The Earth As Modified By Human Action Part 36

Cutting of Marine Isthmuses.

Besides the great enterprises of physical transformation of which I have already spoken, other works of internal improvement or change have been projected in ancient and modern times, the execution of which would produce considerable, and, in some cases, extremely important, revolutions in the face of the earth. Some of the schemes to which I refer are evidently chimerical; others are difficult, indeed, but cannot be said to be impracticable, though discouraged by the apprehension of disastrous consequences from the disturbance of existing natural or artificial arrangements; and there are still others, the accomplishment of which is ultimately certain, though for the present forbidden by economical considerations.

Nature sometimes mocks the cunning and the power of man by spontaneously performing, for his benefit, works which he shrinks from undertaking, and the execution of which by him she would resist with unconquerable obstinacy. A dangerous sand bank, that all the enginery of the world could not dredge out in a generation, may be carried off in a night by a strong river-flood, or by a current impelled by a violent wind from an unusual quarter, and a pa.s.sage scarcely navigable by fishing-boats may be thus converted into a commodious channel for the largest ship that floats upon the ocean. In the remarkable gulf of Liimfjord in Jutland, referred to in the preceding chapter, nature has given a singular example of a ca.n.a.l which she alternately opens as a marine strait, and, by ab.u.t.ting again, converts into a fresh-water lagoon. The Liimfjord was doubtless originally an open channel from the Atlantic to the Baltic between two islands, but the sand washed up by the sea blocked up the western entrance, and built a wall of dunes to close it more firmly.

This natural dike, as we have seen, has been more than once broken through, and it is perhaps in the power of man, either permanently to maintain the barrier, or to remove it and keep a navigable channel constantly open. If the Liimfjord becomes an open strait, the washing of sea-sand through it would perhaps block some of the belts and small channels now important for the navigation of the Baltic, and the direct introduction of a tidal current might produce very perceptible effects on the hydrography of the Cattegat.

When we consider the number of narrow necks or isthmuses which separate gulfs and bays of the sea from each other, or from the main ocean, and take into account the time and cost, and risks of navigation which would be saved by executing channels to connect such waters, and thus avoiding the necessity of doubling long capes and promontories, or even continents, it seems strange that more of the enterprise and money which have been so lavishly expended in forming artificial rivers for internal navigation should not have been bestowed upon the construction of maritime ca.n.a.ls. Many such have been projected in early and in recent ages, and some trifling cuts between marine waters had been actually made; but before the construction of the Suez Ca.n.a.l, no work of this sort, possessing real geographical or even commercial importance, had been effected.

These enterprises are attended with difficulties and open to objections which are not, at first sight, obvious. Nature guards well the chains by which she connects promontories with mainlands, and binds continents together. Isthmuses are usually composed of adamantine rock or of shifting sands--the latter being much the more refractory material to deal with. In all such works there is a necessity for deep excavation below low-water mark--always a matter of great difficulty; the dimensions of channels for sea-going ships must be much greater than those of ca.n.a.ls of inland navigation; the height of the masts or smokepipes of that cla.s.s of vessels would often render bridging impossible, and thus a ship-ca.n.a.l might obstruct a communication more important than that which it was intended to promote; the securing of the entrances of marine ca.n.a.ls and the construction of ports at their termini would in general be difficult and expensive, and the harbors and the channel which connected them would be extremely liable to fill up by deposits washed in from sea and sh.o.r.e. Besides all this there is, in many cases, an alarming uncertainty as to the effects of joining together waters which nature has put asunder. A new channel may deflect strong currents from safe courses, and thus occasion destructive erosion of sh.o.r.es otherwise secure, or promote the transportation of sand or slime to block up important harbors, or it may furnish a powerful enemy with dangerous facilities for hostile operations along the coast. The most colossal project of ca.n.a.lization ever suggested, whether we consider the physical difficulties of its execution, the magnitude and importance of the waters proposed to be united, or the distance which would be saved in navigation, is that of a channel between the Gulf of Mexico and the Pacific, across the Isthmus of Darien. I do not now speak of a lock-ca.n.a.l, by way of the Lake of Nicaragua or any other route--for such a work would not differ essentially from other ca.n.a.ls, and would scarcely possess a geographical character--but of an open cut between the two seas. The late survey by Captain Selfridge, showing that the lowest point on the dividing ridge is 763 feet above the sea-level, must be considered as determining in the negative the question of the possibility of such a cut, by any means now at the control of man; and both the sanguine expectations of benefits, and the dreary suggestions of danger, from the realization of this great dream, may now be dismissed as equally chimerical.

