« PreviousContinue »
Earth,' towards the end of last century; and such the conclusion at which every one must arrive who gives the matter sufficient and enlightened consideration. But this incessant transmutation of the solid framework of the globe is a conception not readily realised by ordinary minds, partly from the restricted range of observation during a single lifetime, and partly from our limited notions of time, which is in itself illimitable and altogether independent of the events that mark the course of its continuity. This difficulty was not unforeseen by the Scotch philosopher, and so he goes on to remark :—" It is not to common observation that it belongs to see the effects of time, and the operation of physical causes, in what is to be perceived upon the surface of the earth. The shepherd thinks the mountain on which he feeds his flock to have always been there, or since the beginning of things; the inhabitant of the valley cultivates the soil as his father had done, and thinks that this soil is coeval with the valley or the mountain. But the man of scientific observation, who looks into the chain of physical events connected with the present state of things, sees great changes that have been made, and foresees a different state that must follow in time, from the continued operation of that which actually is in nature." It is the object of the present Sketch to place this system of waste and reconstruction—of destruction and renovation—in a clear and obvious light, that the "common" as well as "scientific" mind may perceive the means employed by the Creator to keep this world of ours ever young notwithstanding its vast antiquity, and to maintain its stability in the midst of incessant vicissitude.
To the casual observer the hills and valleys that surround him appear unchanged and unchangeable. The plains and battle-fields mentioned in ancient history, the sites of cities and harbours, the courses of rivers, and the contour of mountains, are much the same as when described one thousand, two thousand, or even four thousand years ago. But to him who looks a little more narrowly the case is altogether different. The stream in the valley has cut for itself a deeper channel, and has repeatedly shifted its course—eating away the banks on one side, and laying down spits of new ground on the other. The cliffs in the hills are more weather-worn and rounded, and a larger mound of rock-debris has accumulated at their bases. The lakes of the old historic plain are partly converted into marshes, and the marshes into meadow-land; the site of the old city on the sea-cliff has been partly wasted away by the encroaching waves; and the ancient harbour, once at the river-mouth, is now several miles inland, and separated from the sea by a flat alluvial delta. The Nilotic plain is not precisely the same as when described by Herodotus; the sunderbunds or mud-islands of the Ganges have been largely augmented during the last two hundred years; and many areas that were laid down on the charts of our earlier traders as mudflats, now form fertile portions of the great Chinese plain. Vesuvius has repeatedly changed its aspects since Herculaneum and Pompeii were buried beneath its ejections; and there is scarcely an active volcano that has not materially added to its bulk since the commencement of the current century.river may deepen its channel, gradually as the delta may advance upon the estuary, and little by little as the volcano may pile up its scoriae and lava, yet after the lapse of ages the mountain will be worn down, the river-channel will be eroded into a valley, the estuary converted into an alluvial plain, and the volcano rear its cold and silent dome into the higher atmosphere. All that is necessary is time, and this is an element to which we can see no limit in the future, any more than we can discover a beginning to it in the past. To render these incessant mutations thoroughly intelligible, however, to the ordinary observer, it will be necessary to describe the agents by which they are effected, and at the same time the varying power of these agents according to the latitudes and altitudes within which they operate. We say latitudes and altitudes, for whether it be rainfall, frost, wave-action, or organic growth, every latitude and altitude has its own intensity, and what is scarcely felt in one region may be followed in another by stupendous results. These agencies may be conveniently arranged under five great categories; namely, 1, The Meteoric, or those—like winds, rains, and frosts—depending upon the atmosphere; 2, The Aqueous, or those—like rivers, waves, and tides— arising from the action of water; 3, The Chemical, or those resulting from chemical actions and reactions; 4, The Organic, or those—like peat-mosses and coral-reefs—depending on the growth and decay of plants and animals; and 5, The Igneous, or those—like the volcano and earthquake—connected with the manifestations of heat within the interior of our planet. Each of these agencies has its own mode of working—some chiefly wearing and degrading, some degrading and at the same time accumulating, and others solely reconstructing. Let us now glance at them in detail:—
Such changes are incessant, and though individually they may seem insignificant, yet when viewed in the aggregate, and continued from century to century, they assume a magnitude commensurate with the crust of the globe itself, every portion of which has repeatedly suffered degradation and renovation, been repeatedly spread beneath the waters as sediment, and as repeatedly reconstructed into newer strata and upheaved into dry land. Imperceptibly as the. rains and frosts may wear away the mountain-cliff, slowly as the
The principal effect of the Meteoric or Atmospheric agencies is to weather and wear away. Slowly but surely the gases and moisture of the atmosphere eat into every exposed rock-surface. The disintegrated matter is washed down by the rains, taken up by the runnels and streams, and borne onward by the rivers to the ocean. We often see the effect of heavy rainfalls on exposed soils and surfaces in our own islands—how they batter, loosen, and carry away; but our rainfall, amounting annually to some 30 or 40 inches, is trifling compared with the rainfall of tropical and sub-tropical countries, ranging from 200 to 400 inches, and this concentrated for the most part within one period of the year. It is not uncommon to hear .travellers speak of the soils being converted into mud, and of the rivers running mud rather than water, and this solely through the battering and dissolving influence of the periodical rains. Again, frost in all the higher latitudes and altitudes is annually performing a similar function. The moisture that inserts itself into the pores and interstices of all rock-substances is converted into ice during frost; ice occupies more space than the water of which it consists, or, in other words, water expands during freezing; the particles of rock-matter are distended or forced asunder; and when thaw comes, their cohesion being loosened, they are washed away by the rains and carried down by the streams and rivers. Every winter we see the disintegrating effects of frost on the ploughed soils, road-cuttings, and seacliffs of our own islands; and this effect is manifested a hundredfold in all the colder latitudes and in all the higher mountains, whether within tropical, temperate, or arctic regions. The destructive power of frost is stupendous, whether silently crumbling away the cliffs and precipices; discharging the avalanche and landslip down the mountainslope; slowly grinding its way as the glacier through the Alpine glen; or transporting and dropping, as the iceberg does, its burden of rock-debris over the floor of the ocean. As with the rains and frosts, so to a certain extent with the winds or aerial currents of the atmosphere. Wherever there is rock-matter sufficiently light and loose, thence the winds will remove it and carry it away to some more sheltered locality. And if the set of the wind be constant, or chiefly from one direction, like the trades and sea-breezes, the result in the long-run will be very marked and perceptible. By this means the dry sand of the sea-shore is blown inland and beyond the reach of the tide into mounds and hillocks (sand-dunes, as they are termed); and along every shore in the world there are recently-formed expanses of this nature, often—like the " Landes" of France—of vast extent, and still in the process of augmentation. As with the sands of the sea-shore, so with the sands of the arid deserts; they are driven hither and thither into dunes and ridges, but chiefly forward in one main direction according to the prevailing winds, and this to the obliteration of streams and oases, and to the destruction of fertile valleys that lie in their way. Gentle as it may seem, the drifting of sand over the surface of granite and basalt has been known to wear and polish down their asperities, and even to grind out grooves and furrows like those produced by the long-continued motion of glacier-ice or the flow of running water.*
But perceptible as may be the effects of the meteoric forces, they are far less obvious than those produced through and by the agency of water. The Aqueous are generally on a larger scale; and wherever streams and rivers run, waves
* At the Pass of San Bernardino in California, Mr W. P. Blake (as quoted by Professor Dana) observed the granite rocks not only worn smooth, but covered with scratches and furrows by the sands that were drifted over them. Even quartz was polished, and garnets were left projecting from pedicles of felspar. Limestone was so much worn as to look as if the surface had been removed by solution.