Page images
PDF
EPUB

ceded to its magin, the average depth being far less than the height of the cataract. The changes going on in the basin of the Mississippi, through the action of that magnificent river, the immense erosion going on upon its borders" several acres, thickly covered with wood, being precipitated at a time into the stream"-the islands and banks formed lower down by the accumulation of these materials, and again washed away, perhaps, by the next flood, to be again deposited still nearer to the ocean, confirm and illustrate our remarks. One of the most interesting features of this river is the enormous rafts of drift timber it floats towards the sea, occasionally depositing them for a time, together with vast beds of mud and gravel, in some of its deserted channels. One of these rafts is described by Darby, in 1816, ten miles in length, about two hundred and twenty yards wide, and eight feet deep. It is continually increasing by the addition of fresh drift-wood, and rises and falls with the water on which it floats; evidently waiting only an extraordinary flood to bear it off into the gulf of Mexico, where far greater deposits of the same kind are in progress at the extremity of the delta. The Mississippi offers an example of a remarkable hydrographical Jaw, namely, that the width of a river is by no means proportioned to its volume of water, but, on the contrary, after the junction of two or more confluents, the united stream often occupies less space than either of them did before; the increase of depth and velocity, caused by the greater volume, compensating for the diminished surface.

“The Mississippi is a mile and a half wide at its junction with the Missouri, the latter being half a mile wide; yet the united waters have only, from their confluence to that of the Ohio, a medial breadth of about three-quarters of a mile. The junction of the Ohio seems also to produce no increase, but rather a decrease, of surface. The St. Francis, White, Arkansas, and Red rivers, are also absorbed by the main stream, with scarcely any apparent increase of its width; and, on arriving near the sea at New Orleans, it is scarcely half a mile wide."

Its depth there, however, is enormous, being no less, at the highest water, than one hundred and sixty-eight feet. The basin of this mighty stream exhibits, also, the co-operation of subterranean movements with the power of water, in altering the surface of continents. So late as 1812, the whole valley, from the mouth of the Ohio to that of the St. Francis, was convulsed to such a degree, as to create new islands in the river, and lakes in the al

luvial plain, many of which were twenty miles in extent. Yet, however great the scale on which alterations are here daily going on before our eyes-however enormous must be their combined result during a series of ages-there is no region more richly endowed with the powers of supporting both animal and vegetable life,

Calcareous rocks are dissolved by spring water percolating through them, particularly when charged, as nearly all springs are, more or less, with carbonic acid; and to this cause are to be attributed the innumerable subterranean cavities and winding passages which exclusively occur in limestone formations, in our own as well as in other countries. A subter raneau rill of water flowing through the frequent fissures of such rocks must gradually have enlarged them into caverns or galleries, which, after the stream had shifted to some other chanuel, afforded a retreat to wild animals. Should any further change, occasioned by the processes of excavation or elevation going on in this district, have permitted the waters of any neighbouring rivulet or river to find their way into these winding caves, the animals will have been expelled, mud washed in, and after the water had again drained off, covered with the stalagmitic incrustation that drops from their roof. Thus simply may we explain the bone caves of limestone districts, which have generated so many wonderful theories.

Mr. Lyell proceeds to the consideration of alluvial formations, or the reproductive effects of running water. The formation of deltas, that is, deposits of alluvium at the openings of rivers into stagnant water, goes on equally in lakes as in the ocean, with this difference only, that they tend much more rapidly to fill up the former, from the inferiority of their area and depth. The completion of this process transforms the lake into an alluvial plain, watered by the river, which previously deposited all its drift and sediment there, but now carries them forward into some lower lake, which it proceeds to fill by the same process, or in default of such, into the sea. The Lake of Geneva is thus being gradually filled up by the deposits of the Rhone, which have created a tract of land, a mile and a half in width, between the ancient town Port Vallais, once, as the name implies, on the lake, and its present margin. The filling up of hollows, and cutting through of rocky barriers, is the universal process by which running water ever labours to produce a more uniform declivity. Though the Rhone has not yet obliterated, as it sooner or later will, the Lake of Geneva, many hundreds of alluvial tracts of equal, and

some of greater area, once evidently lakes likewise, may be seen as we follow up this river and its principal tributaries to their

sources.