Suez Ca.n.a.l.

The cutting of the Isthmus of Suez--the grandest and most truly cosmopolite physical improvement ever undertaken by man--threatens none of these dangers, and its only immediate geographical effect will probably be that interchange between the aquatic animal and vegetable life of two seas and two zones to which I alluded in a former chapter.

[Footnote: According to an article by Ascherson, in Petermann's Mitthielungen, vol. xvii., p. 247, the sea-gra.s.s floras of the opposite sides of the Isthmus of Suez are as different as possible. It does not appear whether they have yet intermixed.] A collateral feature of this great enterprise deserves notice as possessing no inconsiderable geographical importance. I refer to the conduit or conduits constructed from the Nile to the isthmus, primarily to supply fresh water to the laborers on the great ca.n.a.l, and ultimately to serve as aqueducts for the city of Suez and other towns on the line of the ca.n.a.l, and for the irrigation and reclamation of a large extent of desert soil. In the flourishing days of the Egyptian empire, the waters of the Nile were carried over important districts cast of the river. In later ages, most of this territory relapsed into a desert, from the decay of the ca.n.a.ls which once fertilized it. There is no difficulty in restoring the ancient channels, or in constructing new, and thus watering not only all the soil that the wisdom of the Pharaohs had improved, but much additional land. Hundreds of square miles of arid sand waste would thus be converted into fields of perennial verdure, and the geography of Lower Egypt would be thereby sensibly changed. Considerable towns are growing up at both ends of the channel, and at intermediate points, all depending on the maintenance of aqueducts from the Nile, both for water and for the irrigation of the neighboring fields which are to supply them with bread. Important interests will thus be created, which will secure the permanence of the hydraulic works and of the geographical changes produced by them, and Suez, or Port Said, or Ismailieh, may become the capital of the government which has been so long established at Cairo. Maritime Ca.n.a.ls in Greece. A maritime ca.n.a.l executed and another projected in ancient times, the latter of which is again beginning to excite attention, deserve some notice, though their importance is of a commercial rather than a geographical character. The first of those is the cut made by Xerxes through the rock which connects the promontory of Mount Athos with the mainland; the other, a navigable ca.n.a.l through the Isthmus of Corinth. In spite of the testimony of Herodotus and Thucydides, the Romans cla.s.sed the ca.n.a.l of Xerxes among the fables of "mendacious Greece," and yet traces of it are perfectly distinct at the present day through its whole extent, except at a single point where, after it had become so choked as to be no longer navigable, it was probably filled up to facilitate communication by land between the promontory and the country in the rear of it. The emperor Nero commenced the construction of a ca.n.a.l across the Isthmus of Corinth, solely to facilitate the importation of grain from the East for distribution among the citizens of Rome--for the encouragement of general commerce was no part of the policy either of the republic or the empire, and though the avidity of traders, chiefly foreigners, secured to the luxury of the imperial city an abundant supply of far-fetched wares, yet Rome had nothing to export in return. The line of Nero's excavations is still traceable for three-quarters of a mile, or more than a fifth of the total distance between gulf and gulf.

