Imágenes de páginas

was there, with russet woods. A kind of remain suspended in the water, the scatmystery attaches itself to gyrating water, tered light will be a uniform green. due perhaps to the fact that we are to Hence the greenness of shoal water. some extent ignorant of the direction of You go to bed with the black Atlantic its force. It is said that at certain points around you. You rise in the morning of the whirlpool pine-trees are sucked and find it a vivid green; and you cordown, to be ejected mysteriously else- rectly infer that you are crossing the where. The water is of the brightest bank of Newfoundland. Such water is emerald green. The gorge through found charged with fine matter in a which it escapes is narrow, and the mo- state of mechanical suspension. The tion of the river swift though silent. light from the bottom may sometimes The surface is steeply inclined, but it is come into play, but it is not necessary. perfectly unbroken. There are no lateral A storm can render the water muddy waves, no ripples with their breaking by rendering the particles too numerous bubbles to raise a murmur, while the and gross. Such a case occurred towards depth is here too great to allow the in- the close of my visit to Niagara. There equality of the bed to ruffle the surface. had been rain and storm in the upperNothing can be more beautiful than this lake regions, and the quantity of sussloping liquid mirror formed by the pended matter brought down quite Niagara in sliding from the whirlpool. extinguished the fascinating green of

the Horseshoe. The green colour is, I think, correctly Nothing can be more superb than the accounted for in “Hours of Exercise in green of the Atlantic waves when the the Alps.” In crossing the Atlantic I circumstances are favourable to the exhad frequent opportunities of testing the hibition of the colour. As long as a explanation there given. Looked pro- wave remains unbroken no colour apperly down upon, there are portions of pears; but when the foam just doubles the ocean to which we should hardly over the crest like an Alpine snowascribe a trace of blue; at the most a cornice, under the cornice we often see hint of indigo reaches the eye. The a display of the most exquisite green. water, indeed, is practically black, and It is metallic in its brilliancy. But the this is an indication both of its depth foam is necessary to its production. and its freedom from mechanically sus. The foam is first illuminated, and it pended matter. In small thicknesses scatters the light in all directions ; the water is sensibly transparent to all kinds light which passes through the higher of light; but as the thickness increases, portion of the wave alone reaches the the rays of low refrangibility are first eye, and gives to that portion its matchabsorbed, and after them the other rays. less colour. The folding of the wave, Where, therefore, the water is very deep producing, as it does, a series of longi. and very pure, all the colours are ab- tudinal protuberances and furrows which sorbed, and such water ought to appear act like cylindrical lenses, introduces black, as no light is sent from its interior variations in the intensity of the light, to the eye. The approximation of the and materially enhances its beauty. Atlantic Ocean to this condition is an indication of its extreme purity.

We have now to consider the genesis Throw a white pebble into such water; and proximate destiny of the Falls of as it sinks it becomes greener and Niagara. We may open our way to this greener, and, before it disappears, it subject by a few preliminary remarks reaches a vivid blue green. Break such upon erosion. Time and intensity are a pebble into fragments, each of these the main factors of geologic change, and will behave like the unbroken mass; they are in a certain sense convertible. grind the pebble to powder, every A feeble force acting through long particle will yield its modicum of green; periods, and an intense force acting and if the particles be so fine as to through short ones, may produce ap

proximately the same results. To Dr. Hooker I have been indebted for some samples of stones, the first examples of which were picked up by Mr. Hackworth on the shores of Lyell's Bay, near Wel. lington, in New Zealand. They have been described by Mr. Travers in the Transactions of the New Zealand Institute. Unacquainted with their origin, you would certainly ascribe their forms to human workmanship. They resemble flint knives and spear-heads, being apparently chiseled off into facets with as much attention to symmetry as if a tool guided by human intelligence had passed over them. But no human instrument has been brought to bear upon these stones. They have been wrought into their present shape by the wind-blown sand of Lyell's Bay. Two winds are dominant here, and they in succession urged the sand against opposite sides of the stone; every little particle of sand chipped away its infinitesimal bit of stone, and in the end sculptured these singular forms."

