to a lower level, while the finer particles taken up by the water are borne out to sea, and if a current traverse the locality may be transported for leagues, till they at last settle to the bottom. The floor of the sea is consequently always receiving additions in the form of fine mud—the waste of the land—derived either from breaker-action, rivers, or icebergs, so that a series of marine deposits exactly similar to those we find among the rocks of our hills and valleys, mustbe constantly in the course of formation. If circumstances be favourable, the shingle of the beach may eventually either be covered over or reach a part of the sea undisturbed by currents or waves, and then consolidate into what we call conglomerate or pudding-stone. The sand, as before, becomes sandstone, and the mud laminated shale or hardened clay. These deposits may go on forming for thousands of years, until at last some slow elevation or some sudden upheaval of the ocean bed brings them to the light of day as part of a new continent. Thus exposed they would differ in no respect from rocks of a similar kind now visible, and the geologist, in tracing out their origin and history, would have no hesitation in ranking them among the ordinary marine formations of the globe.

In fine, we cannot quit the subject without being convinced that these ceaseless changes afford one of the grandest examples of that continuous series of mutations—cycle and epicycle—which has been already alluded to as a distinguishing feature in all the operations of Nature. We are accustomed to think and speak of "the everlasting hills." We look on the solid lands whereon we dwell as the emblem of all that is stable and steadfast, and on the boundless ocean as the type of all that is unsteady and changeful. The traveller who stands on those plains where the human race was cradled, marks still the same valleys with their winding rivers, still the same rocks and hills, still the same blue sky overhead. The dust of centuries has gathered over the graves and the dwellings of the early races, yet the covering is but thin, and if we could conjure from their resting-place some of these venerable patriarchs, they might perhaps see little or no change on the haunts of their boyhood. We feel it otherwise, however, when we contemplate the ocean. In sunshine and in storm its surface never rests. The wave that now breaks against some bald headland of our western shores may have come sweeping across from the coast of America, and the broad swell that rolls into surf along theshores of Newfoundland may have travelled from the frozen seas of the North Pole. And so it has ever been; the "far resounding sea" of Homer is the "far resounding sea" still; and the "countless dimpling of the waves," invoked in his agony by the chained Prometheus, remains restless and playful as ever.

"Firm as a rock," and "fickle as the sea," have therefore become proverbs of universal acceptance. Yet when we investigate the matter as we have done in this and the preceding chapter, it appears that an exactly opposite arrangement would be nearer the truth. It is the sea that remains constant—

"Time writes no wrinkle on its azure brow;"

while the land undergoes a continual change. Hills are insensibly mouldering away, valleys are ever being widened and deepened, rocky coasts and low alluvial shores suffer a constant abrasion, while even within the bowels of the earth the process of decomposition uninterruptedly proceeds. And thus, in place of remaining unchanged from the beginning, we know of nothing more mutable than the land on which we dwell, so that if the waste everywhere so apparent were to go on unchecked or unmodified, island and continent would eventually disappear beneath the waves. Here, however, another principle comes into operation. The debris removed from the land, as we have seen, is not annihilated. Slowly borne seawards, it settles down at river mouths or on the floor of the ocean as an ever-thickening deposit, which eventually hardens into rock, as solid and enduring as that whence it was derived. But it does not always remain there. Owing to the action of subterranean agencies with which we are but slightly acquainted, different parts of the sea-bottom are continually rising. Sometimes this process goes on very slowly, as along the shores of Sweden, where the coast has been ascertained to emerge in some localities at the rate of about thirty inches in a century; sometimes withprodigious rapidity, as on the coast of Chili, where the land was upheaved from two to seven feet in a single night. There can thus be no doubt that the mysterious agency which produces earthquakes and volcanoes on the land affects equally that portion of the earth's crust covered by the waters of the ocean, and must be ceaselessly employed in elevating large areas of sea-bottom into new continents, that will ere long become clothed with vegetation and peopled with animals. In contemplating, therefore, the constant decay in progress on the surface of the land, we see not a mere isolated process of waste, but a provision for future renovation. The sandstone cliffs of the shore are battered down and their debris carried out to sea, but when sea-bottom comes to be land-surface, they may be sandstone cliffs again, lashed once more by the breakers, and once more borne as sediment to the depths of the sea. And thus, in what may seem to us sublime antagonism, land is ever rising in the domain of ocean, and ocean ever encroaching on the regions of land. No sooner does a new island, or mountain peak, or wide area of continent, appear above the waves, than the abrading agencies are at work again. Rain, air, frost, rivers, currents, breakers, all begin anew the process of destruction, and cease not until the land has utterly disappeared, and its worn debris has sunk in mid-ocean to be in process of time once more dry land, and suffer another slow process of obliteration.

Such is the economy of nature around us now, and that such will continue to remain the condition of things in the future, we can affirm with probability from a consideration of the history of the past. The geologist can point to masses of rock several miles in thickness, and occupying a large area of the globe, formed entirely of the worn debris of pre-existing formations. The very oldest rocks with which he is acquainted are made up of hardened sediment, pointing to the existence of some land, even at that early period, worn down by rivers orwasted by the sea. During all the subsequent ages the same principles were at work, and now well-nigh the only evidence of the geological periods is to be gathered from the layers of sediment that successively settled down at the sea-bottom. The records which it is the task of the geologist to decipher, are for the most part written in sand and mud—the deposits of the ocean, for in by far the larger number of formations into which the stratified part of the earth's crust has been divided, and which form his only guide to the history of the past, he can detect no trace of land. Hill and valley have alike disappeared, and the character of their scenery and inhabitants he can often but dimly conjecture from the nature of the sediment and of the drifted terrestrial relics that may chance to be found among strata wholly marine.

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