If the fancy kingdom of Greece shall ever become a sober reality, escape from its tutelage and acquire such a moral as well as political status that its own capitalists--who now prefer to establish themselves and employ their funds anywhere else rather than in their native land--have any confidence in the permanency of its inst.i.tutions, a navigable channel may be opened between the gulfs of Lepanto and AEgina. The annexation of the Ionian Islands to Greece will make such a work almost a political necessity, and it would not only furnish valuable facilities for domestic intercourse, but become an important channel of communication between the Levant and the countries bordering on the Adriatic, or conducting their trade through that sea. SHort as is the distance, the work would be a somewhat formidable undertaking, for the lowest point of the summit ridge of the isthmus is stated to be 246 feet above the water, and consequently the depth of excavation must be not less than 275 feet. As I have said, the importance of this latter ca.n.a.l and of a navigable channel between Mount Athos and the continent would be chiefly commercial, but both of them would be conspicuous instances of the control of man over nature in a field where he has thus far done little to interfere with her spontaneous arrangements. If they were constructed upon such a scale as to admit of the free pa.s.sage of the water through them, in either direction, as the prevailing winds should impel it, they would exercise a certain influence on the coast currents, which are important as hydrographical elements, and also as producing abrasion of the coast and a drift at the bottom of seas, and hence they would be ent.i.tled to rank higher than simply as artificial means of transit. It has been thought practicable to cut a ca.n.a.l across the peninsula of Gallipoli from the outlet of the Sea of Marmora into the Gulf of Saros. It may be doubted whether the mechanical difficulties of such a work would not be found insuperable; but when Constantinople shall recover the important political and commercial rank which naturally belongs to her, the execution of such a ca.n.a.l will be recommended by strong reasons of military expediency, as well as by the interests of trade. An open channel across the peninsula would divert a portion of the water which now flows through the Dardanelles, diminishing the rapidity of that powerful current, and thus in part remove the difficulties which obstruct the navigation of the strait. It would considerably abridge the distance by water between Constantinople and the northern coast of the AEgean, and it would have the important advantage of obliging an enemy to maintain two blockading fleets instead of one.

Ca.n.a.ls Communicating with Dead Sea.

The project of Captain Allen for opening a new route to India by cuts between the Mediterranean and the Dead Sea, and between the Dead Sea and the Red Sea, presents many interesting considerations. [Footnote: The Dead Sea a new Route to India. 2 vols. 12mo, London, 1855.] The hypsometrical observations of Bertou, Roth, and others, render it highly probable, if not certain, that the watershed in the Wadi-el-Araba between the Dead Sea and the Red Sea is not less than three hundred feet above the main level of the latter, and if this is so, the execution of a ca.n.a.l from the one sea to the other is quite out of the question. But the summit level between the Mediterranean and the Jordan, near Jezreel, is believed to be little, if at all, more than one hundred feet above the sea, and the distance is so short that the cutting of a channel through the dividing ridge would probably be found by no means an impracticable undertaking. Although, therefore, we have no reason to believe it possible to open a navigable channel to India by way of the Dead Sea, there is not much doubt that the basin of the latter might be made accessible from the Mediterranean.