The Sphinx of Egypt is nearly covered up by the sand of the desert. The neck of the Sphinx is partly cut across, not, as I am assured by Mr. Huxley, by ordinary weathering,

1 “The stones, which have a strong resem. blance to works of human art, occur in great abundance, and of various sizes, from half an inch to several inches in length. A large number were exhibited showing the various forms, which are those of wedges, knives, arrow-heads, &c., and all with sharp cutting edges.

"Mr. Travers explained that, notwithstanding their artificial appearance, these stones were formed by the cutting action of the winddriven sand as it passed to and fro over an exposed boulder-bank. He gave a minute account of the manner in which the varieties of form are produced, and referred to the effect which the erosive action thus indicated would have on railway and other works executed on sandy tracts.

“Dr. Hector stated that although, as a group, the specimens on the table could not well be mistaken for artificial productions, still the forms are so peculiar, and the edges, in a few of them, so perfect, that if they were discovered associated with human works, there is no doubt that they would have been referred to the so-called 'stone period.'”— Extracted from the Minutes of the Wellington Philosophical Society, Feb. 9, 1869.

but by the eroding action of the fine sand blown against it. In these cases nature furnishes us with hints which may be taken advantage of in art; and this action of sand has been recently turned to extraordinary account in the United States. When in Boston, I was taken by Mr. Josiah Quincey to see the action of the sand-blast. A kind of hopper containing fine silicious sand was connected with a reservoir of compressed air, the pressure being variable at pleasure. The hopper ended in a long slit, from which the sand was blown. A plate of glass was placed beneath this slit, and caused to pass slowly under it; it came out perfectly depolished, with a bright opalescent glimmer, such as could only be produced by the most careful grinding. Every little particle of sand urged against the glass, having all its energy concentrated on the point of impact, formed there a little pit, the depolished surface consisting of innumerable hollows of this description. But this was not all. By protecting certain portions of the surface and exposing others, figures and tracery of any required form could be etched upon the glass. The figures of open iron-work could be thus copied ; while wire gauze placed over the glass produced a reticulated pattern. But it required no such resisting substance as iron to shelter the glass. The patterns of the finest lace could be thus reproduced; the delicate filaments of the lace itself offering a sufficient protection.

All these effects have been obtained with a simple model of the sand-blast devised for me by my assistant. A fraction of a minute suffices to etch upon glass a rich and beautiful lace pattern. Any yielding substance may be employed to protect the glass. By immediately diffusing the shock of the particle, such substances practically destroy the local erosive power. The hand can bear without inconvenience a sand-shower which would pulverize glass. Etchings executed on glass with suitable kinds of ink are accurately worked out by the sand-blast. In fact, within certain limits, the harder the

surface, the greater is the concentration of the sand-blast. To his spontaneous of the shock, and the more effectual is kindness I am indebted for some beautithe erosion. It is not necessary that ful illustrations of his process. In one the sand should be the harder substance thick plate of glass a figure has been of the two; corundum, for example, is worked out to a depth of three-eighths much harder than quartz ; still, quartz- of an inch. A second plate sevensand can not only depolish, but actually eighths of an inch thick is entirely blow a hole through a plate of corundum. perforated. Through a circular plate Nay, glass may be depolished by the of marble, nearly half an inch thick, impact of fine shot; the grains in this open work of the most intricate and case bruising the glass before they have elaborate description has been exetime to flatten and turn their energy cuted. It would probably take many into heat.

days to perform this work by any And here, in passing, we may tie ordinary process ; with the sand-blast it together one or two apparently unrelated was accomplished in an hour. So much facts. Supposing you turn on, at the for the strength of the blast; its delower part of a house, a cock which licacy is illustrated by a beautiful exis fed by a pipe from a cistern at the ample of line engraving, etched on glass top of the house, the column of water, by means of the blast.1 from the cistern downwards, is set in This power of erosion, so strikingly motion. By turning off the cock, this displayed when sand is urged by air, motion is stopped ; and when the turn- renders us better able to conceive ing off is very sudden, the pipe, if not its action when urged by water. The strong, may be burst by the internal erosive power of a river is vastly augimpact of the water. By distributing mented by the solid matter carried along the turning of the cock over half a with it. Sand or pebbles caught in a second of time, the shock and danger of river vortex can wear away the hardest rupture may be entirely avoided. We rock ; “potholes" and deep cylindrical have here an example of the concentra- shafts being thus produced. An extration of energy in time. The sand-blast ordinary instance of this kind of erosion illustrates the concentration of energy is to be seen in the Val Tournanche, in space. The action of flint and steel above the village of this name. The is an illustration of the same principle. gorge at Handeck has been thus cut out. The heat required to generate the spark Such waterfalls were once frequent in is intense, and the mechanical action the valleys of Switzerland; for hardly being moderate, must, to produce fire, any valley is without one or more transbe in the highest degree concentrated. verse barriers of resisting material, over This concentration is secured by the which the river flowing through the collision of hard substances. Calc-spar valley once fell as a cataract. Near will not supply the place of flint, nor Pontresina in the Engadin, there is such lead the place of steel in the production a case, the hard gneiss being now wom of fire by collision. With the softer away to form a gorge through which substances, the total heat produced may

