Page images

excavation goes on in lieu of accumulation, more or less regular curvature according and an estuary, or inlet of the sea, what to these circumstances. It is along this Rennell calls a minus delta, is produced, in sweeping line that the matter drifted by place of a projection. It is easily seen the current subsides, as the momentum of bow a tidal wave, alternately flowing up its particles carries them beyond the line the mouth of a river, and ponding back its which limits the transporting power of waters, and then returning with double the stream; and thus every current, after violence through the added momentum of rushing past opposing headlands, tends to these waters themselves, must scour out form out of their detritus a coast-line corthe channel, and wear away the land on responding with the curve they have im. either side of the mouth. Thus were pro- pressed upon it. The Etangs of the south duced the great estuaries of the Thames, of France, the Haffs of northern Prussia, the Severn, and the Solway, of the Seine, the Fiords of the west coast of Denmark, the Gironde, the Tagns, the Elbe, the and the great Lagoons of the gulf of Delaware, the Chesapeake, and of nu- Mexico, are examples, on a large scale, of merous other rivers flowing into tidal the stagnant pools of water shut out from şeas, which, but for this circumstance, the sea by bars of drifted matter so depowould probably have, long since, filled up sited along the boundary curve of a great the great snbmarine valleys which they in- marine current. The long narrow line of dicate, instead of keeping them open, and coast and string of islands which skirt the indeed widening them daily, as they are north of Holland, seem to have once observed to do now. Where a current formed an extensive bar of this kind, from flows by the mouth of a river, though the the inouth of the Scheldt to that of the whole of the drift matter is not perma- Elbe, having one or more large lagoons nently deposited, yet, at the line of junc- within ; but the bias of the marine cura tion between thc fluviatile and marine cure rent, for some time past, lias set in with rent, where they neutralize each other, a violence against the land (owing to the certain quantity subsides, and a bar, or increase, perhaps, of some of the vast lengthened bank, is the result, extending shoals which are forming in the German across the month of the river. The extent ocean), and these islands have in conseand depth of this bar, and the position it quence, for some centuries, been rapidly takes in tlie opening of the river, are worn away. The Rhine and the ocean determined by the comparative force are here opposed to each other, each disand direction of the antagonist currents puting the ground occupied by north Holof the sea and river. Tlie latter almost land; the one striving to shape out a curved always preserves an opening for its issue line of coast, the other to form a delta. through the bar, at the further extremity “There was evidently a period when from the direction of the marine current; the river obtained the ascendancy, and but where the force of the river is compara- the greater part of Holland is the result of tively triling, the bar is completed, and its depositions; but for the last two thouthe stream either percolates through it, or sand ycars, during which man bas witbeing dammed up into a lake within, overnessed and actively participated in the flows it on one or more points, which are struggle, the result has been in favour of occasionally worn into channels of commu• the ocean, the area of the whole territory pication, admitting the sea-water, and having become more and more circunithen again closed op, so as to occasion the scribed; natural and artificial barriers Jake to be alternately salt and fresh, having given way, one after another, and Bars aud shoals are also formed at the many bundred thousand human beings conflux of two marine currents holding have perished in the waves.” sedimentary inatier in suspension, or of a Even the great gulf of Mexico itself current and an eddy, or along the bound. may be considered as approaching to the ary line of a current lordered by stagnant condition of a vast lagoon; the flat projectwater. The direction of every current ing headlands of Yucatan and Florida—10depends chiefly on the form of the coast gether with the immense submarine shoals past which it flows; and it is deflected by by which they are prolonged two-thirds of projecting headlands, banks, and shoals, the way, at least, across the entrance of just in the manner of a river. Hence be, the gulf-being the extremities of the vast hind such projections the water is undis, bar which is in process of formation by Insbed, except by the eddy occasioned in the action of the great intertropical cur. it through the friction of the current rent. This powerful stream, driven by sweeping by. The boundary lipe of the the tradewinds across the Atlantic, and current and stagnant water is determined along tbe north coast of South America by the momentum and previous direction of where it becomes charged with an enor. the former, and the projcting resistances mous quantity of sediment brought down it meets with, but wiformly assumes a by the rivers Amazon and Orinoco, the sweepings of half the Sonth American con. current of the Mediterranean is deposited linent, is heaped up at the mouth of the on the shores of Syria and Asia Minor as gulf, and deposits there most of its sus- strata of stone, not of loose materials, pended matter, escaping laterally through owing to the abundance of carbonate of the canal of Bahama, with a fall which lime held in solution by the streams and communicates to it a rapidity of four rivers which here flow into the sea. It is miles ao hour. Much of the silt received the opinion of M. Giraid, one of the satans by the gulf-stream from the waters of the who accompanied Napoleon's expedition Amazon is also thrown up on the coast of to Egypt, and were employed on the Guiana, where irmense tracts of new and survey of the ancient canal of Amrou, prodigiously fertile land are forming; communicating between the Nile and the much also is left in the Caribbean sea, on Red Sea, that the isthmus of Suez itself is the shores of Trinidad and Honduras, merely a bar formed by the deposition of which are annually gaining in extent. this current and of the Nile, and that the

