Geological fault

“ When we reflect, therefore, on the fractional state of the annals which are handed down to us, and how little even these have as yet been studied, we may wonder that so many geologists should attribute every break in the series of strata, and every gap in the past history of the organic world, to catastrophes and convulsions of the earth's crust, or to leaps made by the creational force from species to species, or from class to class. ”

“ Had the great oceanic basins, on the other hand, been unstable, changing places with the land at various periods of geological time, they would, almost certainly, again and again have swallowed up the land in their vast abysses, and have thus destroyed all the organic life of the world. ”

“ The phenomena presented by faults, therefore, are just as eloquent of denudation as is the truncated appearance — 43 — of our strata; and only after we have carefully examined the present extension and mutual relations of our rock-masses, their varied inclination, and the size of the dislocations by which they are traversed, can we properly appreciate the degree of erosion which they have sustained. Before we are entitled to express any opinion as to the origin of the surface-features of a country, we must first know its geological structure. ”

“ Everywhere throughout the world we read the same tale of subsidence and accumulation, of upheaval and denudation. The ancient sedimentary deposits which form the major portion of our land-surfaces, are the — 61 — waste materials derived from the demolition of plains, plateaux, and mountains of elevation. In some mountain-regions we read the evidence of successive epochs of uplift, separated by long intervening periods of erosion, followed by depression and accumulation of newer sediments over the denuded surface. Thus the Alps began to be elevated towards the close of Palæozoic times. ”

“ It may be safely affirmed that the quantity of igneous and aqueous action,—of volcanic eruption and denudation,—of subterranean movement and sedimentary deposition,—not only of past ages, but of one geological epoch, or even the fraction of an epoch, has exceeded immeasurably all the fluctuations of the inorganic world which have been witnessed by man. ”

“ We cannot measure the share which human action has had in augmenting the intensity of causes of mountain degradation, and of the formation of plains and marshes below, but we know that the clearing of the woods has, in some cases, produced, within two or three generations, effects as blasting as those generally ascribed to geological convulsions, and has laid waste the face of the earth more hopelessly than if it had been buried by a current of lava or a shower of volcanic sand. New torrents are forming every year in the Alps. ”

“ In the geographical development of our land, movements of elevation and depression have played an important part. But we cannot ignore the work done by other agents of change. If the orographical features of the land everywhere attest the potency of plutonic agents, they no less forcibly assure us that the inequalities of surface resulting from such movements are universally modified by denudation and sedimentation. Elevated plains and mountains are gradually demolished, and the hollows and depressions of the great continental plateau become slowly filled with their detritus. ”

“ The old formations are all greater than the new, receding in extent as they descend in time. And if we are to regard alterations of climate, changes in the constituents of the atmosphere, subsidence of land and elevation of sea-bottom, intrusion of igneous rocks, the escape and circulation of noxious gaseous matter, as among [327] the causes which have led to the extinction of the successive organic tribes in the several geological epochs, so do we find the effects approximating to a scale of corresponding magnitude. ”

“ The erosion of the land is constantly going on, every shower carries its tribute of earthy matter toward the sea, and every wave that strikes against a beach or cliff does some work toward the grinding of shells, pebbles or stone. Thus, everywhere around our continents there is a continuous deposition of beds of earthy matter, and it is this which, when elevated into new land, has given us our chronological series of geological formations. ”

“ We have, therefore, to consider what must have been the ancient condition of a region subjected throughout vast periods of geological time to the utmost force of disintegrating agencies applied to a rock very liable to yield to them, and where, without reckoning the large proportion which must have been carried by rivers to the sea, we see such vast deposits of the disintegrated materials formed out of the same matrix. ”

“ The sedimentary rocks are built from the disintegrated wreckage of older ones. The products of rock decay are picked up and dragged, or carried in suspension or solution, by wind, running water, or moving ice. ”

“ The vast majority of faults give rise to no surface features. The faulted region may be old enough to have been baseleveled, or the rocks on both sides of the line of dislocation may be alike in their resistance to erosion and therefore have been worn down to a common slope. The fault may be entirely concealed by the mantle of waste, and in such cases it can be inferred from abrupt changes in the character or the strike and dip of the strata where they may outcrop near it. ”

“ But, at the same time, it is an equally well-established bet that the denuding forces ever at work upon the earth's surface would have been competent to the removal of existing continents many times over, in the vast periods covered by geological records. Hence we are driven to conclude that the subterranean movements have in past times entirely compensated for the waste produced by the denuding forces ever at work upon our globe. ”

“ A study of the land-surfaces and sea-floors of the present time shows that there are so many chances against the conservation of the remains of either terrestrial or marine animals and plants that if, as is probable, the same conditions existed in former geological periods, we should regard the occurrence of organic remains among the stratified formations of the earth’s crust as generally the result of various fortunate accidents. ”

“ At a very remote period of the past, the rocks comprising these various strata were subjected to some powerful natural force, which resulted in their fracture, dislocation, and the gradual upheaval of a large portion of them, producing at the point of disruption what is known to geologists as a fault. ”

“ Faults are the most common of all the forms of disturbed stratification: but, except in particular cases, they are the least influential on the physical configuration of the country. All the rocks which are disturbed by any fault have experienced on one side the same movement, and to the same extent, excepting only those portions which have been subjected to violent pressure; and the bottom of the faults has never been reached, except when they terminate in another dislocation.
Relation of Faults, Mineral Veins, Dykes, &c., to the great Lines of disturbed Rocks. ”

