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 mass 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 XXXV. 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 masses 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 canals 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 canals 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 quantities 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 circumstances, 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 passing 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.