The ancient bed of the river may still be traced, and in floods the Colorado still sends a part of its overflowing supply into its old channel, and for a time waters a portion of the desert. It is believed that the river might easily be turned back into its original course, and indeed nature herself seems to be now tending, by various spontaneous processes, to accomplish that object. The waters of the Colorado, though perhaps not sufficient to fill the basin and keep it at the sea-level in spite of the rapid evaporation in that climate, [Footnote: The thermometer sometimes rises to 120 degrees F. at Fort Yuma, at the S. E.

angle of California in N. L. 33 degrees.] would at least create a permanent lake in the lower part of the depression, the evaporation from which, Dr. Widney suggests, might sensibly increase the humidity and lower the temperature of an extensive region which is now an arid and desolate wilderness.

Soil below Rock.

One of the most singular changes of natural surface effected by man is that observed by Beechey and by Barth at Lin Tefla, and near Gebel Genunes, in the district of Ben Gasi, in Northern Africa. In this region the superficial stratum originally consisted of a thin sheet of rock covering a layer of fertile earth. This rock has been broken up, and, when not practicable to find use for it in fences, fortresses, or dwellings, heaped together in high piles, and the soil, thus bared of its stony sh.e.l.l, has been employed for agricultural purposes. [Footnote: Barth, Wanderungen durch die Kusten des Mittelmeeres, i., p. 853. In a note on page 380, of the same volume, Barth cites Strabo as a.s.serting that a similar practice prevailed in Iapygia; but the epithet [word in Greek: traxeia], applied by Strabo to the original surface, does not neceasarily imply that it was covered with a continuous stratum of rock.] If we remember that gunpowder was unknown at the period when these remarkable improvements were executed, and of course that the rock could have been broken only with the chisel and wedge, we must infer that land had at that time a very great pecuniary value, and, of course, that the province, though now exhausted, and almost entirely deserted by man, had once a dense population.

Covering Rock with Earth.

If man has, in some cases, broken up rock to reach productive ground beneath, he has, in many other instances, covered bare ledges, and sometimes extensive surfaces of solid stone, with fruitful earth, brought from no inconsiderable distance. Not to speak of the Campo Santo at Pisa, filled, or at least coated, with earth from the Holy Land, for quite a different purpose, it is affirmed that the garden of the monastery of St. Catherine at Mount Sinai is composed of Nile mud, transported on the backs of camels from the banks of that river. Parthey and older authors state that all the productive soil of the Island of Malta was brought over from Sicily. [Footnote: Parthey, Wanderungen durch Sicilen und die Levante, i., p. 404.] The accuracy of the information may be questioned in both cases, but similar practices, on a smaller scale, are matter of daily observation in many parts of Southern Europe.

Much of the wine of the Moselle is derived from grapes grown on earth carried high up the cliffs on the shoulders of men, and the steep terraced slopes of the Island of Teneriffe are covered with soil painfully scooped out from fissures in and between the rocks which have been laid bare by the destruction of the native forests. [Footnote: Mantegazza, Rio de la Plata e Teneriffa, p. 567.] In China, too, rock has been artificially covered with earth to an extent which gives such operations a real geographical importance, and the accounts of the importation of earth at Malta, and the fertilization of the rocks on Mount Sinai with slime from the Nile, may be not wholly without foundation.

Valleys in Deserts.

In the latter case, indeed, river sediment might be very useful as a manure, but it could hardly be needed as a soil; for the growth of vegetation in the wadies of the Sinaitic Peninsula shows that the disintegrated rock of its mountains requires only water to stimulate it to considerable productiveness. The wadies present, not unfrequently, narrow gorges, which might easily be closed, and thus acc.u.mulations of earth, and reservoirs of water to irrigate it, might be formed which would convert many a square mile of desert into flourishing date gardens and cornfields. For example, not far from Wadi Feiran, on the most direct route to Wadi Esh-Sheikh, is a very narrow pa.s.s called by the Arabs El Bueb (El Bab) or, The Gate, which might be securely closed to a very considerable height, with little labor or expense. Above this pa.s.s is a wide and nearly level expanse, filled up to a certain regular level with deposits brought down by torrents before the Gate, or Bueb, was broken through, and they have now worn down a channel in the deposits to the bed of the wadi. If a dam were constructed at the pa.s.s, and reservoirs built to retain the winter rains, a great extent of valley might be rendered cultivable.