“ Great faults have sometimes brought ancient crystalline rocks in contact with weaker and younger sedimentary rocks, and long after erosion has destroyed all fault scarps the harder crystallines rise in an upland of rugged or mountainous country which meets the lowland along the line of faulting. ”

“ Every great Palæozoic limestone, for example, indicates a depression with succeeding elevation. On each elevation marine animals were driven back into the ocean, and on each depression swarmed in over the land, reinforced by new species, either then introduced or derived by migration from other localities. ”

“ There can thus be no doubt that the mysterious agency which produces earthquakes and volcanoes on the land affects equally that portion of the earth's crust covered by the waters of the ocean, and must be ceaselessly employed in elevating large areas of sea-bottom into new continents, that will ere long become clothed with vegetation and peopled with animals. In contemplating, therefore, the constant decay in progress on the surface of the land, we see not a mere isolated process of waste, but a provision for future renovation. ”

“ Several of our first geologists hold, that some of the formidable cataclysms of the remote past may have been occasioned by the sudden upheaval of vast continents, which, by displacing great bodies of water, and rolling them outwards in the character of enormous waves, inundated wide regions elevated hundreds of feet over the sea level, and strewed them over with the rock boulders, clays, gravels, and organic debris of deep sea bottoms. ”

“ The abnormal character of most of the fossiliferous deposits, the sudden world-wide change of climate they record, the marked degeneration in all organic forms in passing from the older to the modern world, together with the great outstanding fact that human beings, with thousands of other living species of animals and plants have at this great world-crisis left their fossils in the rocks all over the world, prove beyond a possible doubt that our once magnificently stocked world met with a tremendous catastrophe some thousands of years ago, before the dawn of history. ”

“ But on the land, no sooner have the subterranean forces upheaved a mountain, a plateau, a continent, or an island, than the processes of subaërial erosion begin to contend against it. Rains, frost and heat, chemical change, subdue the most enduring and solid rock formations, dividing them with fractures, and pulverizing their masses and fragments to sand and clay, which gravitation by the vehicle of running water carries down and away to the sea, there to find rest until another uplift shall renew the cycle of changes. ”

“ The absence, indeed, of chaotic disorder, and the regularity of the plications in geological formations of high antiquity, although not unfrequently adduced to prove the unity and instantaneousness of the disturbing force, might with far greater propriety be brought forward as an argument in favor of the successive application of some irresistible but moderated force, such as that which can elevate or depress a continent. ”

“ Gradational processes belong to two categories—those which level down, degradation, and those which level up, aggradation. The transportation of material from the land, whether by rain, rivers, glaciers, waves, or winds, is degradation and the deposition of material, whether on the land or in the sea, is aggradation. Degradation affects primarily the protuberances of the lithosphere, while aggradation affects primarily its depressions. ”

“ We owe our arable land to that comparatively modern geologic agent, whatever its character, that crushed, as in a mill, the upper parts of the surface-rocks of the kingdom, and then overlaid them with their own debris and rubbish to the depth of from one to forty yards. This debris, existing in one locality as a boulder-clay more or less finely comminuted, in another as a grossly pounded gravel, forms, with few exceptions, that subsoil of the country on which the existing vegetation first found root ”

“ If now a region thus underlaid by what we may call incipient lavas is subjected to the peculiar compressive actions which lead to mountain-building, we should naturally expect that such soft material would be poured forth, possibly in vast quantities through fault fissures, which are so readily formed in all kinds of rock when subject to irregular and powerful strains, such as are necessarily brought about when rocks are moved in mountain-making. ”

“ In many cases this ideal chain is far from realized, but we have instead, a great bulging of the earth’s crust composed on the surface of an inextricable tangle of ridges and valleys of erosion, running in all directions. In all cases, however, the erosion has been immense; for the mountain-chains are the great theatres of erosion as well as of igneous action. As a general fact, all that we see, when we stand on a mountain-chain—every peak and valley, every ridge and cañon, all that constitutes scenery—is wholly due to erosion.”—Le Conte. ”

“ Nor will it avail aught to urge, with certain assertors of a universal deluge, that during the cataclysm, sea and land changed their places, and that what is now land had formed the bottom of the antediluvian ocean, and, vice versa, what is now sea had been the land on which the first human inhabitants of the earth increased and multiplied. No geologist who knows how very various the ages of the several table-lands and mountain chains in reality are could acquiesce in such an hypothesis ”

“ The intricate pattern of river etchings.—The attack of the weather upon the solid lithosphere destroys the integrity of its surface layer, and through reducing it to rock débris makes it the natural prey of any agent competent to carry it along the surface. We have seen how, for short distances, gravity unaided may pile up the débris in accumulations of talus, and how, when assisted by thaw water which has soaked into the material, it may accomplish a slow migration by a peculiar type of soil flow. ”

“ When, therefore, such results are produced by so ordinary a cause, need we go to seek the agency of great debacles to explain the denudation of other parts of the country? It is known that at great depths currents have little effect upon the rocks which they traverse, and that their action is greater as it nears the surface. To account for the phenomena of crag and tail, and the general denudation of the country, we may suppose the land to have been often submerged and re-elevated. ”

“ The deposition of the finer parts of volcanic discharges will be considered in connection with the deposits of the deep sea.
Gradation.—The gradational processes of the land and the sea are in striking contrast. On the land, degradation predominates, and aggradation is subordinate. In the sea, aggradation predominates, and degradation is subordinate. On the land, degradation is, on the whole, greatest where the land is highest, while aggradation is of consequence only where the land is low, or where steep slopes give place334 to gentle ones. ”