Popular Science Monthly/Volume 41/July 1892/The Waste and Gain of the Dry Land
|THE WASTE AND GAIN OF THE DRY LAND.|
THE following letter, respecting a former article in the Monthly, has been addressed to the Editor:
Editor Popular Science Monthly:
Does M. A. de L'Apparent's interesting address on The Future of the Dry Land exhaust all the factors of the inquiry? Is it certain, for example, that some at least of the greatest mountain-chains have not in the main risen in elevation faster than the eroding agencies have depressed them? Are not some of them, and those now among the boldest, admitted to have been uplifted in comparatively recent times, geologically speaking—more recently, for example, than the advent of some of the rivers which intersect them? Erosion must have commenced from the very beginning of the upheaval and have continued to the present time; yet they grew in stature in spite of it, for no one now supposes that the upheaval was a sudden one. Indeed, the persistence of the river's "right of way" proves both the constant action of "the elements" and the extremely gradual character of the upheaval. If our planet, as some think, continues to slowly contract from a once nebulous condition, its advancing age might be expected to be marked by wrinkles, just as we know it to be.
Again, is there not good ground for the conjecture that our globe, however slowly, is approaching a state of desiccation such as is manifested partially in our neighbor Mars and still more notably in the moon? Is there not some lingering continuance of the once active absorption? Is there not, for example, reason to believe that the proportion of sea area has in the main steadily diminished and that upheaval of the land masses has steadily increased since Silurian times?
To the lay reader this masterful and interesting paper of the French savant seems a courteous invitation to one of his compeers to take up the thread of his discourse at the point at which he elects to leave it.George Henry Knight.
- Temple Court, New York.
Other letters on this subject have been received, all of which indicate that the matter has attracted more interest than was anticipated. Our correspondents may find on closer examination of M. de L'Apparent's paper that he avowedly presents it as covering only one side of the question, and that, while he does not discuss the compensating forces, he recognizes their existence and their title to be considered. If we understand him aright, his article was designed to be tentative, and as opening the way to a discussion in which much may be said on both sides. The discussion which follows will help to give more light concerning M. de L'Apparent's views and meaning and their value. M. Jacques Léotard has written in the Revue Scientifique that M. de L'Apparent has in his evidently very curious study arrived at his result only by neglecting several factors of contrary effect. While it is admitted that the earth is swept by powerful atmospheric agents which, if their work was continued without compensatory action on the other side, would ultimately level and submerge all the continents, M. Ldotard insists that there exist other very important causes of increase of the relief, the action of which now counterbalances and may ultimately surpass that of the solvent influences. One of these causes, of which M. de L' Apparent took some notice, is the contribution of volcanic products to the soil It is one of the most minute of the factors, but M. de L'Apparent's estimate of one sixth of a cubic kilometre a year seems too small. The three hundred known active volcanoes on the surface of the earth ought to give out a much larger quantity of their internal products; and it should be remarked that the dejections of craters, besides lavas, comprise various rocks, mud, and ashes. But the importance of this factor, little at the most, is made still less by the occurrence of volcanic explosions on the sea-coast, in which considerable tracts of land have been swallowed up. The chief essential cause of increase of dry land at the expense of the ocean lies in the evolution of our planet. During the geological epochs of thousands of centuries each the upheavals which formed existing continents have come in gradual succession, taking from the primitive sea, which originally extended over the whole earth, a larger and larger part of its immense domain. These upheavals, under the action of internal forces, have continued slowly till our own time in many regions, notably in the north and center of the Scandinavian Peninsula, Spitsbergen, northern Siberia, Turkistan, Scotland, Sardinia, Tunis, on the coasts of the Red Sea, etc., while the depressions of vast countries, which must not be confounded with little local collapses produced by the subterranean work of water, are less numerous. Besides the increase of continents, new islands of volcanic origin rise at times to the surface of the seas, and lands are also gradually formed by the accumulation of sedimentary matter and organic remains. The deltas which rise at the mouths of large rivers in consequence of the deposition of mud and sand transported by the streams, likewise constitute an augmentation of the emerged soil, for the space they occupy is taken from the sea. The sea, moreover, does not eat away all the shores. There are coasts where the waves, instead of carrying away parcels of the dry land, operate to fill up the bays and to add to the littoral and prolong it in the direction of the sea. Thus the ocean every year deposits several million cubic metres of sand along the shores of the Gulf of Gascony. Another accrement which has a right to be regarded as considerable is contributed by the legions of polyps constructing reefs and atolls of coral, to which is due the building up of whole archipelagoes in Oceania and the Indian seas. The islands formed by these minute zoophytes are growing continually in extent and number, and are probably destined ultimately, by joining, to give rise to vast lands, real continents, which will gradually occupy the immense voids of the Pacific. The shells of numerous species of animals and other remains of dead organisms, meteorites and cosmic dusts falling from celestial space, certainly produce further sensible augmentations of the continental mass. Another essential cause of increase of dry land that might be added is the decrease of the ocean itself in consequence of infiltrations of water through the crust of the earth, which is a kind of porous mass, into which the liquid element percolates by innumerable fissures, taking possession of the depths and directing itself slowly toward the center, as the internal fire diminishes and the crusts crack open in consequence. It is understood that the activity of volcanoes and many earthquakes is largely due to this inevitable penetration of the water, which internal heat transforms into vapor under pressure. Some geologists think that the primitive ocean has already diminished in this way one fiftieth of its volume. The water is all destined to disappear from the surface of the globe by being absorbed by the subterranean rocks, with which it will form chemical combinations. The heavenly spheres exhibit sufficiently striking examples of such an evolution. The planet Mars shows what will become of the earth in some thousands of centuries. Its seas are only shallow Mediterraneans of less surface than the continents, and these do not appear to be very high; and in the appearance of the moon, all cracked and dried up, we have a view of the final state of the