Winds often assist in the formation and two seas were formerly uvited. It is increase of bars, by drifting the sand of certain that the istumus is daily gaining the shore up to higher levels than it in width by the accession of fresh dewould otherwise attain, and sometimes posits on the shore of the Mediterranean, into hills of considerable elevation; three Icebergs are probably active instruhundred feet or more, as the Dunes of ments in the transportation of gravel and the north coast of France and Hol- rocks, from the mountainous shores against land, of Norfolk, Cornwall and Moray. which they form in high latitudes, to the But unquestionably the greatest example bottom of the distant seas where the ice is of the transporting power of winds, is the dissolved. “Scoresby counted five honsund-flood of Africa, which, moving gra- dred icebergs in latitnde 69 deg. and dually eastwards, has overwhelmed all the 70 deg. north. Many contained strata of Jands capable of tillage west of the Nile, earth and stone, or were loaded with beds unless sheltered by high mountains, and of rock of great thickness.” Snch ice threatens ultimately to obliterate the rich islands, before they are melted, have been plain of Egypt. It would seem that the known to drift from Baffin's Bay to the formation of the vast central desert of Azores, and from the south pole to the Africa, the Zaliara, may have been ef. neighbourliood of the Cape. fected through the constant westerly winds At the openings of large inland seas drifting along the sands which are thrown into the ocean, currents are sometimes np on the shallow shore on both sides of produced by the influx or efflux of water Cape Blanco, by the powerful and danger- to maintain its uniformity of level, when ous current well known to set in upon it. deranged through the supply of the basins

The fragmentary matter carried away from tributary rivers exceeding or falling by marine currents and spread widely short of the drain upon them from evapoover the bed of the ocean, must infinitely ration. The Baltic may be given as an exceed the deposits of rivers. The bed instance of excessive, the Mediterranean of the German ocean, which is the com- of deficient supply. The former basin dismon receptacle of the detritus swept away charges its redundancy into the German from the eastern coast of Britain, the ocean, through the Sound; and hence it is months of the Rhine, Maes, Scheld, and very inferior in saltness to most seas. In Elbe, and the shores of Holland, Den the north of the gulf of Bothnia, the water mark, and Norway, is encumbered to an is nearly fresh, and the saltness is very inextraordinary degree with sand banks and considerable where it joins the Baltic. shoals, as appears from Mr. Stevenson's The Asediterranean, on the contrary, redetailed and very curious survey. “ The ceives a supply from the Atlantic through Dogger-bank alone is three hundred and the Straits of Gibraltar. It has been supfitty miles in length, and the principal posed that an equal quantity is discharged shoals united occupy an area equal to one- by a counter-current below; but this is an third of Great Britain.” Their average unnecessary and unwarranted hypothesis, height is seventy-eight feet, according to The Mediterranean is, from this cause, Mr. Stevenson; so that, assuming them to salter than the ocean; and as it receives be uniformly composed to this depth of constant accessions of salt from the Atdrift matter, they would cover the whole lantic, as well as its own tributaries, and of England and Scotland to the thickness parts with none, what becomes of the of twenty-eight feet! A great proportion excess ? Mr. Lyell suggests, that in the of these banks consists of siliceous sand enormous depths of the central parts of mixed with fragments of shells and corals, this sea, it is probably precipitated, “on grotud down, the proportion of these the grandest scale, in continuous masses careous matters being very great. The of pure rock-salt, extending, perhaps,

carried eastwards by the great hundreds of miles in length."

