The Earth as Modified by Human Action

by George P. Marsh

To the geological effects of the thickening of the earth's crust in the Bay of Bengal, are to be added those of thinning it on the highlands where the Ganges rises. The same action may, as a learned friend suggests to me, even have a cosmical influence. The great rivers of the earth, taken as a whole, transport sediment from the polar regions in an equatorial direction, and hence tend to increase the equatorial diameter, and at the same time, by their inequality of action, to a continual displacement of the centre of gravity, of the earth. The motion of the globe, and of all bodies affected by its attraction, is modified by every change of its form, and in this case we are not authorized to say that such effects are in any way compensated.]

There are also cases where similar apparent effects are produced by local oceanic currents, by river deposit or erosion, by tidal action, or by the influence of the wind upon the waves and the sands of the seabeach. A regular current may drift suspended earth and seaweed along a coast until they are caught by an eddy and finally deposited out of the reach of further disturbance, or it may scoop out the bed of the sea and undermine promontories and headlands; a powerful river, as the wind changes the direction of its flow at its outlet, may wash away shores and sandbanks at one point to deposit their material at another; the tide or waves, stirred to unusual depths by the wind, may gradually wear down the line of coast, or they may form shoals and coast-dunes by depositing the sand they have rolled up from the bottom of the ocean. These latter modes of action are slow in producing effects sufficiently important to be noticed in general geography, or even to be visible in the representations of coast-line laid down in ordinary maps; but they nevertheless form conspicuous features in local topography, and they are attended with consequences of great moment to the material and the moral interests of men. The forces which produce these limited results are all in a considerable degree subject to control, or rather to direction and resistance, by human power, and it is in guiding, combating, and compensating them that man has achieved some of his most remarkable and most honorable conquests over nature. The triumphs in question, or what we generally call harbor and coast improvements, whether we estimate their value by the money and labor expended upon them, or by their bearing upon the interests of commerce and the arts of civilization, must take a very high rank among the great works of man, and they are fast assuming a magnitude greatly exceeding their former relative importance.

The extension of commerce and of the military marine, and especially the introduction of vessels of increased burden and deeper draught of water, have imposed upon engineers tasks of a character which a century ago would have been pronounced, and, in fact, would have been, impracticable; but necessity has stimulated au ingenuity which has contrived means of executing them, and which gives promise of yet greater performance in time to come.

Indeed, although man, detached from the solid earth, is almost powerless to struggle against the sea, he is fast becoming invincible by it so long as his foot is planted on the shore, or even on the bottom of the rolling ocean; and though on some battle-fields between the waters and the land he is obliged slowly to yield his ground, yet he retreats still facing the foe, and will finally be able to say to the sea, "Thus far shalt thou come and no farther, and here shall thy proud waves be stayed!" [Footnote: It is, nevertheless, remarkable that in the particular branch of coast engineering where great improvements are most urgently needed, comparatively little has been accomplished. I refer to the creation of artificial harbors, and of facilities for loading and discharging ships. The whole coast of Italy is, one may almost say, harborless and even, wharfless, and there are many thousands of miles of coast in rich commercial countries in Europe, where vessels can neither lie in safety for a single day, nor even, in better protected heavens, ship or land their passengers or cargoes except by the help of lighters, and other not less clumsy contrivances. It is strange that such enormous inconveniences are borne with so little effort to remove them, and especially that break-waters are rarely constructed by Governments except for the benefit of the military marine.]

Great Works of Material Improvement.

Men have ceased to admire the vain exercise of power which heaped up the great pyramid to gratify the pride of a despot with a giant sepulchre; for many great harbors, many important lines of internal communication, in the civilized world, now exhibit works which in volume and weight of material surpass the vastest remains of ancient architectural art, and demand the exercise of far greater constructive skill and involve a much heavier pecuniary expenditure than would now be required for the building of the tomb of Cheops. It is computed that the great pyramid, the solid contents of which when complete were about 3,000,000 cubic yards, could be erected for a million of pounds sterling. The breakwater at Cherbourg, founded in rough water sixty feet deep, at an average distance of more than two miles from the shore, contains double the mass of the pyramid, and many a comparatively unimportant canal has been constructed at twice the cost which would now build that stupendous monument.

The description of works of harbor and coast improvement which have only an economical value, not a true geographical importance, does not come within the plan of the present volume, and in treating this branch of my subject, I shall confine myself to such as are designed either to gain new soil by excluding the waters from grounds which they had permanently or occasionally covered, or to resist new encroachments of the sea upon the land. [Footnote: Some notice of great works executed by man in foreign lands, and probably not generally familiar to my readers, may, however, prove not uninteresting.

The desaguadero, or canal constructed by the Viceroy Revillagigedo to prevent the inundation of the city of Mexico by the lakes in its vicinity, besides subsidiary works of great extent, has a cutting half a mile long, 1,000 feet wide, and from 150 to 200 feet deep.—Hoffmann, Encyclopaedie, art. Mexico.

The adit which drains the mines of Gwennap in Cornwall, with its branches, is thirty miles long. Those of the silver mines of Saxony are scarcely less extensive, and the Ernst-August-Stollen, or great drain of the mines of the Harz, is fifteen miles long.

The excavation for the Suez Canal were computed at 75,000,000 cubic metres, or about 100,000,000 cubic yards, and those of the Ganges Canal, which, with its branches, had a length of 3,000 miles, amount to nearly the same quantity.