The shores of the Baltic, and still more of the gulf of Bothnia, are rapidly gaining upon those seas by the accession of new land. The delta of the Rhone advances fast into the Mediterranean. Places which were islands in the ninth century are two leagues from the sea; and a tower erected as a lighthouse, on the shore, so lately as 1737, is now a mile from it. The deposit of this river consists chiefly of solid rock, not loose matter. In the museum of Montpellier is a cannon, taken up from the sea near the mouth of the Rhone, imbedded in a crystalline limestone. An arenaceous rock, cemented by calcarious matter, including multitudes of broken unmineralized shells, is also taken up in large masses, for use as building stone. The delta of the Po is pushed forward still more quickly. Adria was a seaport in the time of Augustus-it is now twenty miles inland. The delta of the Ganges is yet more remarkable, from the extensive scale and vast rapidity of its transformations. Its coast line is two hundred miles in length, and according to Major Rennell, the most newly formed portion of it, a wilderness of islands and creeks, inhabited by tigers and alligators, equals alone in area the whole principality of Wales. So great is the quantity of mud and sand poured into the gulf in the flood season, that the sea only recovers its transparency at the distance of sixty miles from the coast, and the mud is found, by soundings, to be carried at least sixty miles further. Here, then, is a marine formation now in progress, horizontally disposed over an area of at least two hundred miles by one hundred and twenty! In the branches and at the mouth of this mighty river, new islands are constantly forming, and old ones swept off. Mr. Colebrook mentions tracts of land forty square miles in extent, and more than one hundred feet in thickness, as having been washed away within a few years, in one locality. Some of the new islands, says Rennell, formed within a very short period, rival in size and fertility the Isle of Wight. No sooner are they thrown up to the level of the highest floods, than they are overrun with reeds, long grass, and shrubs, composing jungles, where tigers, buffalos, deer, and other wild animals take shelter. Crocodiles swarm on the mud banks and islands at the extremity of the coast. It is easy to perceive that both animal and vegetable remains must be continually imbedded in the sediment which subsides in the delta. flow uncalled for, then, are the

general catastrophes and revolutions resorted to by cosmogonists, to account for the entombing of snccessive races of animals in the older strata, when the same process is obviously going on at present amidst the general tranquility and order that reigns throughout the rich and populous delta of Bengal!

The delta of the Mississippi, as might be expected, increases rapidly. It has advanced many leagues since New Orleans was built. Great submarine deposits are in progress, stretching far and wide over the bottom of the sea, which is become very shallow throughout a vast area. Opposite the opening of the Mississippi large rafts of drift timber are met with, matted into a network, many yards in thickness, and stretching over hundreds of square leagues. They afterwards become covered with a fine mud, on which other layers of trees are deposited the year ensuing, until numerous alteruations of earthy and vegetable matter are accumulated. The geologist will recognize in this relation of Darby the type of the formation of the ancient lignites and coal fields.

The immense deposits of mud and sand at or near the mouths of great rivers will not astonish us, if we reflect on the large proportion of sedimentary matter which their waters carry down, and which never finds its way back again; while the water, on the contrary, is eternally raised by evaporation, and returned in rain upon the land. Manfredi, the celebrated hydrographer, calculated the average proportion of sediment in all the running water on the globe to be as one, to one hundred and seventy-five. Supposing this to be correct, in every one hundred and seventy-five years a quantity of sedimentary matter would be carried into the ocean, equal in bulk to the aggregate volume of water contributed to it in a year by all the streams of the world, which every one will per ceive must be enormous. But the late Major Rennell actually reckoned the quantity of mud, held in suspension by the Ganges during floods, as one-fourth of its bulk. If this were true, as well as the estimate the same eminent hydrographer formed of the volume of the Ganges, this river alone, during the flood-season, carries down daily into the Indian oceau upwards of eight thousand six hundred and fortyone millions of cubic feet of mud! Even if we suppose this greatly exaggerated, there will remain enough to prevent our continuing to make light of the prodigious formations hourly accumulating at the sides and bottom of the ocean, or of the power of running water to excavate and carry off the materials of the land, Few

geologists would be found any longer to speak of the actual erosive agency of water as insignificant, were the immense volume of matter carried into the sea in a given time duly ascertained, since all must admit that the whole, with slight excep. tions, is subtracted from valleys; in other words, that ancient valleys have been excavated, and new ones formed, to the extent of the space which the new deposits, when consolidated, would occupy.