Effects of Mining.

The excavations made by man, for mining and other purposes, may occasion disturbance of the surface by the subsidence of the strata above them, as in the case of the mine of Fahlun, in Sweden, but such accidents have generally been too inconsiderable in extent to deserve notice in a geographical point of view. [Footnote: In March, 1873, the imprudent extension of the excavations in a slate mine near Morzine, in Savoy, occasioned the fall of a ma.s.s of rock measuring more than 700,000 yards in cubical contents. A forest of firs was destroyed, and a hamlet of twelve houses crushed and buried by the slide.] It is said, however, that in many places in the mining regions of England alarming indications of a tendency to a wide dislocation of the superficial strata have manifested themselves. Indeed, when we consider the measure of the underground cavities which miners have excavated, we cannot but be surprised that grave catastrophes have not often resulted from the removal of the foundations on which the crust of our earth is laid. The 100,000,000 tons of coal yearly extracted from British mines require the withdrawal of subterranean strata equal to an area of 20,000 acres one yard deep, or 2,000 acres ten yards deep. These excavations have gone on for several years at this rate, and in smaller proportions for centuries. Hence, it cannot be doubted that by these and other like operations the earth has been undermined and honey-combed in many countries to an extent that may well excite serious apprehensions as to the stability of the surface. In any event such excavations may interfere materially with the course of subterranean waters, and it has even been conjectured that the removal of large bodies of metallic ore from their original deposits might, at least locally, affect in a sensible degree the magnetic and electrical condition of the earth's crust. [Footnote: The exhaustion of the more accessible deposits of coal and other minerals has compelled the miners in Belgium, England, and other countries, to carry their operations to great depths below the surface. At the colliery Des Viviers, at Cilly near Charleroi, in Belgium, coal is worked at the depth of 2,820 feet, and one pit has been sunk to the depth of 3,411 feet. It is supposed that the internal heat of the earth will render mining impossible below 4,000 feet. At Clifford Amalgamated Mines, in Cornwall, the temperature at 1,590 feet stood at 100 degrees, but after the shaft had remained a year open it fell to 83 degrees. In another Cornish mine men work at from 110 degrees to 120 degrees, but only twenty minutes at a time, and with cold water thrown frequently over them.--The last Thirty Years in Mining Districts, p. 95.

Stopponi mentions an abandoned mine at Huttenberg, in Bohemia, of the depth of 3,775 feet.--Corso di Geologia, i., p. 258.]

Hydraulic Mining.

What is called hydraulic mining--a system substantially identical with that described in an interesting way by Pliny the elder, in Book x.x.xV.

of his Natural History, as practised in his time in the gold mines of Spain [Footnote: I have little doubt that the hydraulic mining in Gaul, alluded to by Diodorus Siculus, Bibliotheca Historica, v. 27, as merely a mode of utilizing the effects of water flowing in its natural channels, was really the artificial method described by Pliny.]--is producing important geographical effects in California. Artificially directed currents of water have been long employed for washing down and removing ma.s.ses of earth, but in the Californian mining the process is resorted to on a vastly greater scale than in any other modern engineering operations, and with results proportioned to the means.

Brooks of considerable volume are diverted from their natural channels and conducted to great distances in ca.n.a.ls or wooden aqueducts, [Footnote: In 1867 there were 6,000 miles (including branches) of artificial water-courses employed for mining purposes in California.

