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Popular Science Monthly

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Newbery, Schuchert and others have pointed out that there seem to have been great cycles of sedimentation which may be interpreted as due to the alternate success, first of the factors of elevation, then of those of degradation. Suppose, for instance, that there has been an epoch of elevation, that mountain chains have been lifted far into the sky and volcanoes have sent their floods of lava forth, and fault-scarped cliffs run across the landscape and that then, for a while, the forces of elevation cease their work. Little by little, the mountains will be worn down to a surface of less and less relief, approaching a plain as a hyperbola approaches its asymptote--a surface which W. M. Davis has called peneplain. But where will the material thus worn go? Into the sea. Going into the ocean it will raise the level of the sea slowly but surely. At present, for every four feet of elevation taken off the land, there will be something like one foot rise of the ocean level, and this rise may take only thirty thousand years--a long time in human history, but not so long in the history of the earth. All the time, then, that the forces of the atmosphere are wearing down the surface of the earth to the sea level the sea is rising and its waves are producing a plain of marine denudation which rises slowly to meet the peneplain which is produced by degradation. In the beginning of this cycle, where the forces of degradation have their own way, coarse material may be brought down by torrents from the mountains, and the glaciers, which find their breeding place in these high elevations, may drag down and deposit huge masses of boulder clay. But, little by little as the mountains are lowered, the sediments derived from them will become finer and finer and glaciers will find fewer and fewer sources. Not only that, but the growth of seas extending over the continents will tend to change the climate, we shall have a moister, more insular climate, we shall have a greater surface of evaporation, and thus, on the whole, a more equable temperature throughout the world. We know that, at present, the extremes of cold and hot are found far within the interior of the continents. Continental climates are the climates of extremes, and on the whole extremes are hurtful to life. So then as the forces of degradation tend to lower the continents beneath the sea level glaciers and deserts and desert deposits alike must also disappear. Vegetation will clothe the earth, and marine life swarm in the shallow seas of the broadening continental shelf. Under the mantle of vegetation, mechanical erosion will be less, that is, the breaking up of rocks into small pieces without any very great change, but the rich soil will be charged with carbon dioxide, and chemical activity will still go on. Rivers will still contain carbonates, even though they carry very little mud, and in the oceans the corals and similar living forms will deposit the burden of lime brought into the sea by the rivers. Thus, if forces of degradation have their own way, in time there will be a gradual change in dominant character, from coarse sediments to fine, from rocks which are simply crumbled debris to rocks that are the product of chemical decay and sorting, so that we have the lime deposited as limestone in one place and the alumina and silica, in another. We shall have a change from local deposits, marine on the edges of large continents, or land deposits, very often coarse, with fossils few and far between, to rocks in which marine deposits will spread far over the present land in which will appear more traces of that life that crowded in the shallow warm seas which form on the flooded continents. We shall have a transition from deposits which may be largely formed on the surface of the continents. lakes, rivers, salt beds and gypsum beds, due to the drying up of such lakes and the wind-blown deposits of the steppes, to deposits which are almost wholly marine. Now, I need not say (to those who are familiar with geology) that we have indications of just such alternations in times passed. There are limestones abounding in fossils, with a cosmopolitan life very wide spread to be recognized in every continent, such as used to be known as the Trenton limestone, the mountain limestone, the chalk. Perhaps every proper system and period should be marked by such a limestone in the middle. The time classed as late Permian and Triassic on the other hand was one of uplift, disturbance, volcanic action and extreme climates, which gave us the traps of Mt. Tom, the Palisades of the Hudson, the bold scenery of the Bay of Fundy and the gypsum and red beds which are generally supposed to be quite largely formed beneath the air and beds of tillite formed beneath glaciers. Then in the times succeeding, in many parts of the world, degrading forces were more effective than uplifting so that the mountains became lower, and the seas extended farther over the continents. Then the prevalence of lime sediments was so great that the "chalk" was thought to be characteristic everywhere. And about the time the "chalk" the land was reduced to a peneplain. A similar cycle may be traced from the Keweenawan rocks to the group of limestones so widespread over the North American continent and so full of fossils, which to older geologists and oil drillers have been known, in a broad way, as Trenton. All this introduces a question--to which I wish to suggest an answer--How is it that these cycles came to be? Were the outer rock crust of the earth perfectly smooth the oceans would cover it to the depths of thousands of feet and it is only by the wrinkling of such a crust that any part of it appears above the ocean. If the earth had a cool thin crust upon a hot fluid interior, and that thin crust were able to sustain itself during geologic ages so that the shrinkage should accumulate within, until finally collapse came, giving an era of uplift, it is obvious that we could account for such cycles. There is very clear evidence that the outermost layer of the earth's crust is but a thin shell like the outer shuck or exocarp of a butternut, so thin that it is not at all possible that it can sustain itself for more than a hundred miles or so, or for more than a very few years at the outside. Hayford's[1]

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