The level of the Dead Sea lies 1,316.7 feet below that of the ocean. It is bounded east and west by mountain ridges, rising to the height of from 2,000 to 4,000 feet above the ocean. From its southern end, a depression called the Wadi-el-Araba extends to the Gulf of Akaba, the eastern arm of the Red Sea. The Jordan empties into the northern extremity of the Dead Sea, after having pa.s.sed through the Lake of Tiberias at an elevation of 663.4 feet above the Dead Sea, or 653.3 below the Mediterranean, and drains a considerable valley north of the lake, as well as the plain of Jericho, which lies between the lake and the sea. If the waters of the Mediterranean were admitted freely into the basin of the Dead Sea, they would raise its surface to the general level of the ocean, and consequently flood all the dry land below that level within the basin. I do not know that accurate levels have been taken in the valley of the Jordan above the Lake of Tiberias, and our information is very vague as to the hypsometry of the northern part of the Wadi-el-Araba. As little do we know where a contour line, carried around the basin at the level of the Mediterranean, would strike its eastern and western borders. We cannot, therefore, accurately compute the extent of now dry land which would be covered by the admission of the waters of the Mediterranean, or the area of the inland sea which would be thus created. Its length, however, would certainly exceed one hundred and fifty miles, and its mean breadth, including its gulfs and bays, could scarcely be less than fifteen, perhaps even twenty. It would cover very little ground now occupied by civilized or even uncivilized man, though some of the soil which would be submerged--for instance, that watered by the Fountain of Elisha and other neighboring sources--is of great fertility, and, under a wiser government and better civil inst.i.tutions, might rise to importance, because, from its depression, it possesses a very warm climate, and might supply South-eastern Europe with tropical products more readily than they can be obtained from any other source. Such a ca.n.a.l and sea would be of no present commercial importance, because they would give access to no new markets or sources of supply; but when the fertile valleys and the deserted plains cast of the Jordan shall be reclaimed to agriculture and civilization, these waters would furnish a channel of communication which might become the medium of a very extensive trade. Whatever might be the economical results of the opening and filling of the Dead Sea basin, the creation of a new evaporable area, adding not less than 2,000 or perhaps 3,000 square miles to the present fluid surface of Syria, could not fail to produce important meteorological effects. The climate of Syria would probably be tempered, its precipitation and its fertility increased, the courses of its winds and the electrical condition of its atmosphere modified. The present organic life of the valley would be extinguished, and many tribes of plants and animals would emigrate from the Mediterranean to the new home which human art had prepared for them. It is possible, too, that the addition of 1,300 feet, or forty atmospheres, of hydrostatic pressure upon the bottom of the basin might disturb the equilibrium between the internal and the external forces of the crust of the earth at this point of abnormal configuration, and thus produce geological convulsions the intensity of which cannot be even conjectured.

It is now established by the observations of Rohlf and others that Strabo was right in a.s.serting that a considerable part of the Libyan desert, or Sahara, lay below the level of the Mediterranean. At some points the depression exceeds 325 feet, and at Siwah, in the oasis of Jupiter Ammon, it is not less than 130 feet. It has been proposed to cut a ca.n.a.l through the coast dunes, on the sh.o.r.e south of the Syrtis Major, or Dschnn el Kebrit of the Arabs, and another project is to reopen the communication which appears to have once existed between the Palus Tritonis, or Sebcha el Nandid, and the Syrtis Parva. As we do not know the southern or eastern limits of this depression, we cannot determine the area which would thus be covered with water, but it would certainly be many thousands of square miles in extent, and the climatic effects would doubtless be sensible through a considerable part of Northern Africa, and possibly even in Europe. The rapid evaporation would require a constant influx of water from the Mediterranean, which might perhaps perceptibly influence the current through the Straits of Gibraltar.

Maritime Ca.n.a.ls in Europe.

A great navigable cut across the peninsula of Jutland, forming a new and short route between the North Sea and the Baltic, if not actually commenced, is determined upon. The motives for opening such a communication are perhaps rather to be found in political than in geographical or even commercial considerations, but it will not be without an important bearing on the material interests of all the countries to whose peoples it will furnish new facilities for communication and traffic.

The North Holland ca.n.a.l between the Helder and the port of Amsterdam, a distance of fifty miles, executed a few years since at a cost of $5,000,000, and with dimensions admitting the pa.s.sage of a frigate, was a magnificent enterprise, but it is thrown quite into the shade by the shorter channel now in process of construction for bringing that important city into almost direct communication with the North Sea, and thus restoring to it something at least of its ancient commercial importance. The work involves some of the heaviest hydraulic operations yet undertaken, including the construction of great dams, locks, dikes, embankments, and the execution of draining works and deep cutting under circ.u.mstances of extreme difficulty. In the course of these labors many novel problems have presented themselves for practical solution by the ingenuity of modern engineers, and the now inventions and processes thus necessitated are valuable contributions to our means of physical improvement.