The flumes of these ca.n.a.ls are often of sheet-iron, and in some places are carried considerable distances at a height of 250 feet above the ground.--Raymond, Mineral Statistics west of the Rocky Mountains, 1870, p. 476.] and then directed against hills and large level surfaces of ground which it is necessary to remove to reach the gold-bearing strata, or which themselves contain deposits of the precious mineral. [Footnote: The water is sometimes driven through iron tubes under a hydrostatic pressure of several hundred feet, with a force which cuts away rock of considerable solidity almost as easily as hard earth. In this way of using water, the cutting force might, doubtless, be greatly augmented by introducing sand or gravel into the current.] Naked hills and fertile soils are alike washed away by the artificial torrent, and the material removed--vegetable mould, sand, gravel, pebbles--is carried down by the current and often spread over ground lying quite out of the reach of natural inundations, and burying it to the depth sometimes of twenty-five feet. An orchard valued at $60,000, and another estimated at not less than $200,000, are stated to have been thus sacrificed, and a report from the Agricultural Bureau at Washington computes the annual damage done by this mode of mining at the incredible sum of $12,000,000.

Accidental fires in mines of coal or lignite sometimes lead to consequences not only destructive to large quant.i.ties of valuable material, but which may, directly or indirectly, produce results important in geography. The coal is occasionally ignited by the miners'

lights or other fires used by them, and certain kinds of this mineral, if long exposed to air in deserted galleries, may be spontaneously kindled. Under favorable circ.u.mstances, a stratum of coal will burn until it is exhausted, and a cavity may be burnt out in a few months which human labor could not excavate in many years. Wittwer informs us that a coal mine at St. Etienne in Dauphiny has been burning ever since the fourteenth century, and that a mine near Duttweiler, another near Epterode, and a third at Zwickau, have been on fire for two hundred years. Such conflagrations not only produce cavities in the earth, but communicate a perceptible degree of heat to the surface, and the author just quoted cites cases where this heat has ben advantageously employed in forcing vegetation.

Projects of Agricultural Improvements by Duponchel.

Duponchel's schemes of agricultural improvement are so grandiose in their nature, so vast in their sphere of operation, and so important in their possible effects upon immense tracts of the earth's surface, that they must be considered as projects of geographical revolution, and they therefore merit more than a pa.s.sing notice. In a memoir already quoted, and in a later work, [Footnote: Traite d'Hydraulique et de Geologie Agricole, 1868.] this engineer proposes to construct artificial torrents for the purpose of grinding up calcareous rock, by rolling and attrition along their beds, and thus reducing it into a fine slime; and at the same time these torrents are to transport an argillaceous deposit which is to be mingled with the calcareous slime, and distributed over the Landes by watercourses constructed for the purpose. By this means, he supposes that a very fertile soil may be formed, and so graded in depositing as to secure for it a good drainage.

In order that nothing may be wanting to recommend the project, Duponchel suggests that, as some rivers of Western France are gold-bearing, it is probable that gold enough may be collected by washing the sands to reduce materially the expense of such operations.

In the Landes of Gascony alone, he believes that 3,000,000 acres, now barren, might be made productive at a moderate expense, and that similar methods might be advantageously employed in France over an extent of not less than 30,000,000 acres now almost wholly valueless.

The successful execution of the plan would increase the fertile territory of France by an area of four or five times the extent of Sicily or of Sardinia.

There seems to be no reason why the same method, applied for such different purposes, should necessarily be destructive in the one case while it is so advantageous in the other. A wiser economy might bring about a harmony of action between the miners and the agriculturists of California, and the soil which is removed by the former as an inc.u.mbrance, judiciously deposited, might become for the latter a source of wealth more solid and enduring than the gold now obtained by such a sacrifice of agricultural interests.

Action of Man on the Weather.

Espy's well-known suggestion of the possibility of causing rain artificially, by kindling great fires, is not likely to be turned to practical account, but the speculations of this able meteorologist are not, for that reason, to be rejected as worthless. His labors exhibit great industry in the collection of facts, much ingenuity in dealing with them, remarkable insight into the laws of nature, and a ready perception of a.n.a.logies and relations not obvious to minds less philosophically const.i.tuted. They have unquestionably contributed essentially to the advancement of meteorological science.

The possibility that the distribution and action of electricity may be considerably modified by long lines of iron railways and telegraph wires, is a kindred thought, and in fact rests much on the same foundation as the belief in the utility of lightning-rods, but such influence is too obscure and too uncertain to have been yet demonstrated, though many intelligent observers believe that sensible meteorological effects have been produced by it.