earth for the absorption of the water by the solid nucleus will be followed by that of the atmosphere. We see, therefore, M. Léotard continues, that not only is there no equilibrium in the struggle between the oceans and the continents, but that, inversely to the conclusions of M. de L'Apparent, the event that may be considered very probable in a future presented by millions of years will be, not the disappearance of the dry land, but that of the sea, which, accompanied by all the fluids, will gradually infiltrate through the crust with which our planet is covered. The certain feature in M. de L'Apparent's essay is that, before the eternal drought comes on, the terrestrial relief will be leveled. The continental surface will have become an immense plain, in which the Alps, Himalayas, and Andes will be only little hills. The fertility of the soil will be augmented by the considerable formation of vegetable earth, which will at the same time be deprived of sufficient watering by the rarity of rain. Climates, little as we may suppose them to be modified by the decrease of the luminous and calorific energy of the sun, will be entirely transformed. It seems to M. Leotard, in short, that the phenomena contributing to the destruction of the continents will diminish continuously in intensity, while the natural influences tending to result in the desiccation of the surface of the globe will gather energy in the course of ages, preparing for our planet the curious future just described—a future which will, however, be postponed so far that mankind will not be a witness of that end of terrestrial evolution. Another writer in the Revue Scientifique (H. S.) has called attention to what the land is gaining, believing, in view of the universal stability of affairs, that it must be equal to the losses. The land gains everything, including cosmic dust and meteorites, that falls from space; it gains all the gases that are continually undergoing solidification in flesh and wood, with which they become incorporated; and it gains the shells of all the molluscs, infusoria, etc. With a very insignificant part of what these infinitely small beings have left it has been possible to build cities larger than Paris; and the Great Pyramid may be said to be the work, not of King Cheops, but of the nummulites. If a well-informed person should follow out all the facts bearing on the subject, he would probably find a complete equilibrium, an admirable compensation existing between the gains and the losses of the crust of the earth. M. de L'Apparent has replied to M. Léotard's criticism that it rests on a misunderstanding which can be easily dissipated. In the summer of 1890, he says, "I discussed, in the Geological Society of France, the general question of erosion, not to predict the actual leveling down of the dry land, but simply to arrive at a method of estimating the duration of the geological periods. My reasoning was as follows: If the present causes of destruction (mechanical and chemical action of running waters and marine erosion) continue to act in the same measure as now, without anything intervening to disturb their working, the continental relief will wholly disappear in four or five million years. Geology teaches us that the history of the crust of the earth embraces a period infinitely longer than this. That is enough to prove that another factor does intervene—that is, manifestations of internal energy, which disturb from time to time the acquired states of equilibrium, and restore a new force to the decreasing outer powers. Thus, instead of assuming a regular process of planing down, I laid down in principle that things would not go on in this way. A geologist, besides, could not reason otherwise, except he mistook the daily teachings of science which show him at every instant foldings and contortions of strata, certain signs of an order of things very different from the regular pursuit of external influences. "Seeking, then, to place myself under the most unfavorable conditions for my theory, I supposed the ancient history of the globe divided into tranquil periods, each of four or five million years, separated from one another by so many ruptures of equilibrium. How many of these periods would be required to account for all the known sedimentary formations? In trying to solve this problem, I remarked that each period would have cast into the ocean a cube of débris which, scattered, according to Mr. J. Murray, through only one fifth of its area (the fraction over which, according to the soundings, the sedimentation of detritus extends), would form a bed of some six or seven hundred metres in mean thickness. It seemed to me reasonable to suppose that this thickness, null at the extreme further side of the deposits, would increase slowly at first, and then more rapidly toward the neighborhood of the coast, where it might attain a maximum of two kilometres. Dana having estimated at forty-five thousand metres the united depth of all the sedimentary formations when each is measured at its point of greatest thickness, I drew the conclusion that all geological history could be included within a time certainly less than ninety million years. After the publication of this note I was called upon by the general secretary of the Geographical Society to supply the place of a speaker who was unable to fulfill his engagement. I responded, undertaking to call the attention to the phenomena of erosion which I had been studying, without elaborating the purely geological considerations. I have recently presented an extended paper on these details to the Catholic International Scientific Congress, and in it have examined all the phases of the question. This paper will be published in full, and in it M. Ldotard will be able to learn how greatly my views differ from those which he has mistakenly attributed to me. "I will here only correct a grave error which my critic commits when he charges me with not having taken account of the accrement which volcanic action brings to the dry land. M. Léotard forgets that all the lava that flows to the surface comes from the depths of the crust, where its departure leaves a void which can be compensated for only by the depression of the adjoining territory to such an extent that the emerged relief really gains nothing. More than this—volcanic action, which M. Léotard would make creative, is really, above everything else, destructive. I ask no better proof of this than the great explosions of which the nineteenth century has been the witness; that of 1845, at Temboro, which covered the neighboring country and the surface of the sea with a mass of débris estimated at a hundred cubic kilometres; and the more recent eruption at Krakatoa, which threw into the Strait of Sunda eighteen cubic kilometres of débris and formed an abyss between two and three hundred metres deep, in a place where there had previously stood a volcanic mountain several hundred metres high. While I have felt called upon to make this rectification, I will add that that does not prevent me from believing, with M. Léotard, that the final triumph of the dry land is infinitely more probable than its submersion; and that by reason of the considerable movements of the crust and the wrinkles which lateral compression in consequence of the progress of cooling can not fail to engender from time to time."
- Popular Science Monthly, June, 1891.
- Do not northern Africa and western Asia contain vast regions that have passed from exuberant fertility to hopeless aridity even during the historical period?—G. H. K.