Cape Cod Ca.n.a.l.

The opening of a navigable cut through the narrow neck which separates the southern part of Cape Cod Bay in Ma.s.sachusetts from the Atlantic, was long ago suggested, and there are few coast improvements on the Atlantic sh.o.r.es of the United States which are recommended by higher considerations of utility. It would save the most important coasting trade of the United States the long and dangerous navigation around Cape Cod, afford a new and safer entrance to Boston harbor for vessels from Southern ports, secure a choice of pa.s.sages, thus permitting arrivals upon the coast and departures from it at periods when wind and weather might otherwise prevent them, and furnish a most valuable internal communication in case of coast blockade by a foreign power. The difficulties of the undertaking are no doubt formidable, but the expense of maintenance and the uncertainty of the effects of currents getting through the new strait are still more serious objections. [Footnote: The opening of a channel across Cape Cod would have, though perhaps to a smaller extent, the same effects in interchanging the animal life of the southern and northern sh.o.r.es of the isthmus, as in the case of the Suez Ca.n.a.l; for although the breadth of Cape Cod does not anywhere exceed twenty miles, and is in some places reduced to one, it appears from the official reports on the Natural History of Ma.s.sachusetts, that the population of the opposite waters differs widely in species.

Not having the original doc.u.ments at hand, I quote an extract from the Report on the Invertebrate Animals of Ma.s.s., given by Th.o.r.eau, Excursions, p. 69: "The distribution of the marine sh.e.l.ls is well worthy of notice as a geological fact. Cape Cod, the right arm of the Commonwealth, reaches out into the ocean some fifty or sixty miles. It is nowhere many miles wide; but this narrow point of land has. .h.i.therto proved a barrier to the migration of many species of mollusca. Several genera and numerous species, which are separated by the intervention of only a few miles of land, are effectually prevented from mingling by the Cape, and do not pa.s.s from one side to the other. ... Of the one hundred and ninety-seven marine species, eighty-three do not pa.s.s to the south sh.o.r.e, and fifty are not found on the north sh.o.r.e of the Cape."

Probably the distribution of the species of mollusks is affected by unknown local conditions, and therefore an open ca.n.a.l across the Cape might not make every species that inhabits the waters on one side common to those of the other; but there can be no doubt that there would be a considerable migration in both directions.

The fact stated in the report may suggest an important caution in drawing conclusions upon the relative age of formations from the character of their fossils. Had a geological movement or movements upheaved to different levels the bottoms of waters thus separated by a narrow isthmus, and dislocated the connection between those bottoms, naturalists, in after ages, reasoning from the character of the fossil faunas, might have a.s.signed them to different, and perhaps very widely distant, periods.]

Changes in the Caspian.

The Russian Government has contemplated the establishment of a nearly direct water communication between the Caspian Sea and the Sea of Azoff, partly by natural and partly by artificial channels, and there are now navigable ca.n.a.ls between the Don and the Volga; but these works, though not wanting in commercial and political interest, do not possess any geographical importance. It is, however, very possible to produce appreciable geographical changes in the basin of the Caspian by the diversion of the great rivers which flow from Central Russia. The surface of the Caspian is eighty-three feet below the level of the Sea of Azoff, and its depression has been explained upon the hypothesis that the evaporation exceeds the supply derived, directly and indirectly, from precipitation, though able physicists now maintain that the sinking of this sea is due to a subsidence of its bottom from geological causes.

At Tsaritsin, the Don, which empties into the Sea of Azoff, and the Volga, which pours into the Caspian, approach each other within ten miles. Near this point, by means of open or subterranean ca.n.a.ls, the Don might be turned into the Volga, or the Volga into the Don. If we suppose the whole or a large proportion of the waters of the Don to be thus diverted from their natural outlet and sent down to the Caspian, the equilibrium between the evaporation from that sea and its supply of water might be restored, or its level even raised above its ancient limits. If the Volga were turned into the Sea of Azoff, the Caspian would be reduced in dimensions until the balance between loss and gain should be re-established, and it would occupy a much smaller area than at present. Such changes in the proportion of solid and fluid surface would have some climatic effects in the territory which drains into the Caspian, and on the other hand, the introduction of a greater quant.i.ty of fresh water into the Sea of Azoff would render that gulf less saline, affect the character and numbers of its fish, and perhaps be not wholly without sensible influence on the water of the Black Sea.