It is affirmed that battles and heavy cannonades are generally followed by rain and thunder-storms, and Powers has collected much evidence on this subject, [Footnote: War and the Weather, or the Artificial Production of Rain, Chicago, 1871. Paifer proposed, as early as 1814, arrangements for producing rain by firing cannon and exploding sh.e.l.ls in the air. Ein wunderbarer Traum die Frucht, barkeit durch willkurlichen Regen zu befordern, Metz, 1814. See, on the question of the possibility of influencing the weather by artificial means, London Quarterly Journal of Science, xxix., p. 126, and Nature, Feb. 16, 1871, p. 306.] but the proposition does not seem to be by any means established.

Resistance to Great Natural Forces.

I have often spoken of the greater and more subtile natural forces, and especially of geological agencies, as powers beyond human guidance or resistance. This is no doubt at present true in the main, but man has shown that he is not altogether impotent to struggle with even these mighty servants of nature, and his unconscious as well as his deliberate action may in some cases have increased or diminished the intensity of their energies. It is a very ancient belief that earthquakes are more destructive in districts where the crust of the earth is solid and h.o.m.ogeneous, than where it is of a looser and more interrupted structure. Aristotle, Pliny the elder, and Seneca believed that not only natural ravines and caves, but quarries, wells, and other human excavations, which break the continuity of the terrestrial strata and facilitate the escape of elastic vapors, have a sensible influence in diminishing the violence and preventing the propagation of the earth-waves. In all countries subject to earthquakes this opinion is still maintained, and it is a.s.serted that, both in ancient and in modern times, buildings protected by deep wells under or near them have suffered less from earthquakes than those the architects of which have neglected this precaution. [Footnote: Landgrebe, Geschichte der Vulkane, ii., pp. 19, 20.]

If the commonly received theory of the cause of earthquakes is true--that, namely, which ascribes them to the elastic force of gases acc.u.mulated or generated in subterranean reservoirs--it is evident that open channels of communication between such reservoirs and the atmosphere might serve as a harmless discharge of gases that would otherwise acquire destructive energy. The doubt is whether artificial excavations can be carried deep enough to reach the laboratory where the elastic fluids are distilled. There are, in many places, small natural crevices through which such fluids escape, and the source of them sometimes lies at so moderate a depth that they pervade the superficial soil and, as it were, transpire from it, over a considerable area. When the borer of an ordinary artesian well strikes into a cavity in the earth, imprisoned air often rushes out with great violence, and this has been still more frequently observed, in sinking mineral-oil wells. In this latter case, the discharge of a vehement current of inflammable fluid sometimes continues for hours and even longer periods. These facts seem to render it not wholly improbable that the popular belief of the efficacy of deep wells in mitigating the violence of earthquakes is well founded.

In general, light, wooden buildings are less injured by earthquakes than more solid structures of stone or brick, and it is commonly supposed that the power put forth by the earth-wave is too great to be resisted by any amount of weight or solidity of ma.s.s that man can pile up upon the surface. But the fact that in countries subject to earthquakes many very large and strongly constructed palaces, temples, and other monuments have stood for centuries, comparatively uninjured, suggests a doubt whether this opinion is sound. The earthquake of the first of November, 1755, which is a.s.serted, though upon doubtful evidence, to have been felt over a twelfth part of the earth's surface, was among the most violent of which we have any clear and distinct account, and it seems to have exerted its most destructive force at Lisbon. It has often been noticed as a remarkable fact, that the mint, a building of great solidity, was almost wholly unaffected by the shock which shattered every house and church in the city, and its escape from the common ruin can hardly be accounted for except upon the supposition that its weight, compactness, and strength of material enabled it to resist an agitation of the earth which overthrew all weaker structures. On the other hand, a stone pier in the harbor of Lisbon, on which thousands of people had taken refuge, sank with its foundations to a great depth during the same earthquake; and it is plain that where subterranean cavities exist, at moderate depths, the erection of heavy ma.s.ses upon them would tend to promote the breaking down of the strata which roof them over.