When torrents flow directly into a sea or lake, as along all mountainous coasts, the transported matter consists of coarse gravel, pebbles, and boulders. Vast deposits of this kind are probably forming at present in the deep sea, at the base of the Ligurian Alps, for example, and levelled by the marine currents and waves which wear away this rocky coast. By periodical changes in the rapidity and volume of rivers, or in the direction of marine currents, such coarse deposits are often made to alternate with finer. When two rivers meet in one mouth, the common delta is often successively the receptacle of different sediments derived from the converging streams, whose periods of flood do not always coincide. The one is perhaps charged with calcareous, the other with argillaceous matter, or one may sweep down sand and pebbles, the other mud. These differences may be repeated with considerable regularity, until a thickness of hundreds of feet of alternating beds is accumulated. Again, among the infinite shiftings which occur at the mouths of deltas, it must frequently happen that the same area is alternately, during a considerable period, covered with salt water, and with fresh; and hence occasional alternations and admixtures of fluviatile and -marine deposits must be expected in such situations.

Mr. Lyell proceeds to give instances of the destroying and transporting effect of marine currents, whether caused by tides, or by the heaping up of the surface-water in the direction towards which it is impelled by constant or periodical winds. The amount of excavation and accumulation, carried on by marine currents, is considered by Mr. Lyell to exceed very greatly that of running water on the land. Proofs of the great power of the waves of the sea in removing masses of rock, of enormous weight, are found in the Shetland isles, which are both battered by the waves of the Atlantic, and ground down by a strong current. A block of nine feet by six, and four feet thick, is described by Dr. Hibbert as having been, in the winter of 1818, hurried up an acclivity to a distance of one hundred and fifty feet.

In the Isle of Sheppey, fifty acres of land, from sixty to eighty feet above the sea, have been swept away within the last twenty years. The church of Minster, now near the coast, is said to have been in the middle of the island only fifty years ago; and it is computed that, at the present rate of destruction, the whole of the island will be annihilated in another half cen tury! The tradition, that the Goodwin Sands were once the estates of Earl Goodwin, points, no doubt, to the former existence of an island, or extension of the coast in that direction, which, like Shep pey, has been washed away; and the idea of the former union of England with France gains an appearance of probability, from the proofs of rapid degradation still occurring on our coasts, collected by Mr. Lyell. The French side of the channel is equally corroded by the violence of the great tidal current which flows up this passage in the manner of a vast river.

As a general rule, wherever cliffs or steep escarpments form the shore, there, we may be confident, abrasion is, or has lately been going on, and also that a current sets along the coast, by which its de tritus is carried into deep water. The beating of the waves alone may wear away and break up a rock, but without a current to sweep off the debris, they would accumulate into a permanent talus, which must entirely prevent the formation of a cliff. But, by the shiftings of currents, it often happens that the sea retreats, and leaves a talus or a flat shore of sand or mud beneath the cliffs it once undermined; towards which it may return again, when another change occurs in the circumstances by which the direction of currents, and consequently their erosive and accumulative forces, are locally determined.

[ocr errors]

The existence of currents and tides in the sea at the points where rivers are discharged into it, produces a remarkable effect on the character of their embouchures. We have traced the production of deltas, those flat alluvial projections, by which the detritus carried down by rivers tends to obstruct their mouths when they enter stagnant water, such as inland lakes, or current less seas. But when, on the contrary, they flow into seas where a current sweeps along the coast, the transported matter is hurried away before it can be permanently deposited, and the coast line is prevented from increasing. When, in addition to a current, high tides ascend the mouths of rivers, instead of being obstructed, they are continually enlarged :

+ Mr. Lyell does not seem to know, that one of the prebends in St. Paul's takes its title from these lands now" sub mare."