Diversion of the Nile.

Perhaps the most remarkable project of great physical change, proposed or threatened in earlier ages, is that of the diversion of the Nile from its natural channel, and the turning of its current into either the Libyan Desert or the Red Sea. The Ethiopian or Abyssinian princes more than once menaced the Memlouk sultans with the execution of this alarming project, and the fear of so serious an evil is said to have induced the Moslems to conciliate the Abyssinian kings by large presents, and by some concessions to the oppressed Christians of Egypt.

Indeed, Arabian historians affirm that in the tenth century the Ethiopians dammed the river, and, for a whole year, cut off its waters from Egypt. [Footnote: "Some haue writte, that by certain kings inhabiting aboue, the Nilus should there be stopped; & at a time prefixt, let loose vpon a certaine tribute payd them by the Aegyptians.

The error springing perhaps fro a truth (as all wandring reports for the most part doe) in that the Sultan doth pay a certaine annuall summe to the Abissin Emperour for not diuerting the course of the Riuer, which (they say) he may, or impouerish it at the least."--George Sandys, A Relation of a Journey, etc., p. 98. See, also, Vansles, Voyage en Egypte, p. 61.] The probable explanation of this story is to be found in a season of extreme drought, such as have sometimes occurred in the valley of the Nile.

The Libyan Desert, above the junction of the two princ.i.p.al branches of the Nile at Khartum, is so much higher than the level of the river below that point, that there is no reason to believe a new channel for the united waters of the two streams could be found in that direction; but the Bahr-el-Abiad flows through, if it does not rise in, a great table-land, and some of its tributaries are supposed to communicate in the rainy season with branches of great rivers flowing in quite another direction. Hence it is probable that a portion at least of the waters of this great arm of the Nile--and perhaps a quant.i.ty the abstraction of which would be sensibly felt in Egypt--might be sent to the Atlantic by the Congo or Niger, lost in inland lakes and marshes in Central Africa, or employed to fertilize the Libyan sand wastes.

About the beginning of the sixteenth century, Albuquerque the "Terrible"

revived the scheme of turning the Nile into the Red Sea, with the hope of destroying the transit trade through Egypt by way of Kosseir. In 1525 the King of Portugal was requested by the Emperor of Abyssinia to send him engineers for that purpose; a successor of that prince threatened to attempt the project about the year 1700, and even as late as the French occupation of Egypt, the possibility of driving out the intruder by this means was suggested in England.

It cannot be positively affirmed that the diversion of the waters of the Nile to the Red Sea is impossible. In the chain of mountains which separates the two valleys, Brown found a deep depression or wadi, extending from the one to the other, apparently at no great elevation above the bed of the river, but the height of the summit level was not measured. Admitting the possibility of turning the whole river into the Red Sea, let us consider the probable effect of the change.

First and most obvious is the total destruction of the fertility of Middle and Lower Egypt, the conversion of that part of the valley into a desert, and the extinction of its imperfect civilization, if not the absolute extirpation of its inhabitants. This is the calamity threatened by the Abyssinian princes and the ferocious Portuguese warrior, and feared by the Sultans of Egypt. Beyond these immediate and palpable consequences neither party then looked; but a far wider geographical area, and far more extensive and various human interests, would be affected by the measure. The spread of the Nile during the annual inundation covers, for many weeks, several thousand square miles with water, and at other seasons of the year pervades the same and even a larger area with moisture by infiltration. The abstraction of so large an evaporating surface from the southern sh.o.r.es of the Mediterranean could not but produce important effects on many meteorological phenomena, and the humidity, the temperature, the electrical condition and the atmospheric currents of North-eastern Africa might be modified to a degree that would sensibly affect the climate of Europe.