United States, is forcibly illustrated by the be greater than with the hard ones; but

rapid transfer of men like Mr. Tilghman from to produce the spark, the heat must be

the life of the soldier to that of the civilian. intensely localized.

General McClellan, now a civil engineer, whom But we can go far beyond the mere

I had the honour of frequently meeting in depolishing of glass ; indeed, I have

New York, is a most eminent example of the

same kind. At the end of the war, indeed, a already said that quartz sand can wear

million and a half of men were thus drawn, in a hole through corundum. This leads an astonishingly short time, from military to me to express my acknowledgments to civil life. It is obvious that a nation with General Tilghman, who is the inventor

thesc tendencies can have no desire for war.

1 The sand-blast will be in operation this 1 The absorbent power, if I may use the year at the Kensington International Exhiphrase, exerted by the industrial arts in the bition.

the river from the Morteratsch glacier rushes. The barrier of the Kirchet above Meyringen is also a case in point. Behind it was a lake, derived from the glacier of the Aar, and over the barrier the lake poured its excess of water. Here the rock being limestone was in great part dissolved, but added to this we had the action of the solid particles carried along by the water, each of which, as it struck the rock, chipped it away like the particles of the sand-blast. Thus by solution and mechanical erosion the great chasm of the Fensteraarschlucht was formed. It is demonstrable that the water which flows at the bottoms of such deep fissures once flowed at the level of what is now their edges, and tumbled down the lower faces of the barriers. Almost every valley in Switzerland furnishes examples of this kind; the untenable hypothesis of earthquakes, once so readily resorted to in accounting for these gorges, being now for the most part abandoned. To produce the Cañons of Western America no other cause is needed than the integration of effects individually infinitesimal.

And now we come to Niagara. Soon after Europeans had taken possession of the country, the conviction appears to have arisen that the deep channel of the river Niagara below the falls had been excavated by the cataract. In Mr. Bakewell's “ Introduction to Geology," the prevalence of this belief has been referred to: it is expressed thus by Professor Joseph Henry in the Transactions of the Albany Institute:1—“In viewing the position of the falls and the features of the country round, it is impossible not to be impressed with the idea that this great natural raceway has been formed by the continued action of the irresistible Niagara, and that the falls, beginning at Lewistown, have, in the course of ages, worn back the rocky strata to their present site." The same view is advocated by Sir Charles Lyell, by Mr. Hall, by M. Agassiz, by Prolessor Ramsay, indeed by almost all of those who have inspected the place.

A connected image of the origin and progress of the cataract is easily obtained. Walking northward from the village of Niagara Falls by the side of the river, we have to our left the deep and comparatively narrow gorge through which the Niagara flows. The bounding cliffs of this gorge are from 300 to 350 feet high. We reach the whirlpool, trend to the north-east, and after a little time gradually resume our northward course. Finally, at about seven miles from the present Falls, we come to the edge of a declivity which informs us that we have been hitherto walking on table-land. Some hundreds of feet below us is a comparatively level plain, which stretches to Lake Ontario. The declivity marks the end of the precipitous gorge of the Niagara. Here the river escapes from its steep mural boundaries, and in a widened bed pursues its way to the lake which finally receives its waters. . The fact that in historic times, even within the memory of man, the fall has sensibly receded, prompts the question, how far has this recession gone? At what point did the ledge which thus continually creeps backwards begin its retrograde course ? To minds disciplined in such researches the answer has been and will be, at the precipitous declivity which crossed the Niagara from Lewiston on the American to Queenston on the Canadian side. Over this transverse barrierthe united affluents of all the upper lakes once poured their waters, and here the work of erosion began. The dam, moreover, was demonstrably of sufficient height to cause the river above it to submerge Goat Island; and this would perfectly account for the finding by Mr. Hall, Sir Charles Lyell, and others, in the sand and gravel of the island, the same fluviatile shells as are now found in the Niagara river higher up. It would also account for those deposits along the sides of the river, the discovery of which enabled Lyell, Hall, and Ramsay to reduce to demonstration the popular belief that the Niagara once flowed through a shallow valley.