through the Aodes of Pern and Quito, and IGNEOUS AGENTS OF CHANGE.

thence across the provinces of Pasto, Po. THE igneous agents of change now operat. payan), Guatimala, Nicaragua, and the ing on the surface of the earth, are vol plateau of Mexico, up to the northern exeanos and earthquakes. These, thongh it tremity of the peninsula of California. If may be advisable to divide them, tor the the west coast of North America were sake of classification, are closely mited in explored, we should probably tind this nature, earthquakes being usnally fol. linear series of volcanos prolonged in that lowed hy eruptions from either a new or direction to unite with the yet more resome neighbouring volcano; and no vol. markable train which coinmences in the canic eruption of any magnitude taking vicinity of Cook's Harbour, threads the place without the accompaniment of whole length of the Aleutian isles in an earthquakes, which seem to be merely easterly direction for the space of a thouvibrations of the globe, when rent and up- sand miles, then turns southwards, and beaved by the expansion of the volcanic pursues an winterrupted course of bematter, struggling to find a vent. After tween sixty and seventy degrees of laan issue has been formed, and so long as it titude, through Kamskatchka, the Kurile, keeps suficiently open to allow of an easy Japanese, Loochoo, Philippine isles, and habitual or occasional dischargé, the con- Moluccas, where it branches oíf in differvulsions of the neighbouring soil are of ent directions towards the east and northá mild and harmless character. But, west. One line traverses Java and Suwhere a volcanic vent is wanting, or bas matra, and turns northwards through the been long obstructed by the accumulation Andaman isles to the west coast of the and hardening of the ejected matters, the kingdoin of Ava; the other is prolonged shocks, when they occur at last, are pro. across New Guinea into the Polynesian portionately violent and destructive; so archipelago, which seems to be one, vast that it is not without justice that habitual theatre of igneous action, the greater volcanos bave been called the safety-vulves number, if not all, of the islands being of those districts which are at present formed of coralline reefs, interstratified liable to subterranean convulsions. The with or based upon volcanic rocks. number of principal volcanos kuown to be Throughout the two great lines we have occasionally in eruption is upwards of noticed, which, if they prove, as we suistwo hundred ;-but thousands of moun. pect, to be continuions with each other, tains of similar form and structure, and will be longer than the whole circumference bearing the marks of (geologically speak- of the globe, not only are there a vast puming) exceedingly recent activity, are scat. ber of volcanic apertures, which, within tered around and between them, the fires the last few years, have been in eruption, of which, thongh to all appearance slun. but the intervals are filled by strings of bering, are likely in many instances to eminences evidently produced by similar break forth again, since nothing can be phenomena, all of which have been, and more common than the renewal of erup- many no donbt will again be, habitually tions from volcanic hills which had never active. Sometimes points of eruption are been in activity within the range of tradi- collected in groups, as those of Iceland, tion. The subterranean fire is observed the Canaries, and the Azores; but as to slijft its ont ward development capri- these are uniformly insular, and only, in cio:sly from one point to another, occa. fact, the summits of a group of submarine sionally retorning again to its earlier vents, volcanic mountains, we cannot be certain according to circumstances, with some of that they do not form a part, the inosculawhich we are probably not yet acquainted, tions probably, of one or more lengthened but which seem chiedy to consist in the trains, continued in the depths of the accumulations both of congealed lava and ocean, and not yet raised above its ejected fragments, by which every ha- surface. bitual vent tends continnally to block up The cause of the conical figure so chaits channels of discharge. One remark- racteristic of a volcanic mountain, must able law characterizes the geographical be obvions to all who are acquainted with distribution of points of volcanic ernp. the circumstances of an ordinary eruption; namely, that they almost invariably tion. When the expansion of a subteroccur in linear trains, stretching in some ranean mass of lava has rent the overlying cases across a third of the globe. Such, for crust of rocks, the liquid matter boils up instance, is that which, beginning in the those parts of the fissure which offer least south of Chili, or rather at Cape Horn, if resistance; and, as it approaches the atwe believe the reports of burning moun. mosphere, discharges enormous bubbles of tains in Terra del Fuego and Patagonia, elasiic fluid, chietly steam, which project rúns northwards in an uninterrupted chain into the air showers of red-hot lava and