The Mediterranean, deprived of the contributions of the Nile, would require a larger supply, and of course a stronger current, of water from the Atlantic through the Straits of Gibraltar; the proportion of salt it contains would be increased, and the animal life of at least its southern borders would be consequently modified; the current which winds along its southern, eastern, and north-eastern sh.o.r.es would be diminished in force and volume, if not destroyed altogether, and its basin and its harbors would be shoaled by no new deposits from the highlands of inner Africa.

In the much smaller Red Sea, more immediately perceptible, if not greater, effects, would be produced. The deposits of slime would reduce its depth, and perhaps, in the course of ages, divide it into an inland and an open sea, the former of which, receiving no supply from rivers, would, as in the case of the northern part of the Gulf of California, soon be dried up by evaporation, and its whole area added to the Africo-Arabian desert; the waters of the latter would be more or less freshened, and their immensely rich marine fauna and flora changed in character and proportion, and, near the mouth of the river, perhaps even destroyed altogether; its navigable channels would be altered in position and often quite obstructed; the flow of its tides would be modified by the new geographical conditions; the sediment of the river would form new coast-lines and lowlands, which would be covered with vegetation, and probably thereby produce sensible climatic changes.

Diversion of the Rhine.

The interference of physical improvements with vested rights and ancient arrangements, is a more formidable obstacle in old countries than in new, to enterprises involving anything approaching to a geographical revolution. Hence such projects meet with stronger opposition in Europe than in America, and the number of probable changes in the face of nature in the former continent is proportionally less. I have noticed some important hydraulic improvements as already executed or in progress in Europe, and I may refer to some others as contemplated or suggested.

One of these is the diversion of the Rhine from its present channel below Ragatz, by a cut through the narrow ridge near Sargans, and the consequent turning of its current into the Lake of Wallenstadt. This would be an extremely easy undertaking, for the ridge is but twenty feet above the level of the Rhine, and hardly two hundred yards wide. There is no present adequate motive for this diversion, but it is easy to suppose that it may become advisable within no long period. The navigation of the Lake of Constance is rapidly increasing in importance, and the shoaling of the eastern end of that lake by the deposits of the Rhine may require a remedy which can be found by no other so ready means as the discharge of that river into the Lake of Wallenstadt. The navigation of this latter lake is not important, nor is it ever likely to become so, because the rocky and precipitous character of its sh.o.r.es renders their cultivation impossible. It is of great depth, and its basin is capacious enough to receive and retain all the sediment which the Rhine would carry into it for thousands of years. [Footnote: Many geographers suppose that the dividing ridge between the Lake of Wallenstadt and the bed of the Rhine at Sargans is a fluviatile deposit, which has closed a channel through which the Rhine anciently discharged a part or the whole of its waters into the lake. In the flood of 1868, the water of the Rhine rose to the level of the railway station at Sargans, and for some days there was fear of the giving way of the barrier and the diversion of the current of the river into the lake.]

Improvements in North American Hydrography.

We are not yet well enough acquainted with the geography of Central Africa, or of the interior of South America, to conjecture what hydrographical revolutions might there be wrought; but from the fact that many important rivers in both continents drain extensive table-lands, of moderate elevation and inclination, there is reason to suppose that important changes in the course of those rivers might be accomplished. Our knowledge of the drainage of North America is much more complete, and it is certain that there are numerous points within our territory where the courses of great rivers, or the discharge of considerable lakes, might be completely diverted, or at least partially directed into different channels.