The physics of the problem of exca

Quoted by Bakewell.

vation, which I made clear to my mind Fall are compared together. The Amebefore quitting Niagara, are revealed by rican branch of the upper river is cut a close inspection of the present Horse- at a right angle by the gorge of the shoe Fall. Here we see evidently that Niagara. Here the Horseshoe Fall was the greatest weight of water bends over the real excavator. It cut the rock and the very apex of the Horseshoe. In a formed the precipice over which the passage in his excellent chapter on American Fall tumbles. But since its Niagara Falls, Mr. Hall alludes to this formation, the erosive action of the fact. Here we have the most copious American Fall has been almost nil, and the most violent whirling of the while the Horseshoe has cut its way for shattered liquid ; here the most power- 500 yards across the end of Goat Island, ful eddies recoil against the shale. From and is now doubling back to excavate a this portion of the fall, indeed, the spray channel parallel to the length of the sometimes rises without solution of con- island. This point, I have just learned, tinuity to the region of clouds, becom- has not escaped the acute observation ing gradually more attenuated, and of Professor Ranısay. The river bends; passing finally through the condition of the Horseshoe immediately accommotrue cloud into invisible vapour, which dates itself to the bending, and will is sometimes reprecipitated higher up. follow implicitly the direction of the All the phenomena point distinctly to deepest water in the upper stream, the centre of the river as the place of The flexibility of the gorge, if I may greatest mechanical energy, and from use the term, is determined by the the centre the vigour of the Fall gra- flexibility of the river channel above dually dies away towards the sides. it. Were the Niagara above the Fall The horseshoe form, with the con- sinuous, the gorge would obediently cavity facing downwards, is an obvious follow its sinuosities.. Once suggested, and necessary consequence of this no doubt geographers will be able to action. Right along the middle of the point out many examples of this action, river the apex of the curve pushes its The Zambesi is thought to present a way backwards, cutting along the centre great difficulty to the erosion theory, a deep and comparatively narrow groove, because of the sinuosity of the chasm and draining the sides as it passes below the Victoria Falls. But assuming them. Hence the remarkable dis- the basalt to be of tolerably uniform texcrepancy between the widths of the ture, had the river been examined before Niagara above and below the Horseshoe the formation of this sinuous channel, All along its course, from Lewiston the present zigzag course of the gorge Heights to its present position, the below the Fall could, I am persuaded, form of the Fall was probably that have been predicted, while the sounding of a horseshoe ; for this is merely the of the present river would enable us to expression of the greater depth, and predict the course to be pursued by the consequently greater excavating power, erosion in the future. of the centre of the river. The gorge, But not only has the Niagara river cut moreover, varies in width as the depth the gorge; it has carried away the chips of the centre of the ancient river of its own workshop. The shale being varied, being narrowest where that probably crumbled is easily carried away. depth was greatest.

But at the base of the fall we find the huge The vast comparative erosive energy boulders already described, and by some of the Horseshoe Fall comes strikingly

1 His words are:-"Where the body of into view when it and the American

water is small in the American Fall, the edge

has only receded a few yards (where most 1 In the discourse of which this paper is a eroded) during the time that the Canadian report, the excavation of the centre and Fall has receded from the north corner of drainage of the sides was illustrated by a Goat Island to the innermost curve of the model devised by my assistant, Mr. John Horseshoe Fall.”– Quarterly Journal of GeoloCottrell.

gical Society, May 1859.

« AnteriorContinuar »