Vol. VI.

fragments torn from the sides of the cre- distinguishing appellation of craters of vice through which they escape. These paroxysmal explosion. Nor are they ejected matters, on failing, accumulate broken through volcanic mountains alone, round the opening into a circular bank, but not unfrequently through granite or which, by the continuance of the process, stratified rocks, which may be seen surbeconies a truncated cone, with an in- rounding them in deep escarpments, and ternal funnel. This is the common form supporting the fragments of ihose rocks of a volcanic cone, thrown up by the explo- and scoriæ thrown out. The width of a sions of a single eruption. If lava flows crater seems to depend on the bulk of the from the same orifice, after the formation volumes of vapour discharged at once, and of the cone, it breaks down the side; if does not always correspond with the quan. before, the cone is often raised upon the tity of matter ejected, or the duration of hardened surface of the lava-current, the eruption. After the formation of a which flows underneath, in a sort of ca- crater of great size, in the manner we nal, without damaging the bank above. have described, succeeding eruptions, Should subsequent ernptions take place from the same central vent, only throw up on the same point, the billock becomes secondary cones and lava streams at the more complicated in its structure, but the bottom of this golph, which, accumulating conical form is still preserved with suf- on one another, by degrees fill it op enficient regularity, the ejected matters tirely. At this time the volcanic mounmantling round the outside of the hill, tain may exhibit no crater at all; and and the lava, which pours over the lips of this is by no means an frequent conthe crater, or forces its way through cre- dition of extinct or dormant volcanos, vices in the sides of the cone, hardening But the weight and coherence of these into massive ribs or coatings, by which its accumulations over the mouth of the volbulk is at the same time increased, and cano seem, by repressing, to increase its a durable skeleton supplied. After re- latent energy ; and it often again bursts peated eruptions from the same opening, forth in a paroxysm of explosions, which the simple cone becomes in this way en- blow off the whole summit of the mounlarged into the volcanic mountain.

tain, and leave a fresh central cavity, of Vague and incorrect ideas are often proportionate dimensions, sometimes seattached to what is called the crater of a veral miles in diameter. Almost every volcano. Some have erroneously sup. volcanic mountain, habitually eruptive, is posed that every volcano must at all times thus unilergoing a succession of destruchave a crater-confounding it with the tions and repairs, and none conld better vent of the erupted matter, which is often illustrate this law than Vesuvius during no more than a narrow crevice, and, being the past century. Those who will take filled up by the products of the ernption, the trouble to consult Hamilton's plates is not easily to be discovered afterwards. and relations, will trace the process we A crater is the cup-shaped hollow left by have described several times repeated, np the repeated explosions of elastic fluids, to the publication of his work. The last which iisnally, but not always, accompany phenomenon, described by him, was the the emission of lava from a crevice, and paroxysmal eruption of 1791, which gutted often occur without any overflow of lava. the cone, and left a vast crater, three The crater of a simple cone, formed of miles in circumference. This cavity was fragmentary matter alone, is, as we have gradually filled by the falling in of its seen, a hollow inverted cone, circum- sides, and the subsequent minor eroptions scribed by the talus of debris heaped up froin that time to 1822, when a higli conround the veut. But, in volcanic moun- vexity had replaced the hollow on the tains, after explosions of paroxysmal vio- summit of the cone. In October of that Jence, the whole solid centre of the moun- year, an eruption occurred, accompanied tain is often blown into the air, and its by explosions of great violence, which contents scattered over the outer slopes, lasted twenty days, and once more holor worn to powder by repeated ejections, lowed out the cone, leaving a crater a and carried by winds to vast distances. mile in diameter, and two thousand fect The crater left by such an eruption is a deep. Since that time, fresh eruptions deep and often wide cavity, bordered by bave been going on from the bottom of abrupt rocky precipices, in which sections the crater : a secondary cone is thrown are exposed of the successively-accumul- up there, and produces lava and scoriæ, lated beds that form the substance of the which already bave half filled the great mountain. Such a crater is wholly dif- crater. ferent in appearance from the smooth. The cliff-range of Somma, which halt sided and regularly-sloping funnel of a encircles the upper cone of Vesuvius, is, simple cone. The former deserve the without doubt, the remaining segment of tle walls of the vast crater produced by seems that some vast and ancient crater of the explosions of 79 A. D., which en this mountain lias been nearly filled with tombed Herculaneum and Pompeii be- a sort of bath, or pool, of liquid lava, on neath the fragments of the shattered the surface of which a crust forms, but as mountain,