excavation goes on in lieu of accumulation, and an estuary, or inlet of the sea, what Rennell calls a minus delta, is produced, in place of a projection. It is easily seen how a tidal wave, alternately flowing up the mouth of a river, and ponding back its waters, and then returning with double violence through the added momentum of these waters themselves, must scour out the channel, and wear away the land on either side of the mouth. Thus were produced the great estuaries of the Thames, the Severn, and the Solway, of the Seine, the Gironde, the Tagus, the Elbe, the Delaware, the Chesapeake, and of numerous other rivers flowing into tidal seas, which, but for this circumstance, would probably have, long since, filled up the great submarine valleys which they indicate, instead of keeping them open, and indeed widening them daily, as they are observed to do now. Where a current flows by the mouth of a river, though the whole of the drift matter is not permanently deposited, yet, at the line of junction between the fluviatile and marine current, where they neutralize each other, a certain quantity subsides, and a bar, or lengthened bank, is the result, extending across the mouth of the river. The extent and depth of this bar, and the position it takes in the opening of the river, are determined by the comparative force and direction of the antagonist currents of the sea and river. The latter almost always preserves an opening for its issue through the bar, at the further extremity from the direction of the marine current; but where the force of the river is comparatively trifling, the bar is completed, and the stream either percolates through it, or being dammed up into a lake within, overflows it on one or more points, which are occasionally worn into channels of communication, admitting the sea-water, and then again closed up, so as to occasion the lake to be alternately salt and fresh, Bars and shoals are also formed at the conflux of two marine currents holding sedimentary matter in suspension, or of a current and an eddy, or along the boundary line of a current bordered by stagnant water. The direction of every current depends chiefly on the form of the coast past which it flows; and it is deflected by projecting headlands, banks, and shoals, just in the manner of a river. Hence behind such projections the water is undisturbed, except by the eddy occasioned in it through the friction of the current sweeping by. The boundary line of the current and stagnant water is determined by the momentum and previous direction of the former, and the projcting resistances it meets with, but uniformly assumes a

more or less regular curvature according to these circumstances. It is along this sweeping line that the matter drifted by the current subsides, as the momentum of its particles carries them beyond the line which limits the transporting power of the stream; and thus every current, after rushing past opposing headlands, tends to form out of their detritus a coast-line corresponding with the curve they have im pressed upon it. The Etangs of the south of France, the Haffs of northern Prussia, the Fiords of the west coast of Denmark, and the great Lagoons of the gulf of Mexico, are examples, on a large scale, of the stagnant pools of water shut out from the sea by bars of drifted matter so deposited along the boundary curve of a great marine current. The long narrow line of coast and string of islands which skirt the north of Holland, seem to have once formed an extensive bar of this kind, from the mouth of the Scheldt to that of the Elbe, having one or more large lagoons within; but the bias of the marine current, for some time past, has set in with violence against the land (owing to the increase, perhaps, of some of the vast shoals which are forming in the German ocean), and these islands have in consequence, for some centuries, been rapidly worn away. The Rhine and the ocean are here opposed to each other, each disputing the ground occupied by north Holland; the one striving to shape out a curved line of coast, the other to form a delta.

"There was evidently a period when the river obtained the ascendancy, and the greater part of Holland is the result of its depositions; but for the last two thousand years, during which man has witnessed and actively participated in the struggle, the result has been in favour of the ocean, the area of the whole territory having become more and more circumscribed; natural and artificial barriers having given way, one after another, and many hundred thousand human beings have perished in the waves."

Even the great gulf of Mexico itself may be considered as approaching to the condition of a vast lagoon; the flat projecting headlands of Yucatan and Florida-together with the immense submarine shoals by which they are prolonged two-thirds of the way, at least, across the entrance of the gulf-being the extremities of the vast bar which is in process of formation by the action of the great intertropical current. This powerful stream, driven by the tradewinds across the Atlantic, and along the north coast of South America where it becomes charged with an enor mous quantity of sediment brought down by the rivers Amazon and Orinoco, the

sweepings of half the South American continent, is heaped up at the mouth of the gulf, and deposits there most of its suspended matter, escaping laterally through the canal of Bahama, with a fall which communicates to it a rapidity of four miles an hour. Much of the silt received by the gulf-stream from the waters of the Amazon is also thrown up on the coast of Guiana, where immense tracts of new and prodigiously fertile land are forming; much also is left in the Caribbean sea, on the shores of Trinidad and Honduras, which are annually gaining in extent.