No physicist, I believe, has supposed that man can avert the eruption of a volcano or diminish the quant.i.ty of melted rock which it pours out of the bowels of the earth; but it is not always impossible to divert the course of even a large current of lava. "The smaller streams of lava near Catania," says Ferrara, in describing the great eruption of 1669, "were turned from their course by building dry walls of stone as a barrier against them. ... It was proposed to divert the main current from Catania, and fifty men, protected by hides, were sent with hooks and iron bars to break the flank of the stream near Belpa.s.so. [Footnote: Soon after the current issues from the volcano, it is covered above and at its sides, and finally in front, with scoriae, formed by the cooling of the exposed surface, which bury and conceal the fluid ma.s.s. The stream rolls on under the coating, and between the walls of scoriae, and it was the lateral crust which was broken through by the workmen mentioned in the text.

The distance to which lava flows, before its surface begns to solidify, depends on its volume, its composition, its temperature and that of the air, the force with which it is ejected, and the inclination of the declivity over which it runs. In most cases it is difficult to approach the current at points where it is still entirely fluid, and hence opportunities of observing it in that condition are not very frequent.

In the eruption of February, 1850, on the east side of Vesuvius, I went quite up to one of the outlets. The lava shot out of the orifice upwards with great velocity, like the water from a fountain, in a stream eight or ten feet in diameter, throwing up occasionally volcanic bombs three or four feet in diameter, which exploded at the height of eight or ten yards, but it immediately spread out on the declivity down which it flowed, to the width of several yards. It continued red-hot in broad daylight, and without a particle of scoriae on its surface, for a course of at least one hundred yards. At this distance, the suffocating, sulphurous vapors became so dense that I could follow the current no farther. The undulations of the surface were like those of a brook swollen by rain. I estimated the height of the waves at five or six inches by a breadth of eighteen or twenty. To the eye, the fluidity of the lava seemed as perfect as that of water, but ma.s.ses of cold lava weighing ten or fifteen pounds floated upon it like cork.

The heat emitted by lava currents seems extremely small when we consider the temperature required to fuse such materials and the great length of time they take in cooling. I saw at Nicolosi ancient oil-jars, holding a hundred gallons or more, which had been dug out from under a stream of old lava above that town. They had been very slightly covered with volcanic ashes before the lava flowed over them, but the lead with which holes in them had been plugged was not melted. The current that buried Mompiliere in 1669 was thirty-five feet thick, but marble statues, in a church over which the lava formed an arch, were found uncalcined and uninjured in 1704, See Scrope, Volcanoes, chap. vi. Section 6.]

When the opening was made, fluid lava poured forth and flowed rapidly towards Paterno; but the inhabitants of that place, not caring to sacrifice their own town to save Catania, rushed out in arms and put a stop to the operation." [Footnote: Ferrara, Descrizione dell' Etna, p.

108.] In the eruption of Vesuvius in 1794, the viceroy saved from impending destruction the town of Portici, and the valuable collection of antiquities then deposited there but since removed to Naples, by employing several thousand men to dig a ditch above the town, by which the lava current was carried off in another direction. [Footnote: Landgrebe, Naturgeschichte der Vulkane, ii., p. 82.]

Incidental Effects of Human Action.

I have more than once alluded to the collateral and unsought consequences of human action as being often more momentous than the direct and desired results. There are cases where such incidental, or, in popular speech, accidental, consequences, though of minor importance in themselves, serve to ill.u.s.trate natural processes; others, where, by the magnitude and character of the material traces they leave behind them, they prove that man, in primary or in more advanced stages of social life, must have occupied particular districts for a longer period than has been supposed by popular chronology. "On the coast of Jutland,"

says Forchhammer, "wherever a bolt from a wreck or any other fragment of iron is deposited in the beach sand, the particles are cemented together, and form a very solid ma.s.s around the iron. A remarkable formation of this sort was observed a few years ago in constructing the sea-wall of the harbor of Elsineur. This stratum, which seldom exceeded a foot in thickness, rested upon common beach sand, and was found at various depths, less near the sh.o.r.e, greater at some distance from it.