fast as fresh lava wells up from below, the From wliat we have said, it will appear crust is broken through by minor erupbow incorrect is the popular notion, that, tions. As this mass of lava rose in the in every eruption, the crater of a volcano crater, the weight of its increasing column is filled to the brim with lava, which pour's has, at intervals, burst a lateral crevice in thence over the outer slope. The violent the side of the mountain, through which explosions of a single eruption occasion, the reservoir of lava has been tapped of its ally blow nearly the whole mountain into excess, and circular subsidences been sacthe air, leaving only its skirts as a low cessively formed in the crust above the truncated cone, surrounding a basin, se- broken edges of which form a series of veral miles in diameter. After such a pa. terraced ledges, at different heights, surroxysm, hundreds of cruptions may take rounding the present hollow. This is a place within this vast crater before it is remarkable, but very intelligible, variation tilled, and a new moutain reared in place of the volcanic phenomena, perfectly in of the oid one. We may mention here harmony with their known laws of operathat we are very sceptical as to the ac- tion. cuaints received, tiom popular report, of Immense volumes of aqueous vapours the sinking in of volcanic mountains during are evolved from a crater during eruperuptions.' We kuow the ordinary course tions, and for a long time after the disto be, that they are blown outwards, and charge of lava and scoriæ has ceased. their fragments scaltered on all sides by They are condensed in the cold atmothe violence of the aeriform explosions, sphere suurrounding the volcanic peak, and which sometimes continue for weeks, and heavy rains are often caused, even in reduce the wreck of the mountain to an countries whiere, under other circumimpalpable powder, which the winds bear stances, raiu is uwkuown. Falling on a off to enormous distances. Nor do we surface which the eruption has thickly recollect any relation of the disappearance coated with fine ashes and loose fragments of a mountain, and the substitution of a of all sizes, the rains sweep them along in cavity, perhaps a lake (as the Peak of a flood of mud and stones, which often Timor, destroyed in 1637 ; Papandayang, does far more mischief than the ignited in Java, 1772), without the accompani. Java or earthquakes, and deposit at the ment of tremendous discharges of frag- foot of the mountain massive beds of conmentary matter, which is described as glomerate. If snow covers the cone, still covering the whole face of the country more extensive deluges are produced around, to a distance sometimes of hume through its sudden melting by contact dreds of miles : froin which circunstances with the red-hot lava. Etna, as might we conclude, that the bulk of the moun- be expected, presents many traces of such tain was broken up and scattered to the floods; but it is in Iceland that they are winds by repeated explosions, not that it exhibited on the most powerful scale. fell in; though it is natural that the in- Conglomerates of immense extent and Jiabitants, finding on their return a deep thickness have been spread in this manner cavity in place of a mountain, sliould within a late period, over the plains at imagwe it the effect of subsidence rather the base of Hecla, On Etna itself a than explosion. In fact, all the pheno. thick bed of solid ice has lately been found mena of volcanos tend to show their origin under an ancient current of lava. It is in a mass of matter, confined at an intense very conceivable that a coating of sand temperature, and struggling to escape; and scoriæ, the best possible non-conand, therefore, make it very improbable ductors of heat, may enable snow to bear that any vast subterranean caverns can a stream of red-hot lava over it without exist, into which the mountain could be being melted. It is probable, that in Iceprecipitated. That the cliffs, surround- land the circumstance has been often reing a deep crater, occasionally fall in- peated, and we may expect to find glaciers wards during earthquakes, so as to soften alternating there with beds of lava and their declivity, and truncate the mountain volcanic conglomerate. at a lower pomt, is very true, and this has O ne continuous eruption will frequently probably given rise to some of the stories throw up a number of simple cones. as to the engulphing of mountains. The Every considerable eruption is described appearances of the volcano of Kirauea, in as commencing with the splitting of the O whyhee, described by Mr. Ellis, are solid ground, and the production of a very peculiar, but afford no countenance crevice prolonged sometimes many miles. to the idea of subterranean cavities. It The explosions, as well as the lava

« PreviousContinue »