Winds often assist in the formation and increase of bars, by drifting the sand of the shore up to higher levels than it would otherwise attain, and sometimes into hills of considerable elevation; three hundred feet or more, as the Dunes of the north coast of France and Holland, of Norfolk, Cornwall and Moray. But unquestionably the greatest example of the transporting power of winds, is the sand-flood of Africa, which, moving gradually eastwards, has overwhelmed all the lands capable of tillage west of the Nile, unless sheltered by high mountains, and threatens ultimately to obliterate the rich plain of Egypt. It would seem that the formation of the vast central desert of Africa, the Zahara, may have been effected through the constant westerly winds drifting along the sands which are thrown up on the shallow shore on both sides of Cape Blanco, by the powerful and danger ous current well known to set in upon it.

The fragmentary matter carried away by marine currents and spread widely over the bed of the ocean, must infinitely exceed the deposits of rivers. The bed of the German ocean, which is the common receptacle of the detritus swept away from the eastern coast of Britain, the mouths of the Rhine, Maes, Scheld, and Elbe, and the shores of Holland, Denmark, and Norway, is encumbered to an extraordinary degree with sand banks and shoals, as appears from Mr. Stevenson's detailed and very curious survey. "The Dogger-bank alone is three hundred and fifty miles in length, and the principal shoals united occupy an area equal to onethird of Great Britain." Their average height is seventy-eight feet, according to Mr. Stevenson; so that, assuming them to be uniformly composed to this depth of drift matter, they would cover the whole of England and Scotland to the thickness of twenty-eight feet! A great proportion of these banks consists of siliceous sand mixed with fragments of shells and corals, ground down, the proportion of these lcareous matters being very great. The

carried eastwards by the great

current of the Mediterranean is deposited on the shores of Syria and Asia Minor as strata of stone, not of loose materials, owing to the abundance of carbonate of lime held in solution by the streams and rivers which here flow into the sea. It is the opinion of M. Giraid, one of the savans who accompanied Napoleon's expedition to Egypt, and were employed on the survey of the ancient canal of Amrou, communicating between the Nile and the Red Sea, that the isthmus of Suez itself is merely a bar formed by the deposition of this current and of the Nile, and that the two seas were formerly united. It is certain that the istumus is daily gaining in width by the accession of fresh deposits on the shore of the Mediterranean.

66

Icebergs are probably active instruments in the transportation of gravel and rocks, from the mountainous shores against which they form in high latitudes, to the bottom of the distant seas where the ice is dissolved. Scoresby counted five hundred icebergs in latitude 69 deg. and 70 deg. north. Many contained strata of earth and stone, or were loaded with beds of rock of great thickness." Such ice islands, before they are melted, have been known to drift from Baffin's Bay to the Azores, and from the south pole to the neighbourhood of the Cape.

At the openings of large inland seas into the ocean, currents are sometimes produced by the influx or efflux of water to maintain its uniformity of level, when deranged through the supply of the basins from tributary rivers exceeding or falling short of the drain upon them from evaporation. The Baltic may be given as an instance of excessive, the Mediterranean of deficient supply. The former basin discharges its redundancy into the German ocean, through the Sound; and hence it is very inferior in saltness to most seas. In the north of the gulf of Bothnia, the water is nearly fresh, and the saltness is very inconsiderable where it joins the Baltic. The Mediterranean, on the contrary, receives a supply from the Atlantic through the Straits of Gibraltar. It has been supposed that an equal quantity is discharged by a counter-current below; but this is an unnecessary and unwarranted hypothesis, The Mediterranean is, from this cause, salter than the ocean; and as it receives constant accessions of salt from the Atlantic, as well as its own tributaries, and parts with none, what becomes of the excess? Mr. Lyell suggests, that in the enormous depths of the central parts of this sea, it is probably precipitated, "on the grandest scale, in continuous masses of pure rock-salt, extending, perhaps, hundreds of miles in length."

« PreviousContinue »