It was composed of pebbles and sand, and contained a great quant.i.ty of pins, and some coins of the reign of Christian IV., between the beginning and the middle of the seventeenth century. Here and there, a coating of metallic copper had been deposited by galvanic action, and the presence of completely oxydized metallic iron was often detected.

Investigation made it in the highest degree probable that this formation owed its origin to the street sweepings of the town, which had been thrown upon the beach, and carried off and distributed by the waves over the bottom of the harbor." [Footnote: Geognostische Studien am Meeres Ufer, Leonhard und Bronn, 1841, pp. 25, 26.] These and other familiar observations of the like sort show that a sandstone reef, of no inconsiderable magnitude, might originate from the stranding of a ship with a cargo of iron, [Footnote: Kohl, Schleswig-Holstein, ii., p. 45.]

or from throwing the waste of an establishment for working metals into running water which might carry it to the sea.

Parthey records a singular instance of unforeseen mischief from an interference with the arrangements of nature. A landowner at Malta possessed a rocky plateau sloping gradually towards the sea, and terminating in a precipice forty or fifty feet high, through natural openings in which the sea-water flowed into a large cave under the rock.

The proprietor attempted to establish salt-works on the surface, and cut shallow pools in the rock for the evaporation of the water. In order to fill the salt-pans more readily, he sank a well down to the cave beneath, through which he drew up water by a windla.s.s and buckets. The speculation proved a failure, because the water filtered through the porous bottom of the pans, leaving little salt behind. But this was a small evil, compared with other destructive consequences that followed.

When the sea was driven into the cave by violent west or north-west winds, it shot a jet d'eau through the well to the height of sixty feet, the spray of which was scattered far and wide over the neighboring gardens and blasted the crops. The well was now closed with stones, but the next winter's storms hurled them out again, and spread the salt spray over the grounds in the vicinity as before. Repeated attempts were made to stop the orifice, but at the time of Parthey's visit the sea had thrice burst through, and it was feared that the evil was without remedy. [Footnote: Wanderungen durch Sicilien und die Levante, i., p.

406.]

I have mentioned the great extent of the heaps of oyster and other sh.e.l.ls left by the American Indians on the Atlantic coast of the United States. Some of the Danish kitchen-middens, which closely resemble them, are a thousand feet long, from one hundred and fifty to two hundred wide, and from six to ten high. These piles have an importance as geological witnesses, independent of their bearing upon human history.

Wherever the coast line appears, from other evidence, to have remained unchanged in outline and elevation since they were acc.u.mulated, they are found near the sea, and not more than about ten feet above its level. In some cases they are at a considerable distance from the beach, and in these instances, so far as yet examined, there are proofs that the coast has advanced in consequence of upheaval or of fluviatile or marine deposit. Where they are altogether wanting, the coast seems to have sunk or been washed away by the sea. The constancy of these observations justifies geologists in arguing, where other evidence is wanting, the advance of land or sea respectively, or the elevation or depression of the former, from the position or the absence of these heaps alone.

Every traveller in Italy is familiar with Monte Testaccio, the mountain of potsherds, at Rome; [Footnote: Untill recently this hillock was supposed to consist of shards of household pottery broken in using, but it now appears to be ascertained that it is composed of fragments of earthenware broken in transportation from the place of manufacture to the emporium on the Tiber where such articles were landed.] but this deposit, large as it is, shrinks into insignificance when compared with ma.s.ses of similar origin in the neighborhood of older cities. The castaway pottery of ancient towns in Magna Grecia composes strata of such extent and thickness that they have been dignified with the appellation of the ceramic formation. The Nile, as it slowly changes its bed, exposes in its banks ma.s.ses of the same material, so vast that the population of the world during the whole historical period would seem to have chosen this valley as a general deposit for its broken vessels.

The fertility imparted to the banks of the Nile by the water and the slime of the inundations, is such that manures are little employed.

Hence much domestic waste, which would elsewhere be employed to enrich the soil, is thrown out into vacant places near the town. Hills of rubbish are thus piled up which astonish the traveller almost as much as the solid pyramids themselves. The heaps of ashes and other household refuse collected on the borders and within the limits of Cairo were so large, that the removal of them by Ibrahim Pacha has been looked upon as one of the great works of the age.