The surface of Lake Erie is 565 feet above that of the Hudson at Albany, and it is so near the level of the great plain lying east of it, that it was found practicable to supply the western section of the ca.n.a.l, which unites it with the Hudson, with water from the lake, or rather from the Niagara which flows out of it. The greatest depth of water yet sounded in Lake Erie is but two hundred and seventy feet, the mean depth one hundred and twenty. Open ca.n.a.ls parallel with the Niagara, or directly towards the Genesee, might be executed upon a scale which would exercise an important influence on the drainage of the lake, if there were any adequate motive for such an undertaking. Still easier would it be to enlarge the outlet for the waters of Lake Superior at the Saut St.

Mary--where the river which drains the lake descends twenty-two feet in a single mile--and thus to produce incalculable effects, both upon that lake and upon the great chain of inland waters which communicate with it.

The summit level between the surface of Lake Michigan at its mean height and that of the River Des Plaines, a tributary of the Illinois, at a point some ten miles west of Chicago, is but ten and a half feet above the lake. The lake once discharged a part or the whole of its waters into the valley of the Des Plaines. A slight upheaval, at an unknown period, elevated the bed of the Des Plaines, and the prairie between it and the lake, to their present level, and the outflow of the lake was turned into a new direction. The bed of the Des Plaines is higher than the surface of the lake, and in recent times the Des Plaines, when at flood, has sent more or less of its waters across the ridge into the bed of the South Branch of Chicago River, and so into Lake Michigan.

A navigable channel has now been cut, admitting a constant flow of water from the lake, by the valley of the Des Plaines, into the Illinois. The mean discharge by this channel does not much exceed 23,000 cubic feet per minute, but it would be quite practicable to enlarge its cross-section indefinitely, and the flow through it might be so regulated as to keep the Illinois and the Mississippi at flood at all seasons of the year. The increase in the volume of these rivers would augment their velocity and their transporting power, and, consequently, the erosion of their banks and the deposit of slime in the Gulf of Mexico, while the opening of a communication between the lake and the affluents of the Mississippi, un.o.bstructed except by locks, and the introduction of a large body of colder water into the latter, would very probably produce a considerable effect on the animal life that peoples them. The diversion of water from the common basin of the great lakes through a new channel, in a direction opposite to their present discharge, would not be absolutely without influence on the St.

Lawrence, though probably this effect might be too small to be readily perceptible. [Footnote: From Reports of the Ca.n.a.l Commissioners of the State of Illinois, and especially from a very interesting private letter from William Gooding, Esq., an eminent engineer, which I regret I have not s.p.a.ce to print in full, I learn that the length of the present ca.n.a.l, from the lake to the River Illinois, is 101 miles, with a total descent of a trifle more than 145 feet, and that it is proposed to enlarge this channel to the width of one hundred and sixty feet, with a minimum depth of seven, and to create a slack-water navigation in the Illinois by the construction of five dams, one of which is already completed. The descent for the outlet of the ca.n.a.l at La Salle on the Illinois to the Mississippi is twenty-eight feet, the distance being 230 miles. The ca.n.a.l thus enlarged would cost about $16,000,000, and it would establish a navigation for vessels of 1,200 to 1,500 tons burden between Lake Michigan and the Mississippi, and consequently, by means of the great lakes and the Welland ca.n.a.l, between the St. Lawrence and the Gulf of Mexico.]

In an able and interesting article in a California magazine, Dr. Widney has suggested a probable cause and a possible remedy for the desiccation of south-eastern California referred to in a former chapter. The Colorado Desert which lies considerably below the level of the waters of the Gulf of California, and has an area of about 4,000 square miles, evidently once formed a part of that gulf. This northern extension of the gulf appears to have been cut off from the main body by deposits brought down by the great river Colorado, at no very distant period.

These deposits at the same time turned the course of the river to the south, and it now enters the gulf at a point twenty miles distant from its original outlet.

When this northern arm of the gulf was cut off from the sea, and the river which once discharged itself into it was diverted, it was speedily laid dry by evaporation, and now yields no vapor to be condensed into fog, rain, and snow on the neighboring mountains, which are now parched and almost bare of vegetation.